Problem

You want to work out the value of a resistor.

Solution

On a through-hole resistor (a resistor with leads) that has colored stripes on it, use the resistor color code.

If your resistor has stripes in the same positions as Figure 2-1 then the three stripes together on the left determine the resistor’s value and the single stripe on the right determines the accuracy of the value.

Figure 2-1. A Three-Stripe Resistor

Each color has a value as listed in Table 2-1.

For a three-stripe resistor such as this, the first two stripes determine the basic value (say 27 in Figure 2-1) and the third stripe determines the number of zeros to add to the end. In the example of Figure 2-1, the value of a resistor with stripes red, purple, and brown is 270Ω. I said before that this stripe indicates the number of zeros, but actually, to be more accurate, it is a multiplier. If it has a value of gold, then this means ⅒ of the value indicated by the first two stripes. So brown, black, and gold would indicate a 1Ω resistor.

The stripe on its own specifies the tolerance of the resistor. Silver (rare these days) indicates ±10%, gold ±5%, and brown ±1%.

If your resistor has stripes as shown in Figure 2-2 then the value of the resistor is specified with an extra digit of precision. In this case, the first three stripes determine the basic value (in the case of Figure 2-2, 270) and the final digit the number of zeros to add (in this case, 0). This resistor is also 270Ω.

Figure 2-2. A Four-Stripe Resistor

For low-value resistors, gold is used as a multiplier of 0.1 and silver of 0.01. A 1Ω four-stripe resistor would have value stripes of brown, black, black, and silver (100 x 0.01).

Discussion

Although tiny, surface mount technology (SMT) resistors normally have their value of resistance printed on them. However, the same system of base value followed by multiplier is used so a 270Ω SMT resistor would have the number 2700 printed on it and a 1kΩ resistor 1001.

See Also

Through-hole capacitors also have value labels similar to SMT resistors (see Recipe 3.3).

Problem

You’ve done your calculations and you need a 239Ω resistor, but how do you match this to a standard value that you can actually buy?

Solution

Buy a resistor from ±5% E24 series.

Values in the E24 series have basic values of 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, and 91, with as many zeros after them as you need.

Discussion

The ±1% E96 series includes all the base values of the E24 series, but has four times as many values. However, it is very rare to need such precise values of resistor.

If your resistor is to limit current to some other component that might be damaged, perhaps limiting the power to an LED (Recipe 4.4) or into the base of a bipolar transistor (Recipe 5.1), then pick the next largest value of resistor from the E24 series.

For example, if your calculations tell you that the resistor should be a 239Ω resistor, then pick a 240Ω resistor from the E24 series.

In reality, you may decide to limit yourself even further to avoid having to gradually collect every conceivable value of resistor, especially as they are often sold in packs of 100. I generally keep the following resistor values in stock: 10Ω, 100Ω, 270Ω, 470Ω, 1k, 3.3k, 4.7k, 10k, 100k, and 1M.

See Also

For full details on all of the resistor series available, see http://www.logwell.com/tech/components/resistor_values.html.

Problem

You want to understand how variable resistors work.

Solution

A variable resistor, a.k.a. a pot or potentiometer, is made from a resistive track and a slider that varies its position along the track. By varying the position of the slider you can vary the resistance between the slider and either of the terminals at the ends of the track. The most common pots are rotary like the one shown in Figure 2-3.

Figure 2-3. A Rotary Pot

Discussion

Pots come in a large variety of shapes and sizes. Figure 2-4 shows a selection of pots.

Figure 2-4. Potentiometers

The two pots on the left of Figure 2-4 are called trimmers or trimpots. These devices are designed to be turned using a screwdriver or by twiddling the tiny knob between the thumb and forefinger.

The next pot is a pretty standard device with a threaded barrel that allows the pot to be fixed into a hole. The shaft can be cut to the length required before fixing a knob to it.

There is a dual-gang pot in the middle of Figure 2-4. This is actually two pots with a common shaft and is often employed in stereo volume controls. After that is a similar looking device that combines a pot with an on/off switch. Finally, on the right is a sliding type of pot, the kind that you might find on a mixing desk.

Pots are available with two types of tracks. Linear tracks have a close to linear resistance across the whole range of the pot. So, at the half-way point the resistance will be half of the full range.

Pots with a logarithmic track increase in resistance as a function of the log of the slider position rather than in proportion to the position. This makes them more suitable to volume controls as human perception of loudness is logarithmic. Unless you are making a volume control for an audio amplifier, you probably want a linear pot.

See Also

To connect a pot to an Arduino or Raspberry Pi, see Recipe 12.9.

A potentiometer lends itself to being a variable voltage divider (see Recipe 2.6).

Problem

You want to understand the overall resistance and power-handling implications of placing a number of resistors in series.

Solution

The overall resistance of a number of resistors in series is just the sum of the separate resistances.

Discussion

Figure 2-5 shows two resistors in series. The current flows through one resistor and then the second. As a pair, the resistors will be equivalent to a single resistor of 200Ω.

Figure 2-5. Resistors in Series

The heating power of each resistor will be .

If you used a single resistor of 200Ω then the power would be:

So, by using two resistors, you can double the power.

You may be wondering why you would ever want to use two resistors in series when you could just use one. Power dissipation may be one reason, if you cannot find resistors of sufficient power.

But there are other situations, such as the one shown in Figure 2-6, where you use a variable resistor (pot) with a fixed resistor to make sure that the combination’s resistance does not fall below the value of the fixed resistor.

Figure 2-6. A Pot and Fixed Resistor

See Also

Resistors in series are often used to form a voltage divider (see Recipe 2.6).

Problem

You want to understand the overall resistance and power-handling implications of placing a number of resistors in parallel.

Solution

The combined resistance of a number of resistors in parallel is the inverse of the sum of the inverses of the resistors. That is, if there are two resistors R1 and R2 in parallel, then the overall resistance is given by:

Discussion

In the example shown in Figure 2-7 with two 100Ω resistors in parallel, the arrangement is equivalent to a single resistor of:

Intuitively, this makes perfect sense, because there are now two equally resistive paths through the resistors instead of one as would be the case with a single resistor.

Figure 2-7. Resistors in Parallel

In Figure 2-7 a single 50Ω resistor is equivalent to the two 100Ω resistors in parallel, but what implications does this have for the power ratings of the two resistors?

Intuitively, you would expect the total power dissipation of two 100Ω resistors to be the same as the two 50Ω resistors, but just to be sure let’s do the math.

For each 100Ω resistor, the power will be:

So the total for the two resistors will be 45mW, allowing you to use lower power and more common resistors.

Just as you’d expect, performing the calculation for the single 50Ω resistor you get:

See Also

See Recipe 2.4 for resistors in series.

Problem

You have a DC or AC voltage that you want to reduce.

Solution

Use two resistors in series as a voltage divider (also called potential divider). The word “potential” indicates the voltage has the potential to do work and make current flow.

Figure 2-8 shows a pair of resistors being used as a voltage divider.

Figure 2-8. A Voltage Divider

The output voltage (Vout) will be a fraction of the input voltage (Vin) according to the formula:

For example, if R1 is 470Ω, R2 is 270Ω, and Vin is 5V:

Discussion

Note that if R1 and R2 are equal then the voltage is divided by 2.

A pot naturally forms a potential divider as you can think of it as two resistors in series whose overall resistance adds up to the same value, but the proportion of the resistance of R1 to R2 varies as you turn the knob. This is just how a pot is used in a volume control.

It may be tempting to think of a voltage divider as useful for reducing voltages in power supplies. However, this is not the case, because as soon as you try to use the output of the voltage divider to power something (a load), it is as if another resistor is being put in parallel with R2. This effectively reduces the resistance of the bottom half of the voltage divider and therefore also reduces the output voltage. This will only work if R1 and R2 are much lower than the resistance of the load. This makes them great for reducing signal levels, but of no use for high-power circuits.

See Also

See Chapter 7 for various techniques for reducing voltages in power supplies.

For level shifting with a voltage divider, see Recipe 10.17.

Problem

You want to know what power rating to use for a resistor to avoid overheating and failing it.

Solution

Calculate the power (Recipe 1.6) your resistor will be converting into heat and pick a resistor with a power rating comfortably greater than this value.

For example, if you have a 10Ω resistor connected directly to the terminals of a 1.5V battery, then the power that the resistor converts to heat can be calculated as:

This means that a standard ¼W resistor will be OK, but you may wish to take the next step up to ½W.

Discussion

The most common power rating of resistor used by hobbyists is the ¼W (250mW) resistor. These resistors are not so tiny that they are hard to handle or the leads too thin to make good contact in breadboard (Recipe 20.1) yet they are capable of handling enough power for most uses, such as limiting current to LEDs (Recipe 14.1) or used as voltage dividers for low current (Recipe 2.6).

Other common power ratings for through-hole resistors with leads are ½W, 1W, 2W, 5W, 10W, and even higher power resistors.

Figure 2-9 shows a selection of resistors of different power ratings.

When it comes to the tiny surface-mount “chip resistors” that are surface mounted onto circuit boards, power ratings start much lower.

See Also

To understand power, see Recipe 1.6.

Problem

You want to measure the intensity of light electronically.

Solution

Use a photoresistor.

A photoresistor (Figure 2-10) is a resistor in a clear plastic package whose resistance varies depending on the amount of light falling on it. The more brightly the photoresistor is illuminated, the lower the resistance.

A typical photoresistor might have a resistance in direct sunlight of 1kΩ increasing to several MΩ in total darkness.

Discussion

Photoresistors are often used in a voltage-divider arrangement (Recipe 2.6) with a fixed-value resistor to convert the resistance of the photoresistor into a voltage that can then be used with a microcontroller (Recipe 12.6) or comparator (Recipe 17.10).

Figure 2-10. A Photoresistor

See Also

You will find more details on using a photoresistor in Recipe 12.6.

Problem

You want to measure temperature electronically.

Solution

One method is to use a thermistor. Other ways can be found in Recipe 12.10 and Recipe 12.11.

All resistors are slightly sensitive to changes in temperature. However, thermistors (Figure 2-11) are resistors whose resistance is very sensitive to changes in temperature. As with photoresistors (Recipe 2.8), they are often used in a voltage divider (Recipe 2.6) to convert the resistance to a voltage reading that is more convenient.

Figure 2-11. Two Thermistors

Discussion

There are two types of thermistors. NTC (negative temperature coefficient) thermistors are the more common type and their resistance decreases as the temperature increases. PTC (positive temperature coefficient) thermistors have a resistance that increases as the temperature increases.

In addition to being used to measure temperature (see Recipe 12.7) PTC thermistors are also used to limit current. As the current through the thermistor increases, the resistor heats up and thus its resistance increases, reducing the current.

See Also

For practical circuits that use a thermistor to measure temperature, see Recipe 12.7 and Recipe 12.8.

Problem

The idealized wire has zero resistance. In reality wires do have resistance and you want to know how to account for this in designs as well as know about different types of wire.

Solution

Wires all have resistance. A thick copper wire will have a much lower resistance than the same length of a much thinner wire. As someone once said, “the great thing about standards are that there are always lots to choose from.” Nowhere is this more true than for wire thicknesses or gauges. One of the most common standards is AWG (American Wire Gauge) mostly used in the US, and the SWG (Standard Wire Gauge) mostly used in the UK and most logically just the diameter of the wire in mm.

Nearly all wiring used in electronics is made of copper. If you strip the insulation off a wire and it looks silver-colored, then it’s probably still copper, but copper that has been “tinned” to stop it from oxidizing and make it easier to solder.

Table 2-2 shows some commonly used wire gauges along with their resistances in Ω per foot and meter for copper wiring.

The larger the AWG number the thicker the wire. Wires thinner than 24AWG are most likely to be thin enameled wire designed for winding transformers and indictors, like the wire shown in Figure 2-12.

Figure 2-12. Enamel-Insulated Wire for Winding Inductors 30-22 AWG

Solid-core wire is made of a single strand of copper insulated in plastic (Figure 2-13). It is useful with solderless breadboards (Recipe 20.1) but should not be used in situations where it is likely to be flexed as it will break through metal fatigue if bent back and forth. I always have this wire in at least three colors so that I can use black for negative, red for positive, and other colors for nonpower connections.

Figure 2-13. Single-core Hookup Wire (24 AWG)

For general-purpose wire where some movement is likely, use multicore wire made up of a number of strands of wire twisted about each other and encased in plastic insulation. Again, it’s useful to have a selection of colors.

Figure 2-14. 19 and 15 AWG Multicore Wire with Twin “Bell-wire”

Discussion

The currents listed in Table 2-2 are only suggestions. The actual current that each gauge of wire can carry without getting too hot depends on many factors, including how well ventilated the project’s case is and whether the wires are all grouped together with a whole load of other wires all getting warm. So treat Table 2-2 as a guide.

When you buy wire, the maximum temperature of the insulation will usually be specified. This is not just because of internal heating of the wire but also for situations where the insulation is to be used in hot environments such as ovens or furnaces.

If you are looking for wires to carry high voltages then you will need a good insulation layer. Again, wires will normally have a breakdown voltage specified for the insulation.

See Also

For a table comparing wire gauges, see http://bit.ly/2lOyPIh.

For the current-handling capabilities of wire by gauge, see http://bit.ly/2mbgZS8.

Problem

You need an electronic component that can store energy for short periods of time, perhaps to create pulses, or to insulate other components from voltage spikes.

Solution

Use a capacitor.

In construction, capacitors are just two conductive surfaces separated by an insulating layer (Figure 3-1).

Figure 3-1. A Capacitor

In fact, the insulating layer between the conductive surfaces of the capacitor can just be air, although a capacitor using an air gap will be of very low value. In fact, the value of the capacitor depends on the area of the conducting plates, how close they are together, and how good an insulator separates them. So the greater the area of the plates and the smaller the distance between them the greater the capacitance (the more charge it can store).

Individual electrons do not flow through a capacitor, but those on one side of the capacitor influence those on the other. If you apply a voltage source like a battery to a capacitor, then the plate of the capacitor connected to the positive supply of the battery will accumulate positive charge and the electric field this generates will create a negative charge of equal magnitude on the opposite plate.

In water terms, you can think of a capacitor as an elastic membrane in a pipe (Figure 3-2) that does not allow water to pass all the way through the pipe, but will stretch, allowing the capacitor to be charged. If the capacitor is stretched too far then the elastic membrane will rupture. This is why exceeding the maximum voltage of a capacitor is likely to destroy it.

Discussion

When you apply a voltage across a capacitor it will almost instantly charge to that voltage. But if you charge it through a resistor then it will take time to become full of charge. Figure 3-3 shows how a capacitor can be charged or discharged through the use of the switches S1 and S2.

Figure 3-2. Water and Pipe Analogy for a Capacitor

Figure 3-3. Charging and Discharging a Capacitor

When you close switch S1, C1 will charge through R1 until C1 reaches the battery voltage. Open S1 again and the now charged capacitor will retain its charge. Eventually the capacitor will lose its charge through self-discharge.

If you now close S2, C1 will now discharge through R2 and LED1, which will light brightly at first and then more dimly as C1 is discharged.

If you want to experiment with the schematic diagram of Figure 3-3, then build up the breadboard diagram shown in Figure 3-4. For an introduction to breadboard, see Recipe 20.1. Use 1kΩ resistors and a 100µF capacitor.

Figure 3-4. Breadboard Layout for Capacitor Experimentation

Press the button labeled CHARGE for a second or two to charge the capacitor then release the button and press the DISCHARGE button. The LED should glow brightly for a second or so and then dim until it extinguishes after a second or so.

If you were to monitor the voltage across the capacitor while it was first being charged and then discharged, you would see something similar to Figure 3-5.

Figure 3-5. Charging and Discharging a Capacitor

In Figure 3-5 the square-shaped waveform is the voltage applied to the capacitor through a 1kΩ resistor. For the first 400ms this is 9V. But as you can see the voltage across the capacitor does not increase linearly, but rather increases faster at first and then gradually tails off as the voltage across the capacitor gets closer to the battery voltage.

Similarly, when the capacitor discharges, the voltage decreases sharply at first and then starts to tail off.

So, if a capacitor stores electrical energy, you may be wondering how this differs from a rechargeable battery. Well, in fact a special type of very high capacitance capacitor called a supercapacitor is sometimes used in place of a rechargeable battery for applications that require a very rapid storage and release of energy. The differences between a capacitor and a battery include:

• A rechargeable battery uses a chemical reaction to generate electricity’ a capacitor stores charge directly. 
• A rechargeable battery must be charged and discharged over a period of minutes or hours. A capacitor can charge and discharge in fractions of a second.
• The voltage across a capacitor falls off sharply as it starts to discharge, whereas the voltage across a battery stays relatively constant until most of the energy is used.
• Per unit size, a battery can store around ten times as much energy as the best supercapacitor.

See Also

For information on using a solderless breadboard, see Recipe 20.1.

The voltage curves of Figure 3-5 were created using a circuit simulator (Recipe 21.11). You can experiment with this simulation online using PartSim at  http://bit.ly/2mrtrhs.

Problem

You need to navigate the bewildering array of capacitor types to select one that is appropriate for your application.

Solution

Unless your application needs capacitors with special features, the following rule of thumb applies.

In most cases, for capacitors between 1pF and 1nF use a disc capacitor (Figure 3-6a). For capacitors between 1nF and 1µF use a multilayer ceramic capacitor (MLC; Figure 3-6b) and for capacitors above 1µF use an aluminum electrolytic capacitor (Figure 3-6c). The rightmost capacitor is a tantulum electrolytic capacitor.

Figure 3-6. Capacitor Types: (a) Disc Ceramic, (b) MLC, (c) Aluminum Electrolytic, and (d) Tantulum

Discussion

Although disc ceramic, MLC, and aluminum electrolytic capacitors are the most commonly used types of capacitor, there are other types:

• Glass and mica capacitors offer a wide temperature range, but are expensive compared to other types of capacitor.
• Tantulum electrolytic capacitors are polarized capacitors that have values around the overlap between the ranges of MLC and electrolytic capacitors. They are small, but relatively expensive and available in values up to a few tens of µF. They suffer from the disadvantage that when they fail, they generally fail in such a way that the terminals of the capacitor become connected to each other, often with fairly explosive consequences. Improvements in MLC capacitors have led to upper values of capacitance of hundreds of µF making tantulum capacitors something of a rarity.

Capacitors are less reliable than resistors. Exceed the voltage rating and you are likely to damage the insulating layer. Electrolytic capacitors use an electrolyte contained in an aluminum can that creates a very thin layer of oxide as the insulator. These are especially prone to failure due to overvoltage, or overtemperature, or just age. If vintage HiFi equipment fails, it is usually the large electrolytic capacitors in the power supplies that are the cause of the failure. In addition, if an electrolytic capacitor does fail then it can spray out the electrolyte in a somewhat messy manner.

See Also

For the use of electrolytic capacitors to smooth out voltage ripple from power supplies, see Recipe 7.4.

Problem

You have a capacitor and want to identify its value.

Solution

Small, low-value SMD capacitors are generally unmarked, and so you should label them as soon as you buy them.

Electrolytic capacitors usually have their capacitance value and voltage rating printed on the package. Polarized through-hole electrolytic capacitors are also usually supplied with the positive lead longer than the negative lead and the negative lead marked with a minus sign or a diamond symbol.

Most other capacitors use a numbering system similar to that of SMD resistors. The value is usually three digits and a letter. The first two digits are the base value and the third digit is the number of zeros to follow. The base value being the value in pF (pico Farad; see Appendix D).

For example, a 100pF capacitor would have the three digits 101 (a 1, a 0, followed by one further 0). A 100nF capacitor would be marked 104 (a 1, a 0, followed by four further 0s); that is, 100,000 pF or 100nF.

The letter after the digits indicates the tolerance (J, K, or M for ±5%, ±10%, and ±20%, respectively).

Discussion

The number of standard values for capacitors is much smaller than for resistors. Generally the values available are 10, 15, 22, 33, 47, and 68 followed by the necessary number of zeros.

See Also

To read resistor color codes, see Recipe 2.1.

Problem

You want to combine capacitors to increase their overall capacitance.

Solution

Looking at Figure 3-7, you can see two capacitors in parallel double up on the surface area of the conductive plates and therefore may correctly assume that the overall capacitance is the sum of the two capacitances.

Figure 3-7. Two Capacitors in Parallel

Discussion

It is actually quite common to place a number of capacitors in parallel to increase the overall capacitance. It is especially common when smoothing a high-power transformer-based power supply for, say, an audio amplifier where it is important to remove as much ripple from the power supply as possible (see Recipe 7.2).

In such systems it is common to use a number of different capacitors of different types and values in parallel to minimize the effects of ESR (see Recipe 3.2).

See Also

For capacitors in series, see Recipe 3.5.

Problem

You want to know why someone has combined capacitors in this unusual fashion.

Solution

When you connect two or more capacitors in series, the total value of capacitance is calculated according to the following formula, which, interestingly, is very similar to resistors in parallel:

Discussion

It is unusual to connect capacitors in series. Occasionally, this will be done as part of a more complex circuit such as in Recipe 7.12.

See Also

For capacitors in parallel, see Recipe 3.4.

Problem

Regular capacitors are not big enough for you.

Solution

Supercapacitors are low-voltage capacitors that have extremely high capacitances. They are primarily used as energy-storage devices in situations that would otherwise use rechargeable batteries.

They can have values up into the hundreds of F (Farads). Note that the top of the capacitance range for an aluminum electrolytic is around 0.22F.

Discussion

Low (comparatively) value supercapacitors of perhaps a few F are sometimes used as alternatives to rechargeable batteries or long-life lithium batteries to power ICs in standby mode to retain memory in static RAM that would otherwise be lost, or to power real-time clock (RTC) chips so a device using an RTC IC will keep the time if it’s powered off for a while.

Extremely high-value supercapacitors are available that offer an alternative to rechargeable batteries for larger capacity energy storage.

Supercapacitors with values of 500F or more can be bought for just a few dollars. The maximum voltage for a supercapacitor is 2.7V so, for higher voltage use, the capacitors can be placed in series with special protection circuitry to ensure the 2.7V limit is not exceeded as the bank of capacitors are charged.

Supercapacitors generally look like standard aluminum electrolytic capacitors. At present, their energy storage is still quite a long way from that of rechargeable batteries and because they are capacitors, the voltage decreases much more quickly than when discharging a battery.

See Also

See Recipe 3.7 to calculate the energy stored in super capacitors and normal capacitors.

Problem

You have charged up a capacitor to a certain voltage and want to know how much energy the capacitor is storing.

Solution

The energy stored in a capacitor in J is calculated as:

Discussion

Taking a medium-sized electrolytic of 470µF at 35V the energy stored would be:

which is not very much energy. Since the energy storage is proportional to the square of the voltage, the results for a capacitor of the same value but at 200V are much more impressive:

For a 500F 2.7V supercapacitor, the results are even more impressive:

By way of comparison, a single 1.5V AA battery of 2000mAH stores around:

See Also

For information on rechargeable batteries, see Recipe 8.3.

Problem

You need a component that can filter parts of a signal or smooth out fluctuations.

Solution

An inductor is, at its simplest, just a coil of wire. At DC, it behaves just like any length of wire and will have some resistance, but when AC flows through it, it starts to do something interesting.

A change in current in one direction in an inductor causes a change in voltage in the opposite direction. This effect is more marked the higher the frequency of the AC flowing through the inductor. The net result of this is that the higher the frequency of the AC, the more the inductor resists the flow of current. So as not to confuse this effect with ordinary resistance, it is called reactance but it still has units of Ω.

The reactance X of an inductor can be calculated by the formula:

Where f is the frequency of the AC (cycles per second) and L is the inductance of the inductor whose units are the Henry (H). This resistive effect does not generate heat like a resistor, but rather returns the energy to the circuit.

The inductance of an inductor will depend on the number of turns of wire as well as what the wire is wrapped around. So, very low-value inductors may be just a couple of turns of wire without any core (called air-core inductors). Higher value inductors will generally use an iron or more likely ferrite core. Ferrite is a magnetic ceramic material.

The current-carrying capability of an inductor generally depends on the thickness of the wire used in the coil.

Discussion

Inductors are used in switched mode power supplies (SMPS) where they are pulsed at a high frequency (see Recipe 7.8 and Recipe 7.9). They are also used extensively in radio-frequency electronics, where they are often combined with a capacitor to form a tuned circuit (see Chapter 19).

A type of inductor called a choke is designed to let DC pass while blocking the AC part of a signal. This prevents unwanted radio-frequency noise from infiltrating a circuit. You will often find a USB lead has a lump in it near one end. This is a ferrite choke, which is just a cylinder of ferrite material that encloses the cables and increases the inductance of the wire to a level where it can suppress high-frequency noise.

See Also

For more information on the use of inductors in SMPSs, see Recipe 7.8 and Recipe 7.9.

See Recipe 3.9 for more information on transformers.

Problem

You need a component that can convert AC voltages.

Solution

A transformer is essentially two or more inductor coils wrapped onto a single core. Figure 3-8 shows the schematic diagram for a transformer that also gives a clue as to how it works.

Figure 3-8. The Transformer

A transformer has primary coils and secondary coils. Figure 3-8 shows one of each. The primary coil is driven by AC, say the 110V from an AC outlet, and the secondary is connected to the load.

The voltage at the secondary is determined by the ratio of the number of turns on the primary to the number of turns on the secondary. Thus if the primary has 1000 turns and the secondary just 100, then the AC voltage will be reduced by a factor of 10.

Figure 3-9 shows a selection of transformers. As you can see they come in a wide range of sizes.

In Figure 3-9 there is a small high-frequency transformer on the left that was taken from a disposable flash camera where it was used to step up pulsed DC (almost AC) from a 1.5V battery to the 400V needed by a Xenon flash tube.

The type of transformer shown in the center is commonly used to step-down 110V AC to a low voltage, say 6V or 9V.

Figure 3-9. A Selection of Transformers

The transformer on the right is also designed to step-down AC outlet voltages to lower voltages. It is called a torroidal transformer and the primary and secondary coils are wrapped around a single torroidal former (donut shaped arrangement of iron layers), on top of each other. These transformers are often used in HiFi equipment where the noise found in most SMPSs is considered too high for high-end HiFi amplifiers.

Discussion

It used to be that if you wanted to power a low-voltage DC appliance such as a radio receiver from an AC outlet, then you would first use a transformer to drop the 100V AC at 60Hz to, say, 9V. You would then rectify and smooth the low-voltage AC into DC.

These days, an SMPS (see Recipe 7.8) is normally used as transformers are expensive and heavy items with iron and long lengths of copper-winding wire. However, transformers are still used in SMPS but operate at a very much higher frequency than 60Hz. Operating at a high frequency (often 100s of kHz) allows the transformers to be much smaller and lighter than low-frequency transformers while retaining good efficiency.

See Also

This video shows a torroidal transformer winding machine in action: https://youtu.be/82PpCzM2CUg.

Recipe 7.1 describes how to use a transformer to convert one AC voltage to another.

Problem

You want a component that allows current to flow one way but not the other.

Solution

A diode is a component that only allows current to flow through it in one direction. It’s a kind of one-way valve if you want to think of it in terms of water running through pipes, which of course is a simplification. In reality the diode offers very low resistance in one direction and very high resistance in the other. In other words, the one-way valve restricts the flow a tiny bit when open and also leaks slightly when closed. But most of the time, thinking of a diode as a one-way valve for electrical current works just fine.

There are lots of specialized types of diodes, but let’s start with the most common and basic diode, the rectifier diode. Figure 4-1 shows such a diode in a circuit with a battery and resistor.

Figure 4-1. A Forward-biased Diode

In this case, the diode allows flow of current and is considered forward-biased. The two leads of the diode are called the anode (abbreviated as “a”) and cathode (abbreviated rather confusingly as “k”). For a diode to be forward-biased, the anode needs to be at a higher voltage than the cathode, as shown in Figure 4-1.

One interesting property of a forward-biased diode is that unlike a resistor, the voltage across it does not vary in proportion to the current flowing through it. Instead, the voltage remains almost constant, no matter how much current flows through it. This varies depending on the type of diode, but is generally about 0.5V.

In the case of Figure 4-1, we can calculate that the current flowing through the resistor will be:

This is only 0.5mA less than if the diode were to be replaced with wire.

In Figure 4-2 the diode is facing the other direction. It is reverse-biased and consequently almost no current will flow through the resistor.

Figure 4-2. A Reverse-biased Diode

Discussion

The diode’s one-way effect can be used to convert AC (Recipe 1.7) into DC. Figure 4-3 shows the effect of a diode on an AC voltage source.

Figure 4-3. Rectification

This effect is called rectification (see Recipe 7.2). The negative part of the cycle is unused. This still isn’t true DC because even though the voltage never becomes negative it still swings from 0V up to a maximum and back down rather than remain constant. The next stage would be to add a capacitor in parallel with the load resistor that would smooth out the signal to a flat and almost constant DC voltage.

See Also

For information on using diodes in power supplies, see Recipe 7.2 and Recipe 7.3.

Problem

You want to know about the different types of diodes and their uses.

Solution

Figure 4-4 shows different types of diodes. Generally, the bigger the diode package, the greater its power-handling capability. Most diodes are a black plastic cylinder with a line at one end that points to the cathode (the end that should be more negative for forward-biased operation).

The diode on the left of Figure 4-4 is an SMD. The through-hole diodes to the right get larger, the higher the current rating.

Figure 4-4. A Selection of Diodes

Discussion

There are many different types of diodes. Unlike resistors that are bought as a particular value resistor (say, 1kΩ) diodes are identified by the manufacturer’s part number.

Some of the most commonly used rectifier diodes are listed in Table 4-1.

The forward voltage, often abbreviated as Vf, is the voltage across the diode when forward-biased. The DC-blocking voltage is the reverse-biased voltage that if exceeded may destroy the diode.

The recovery time of a diode refers to how quickly the diode can switch from forward-biased conducting to reverse-biased blocking. This is not instantaneous in any diode and in some applications fast switching is needed.

The 1N5819 diode is called a Schottky diode. These types of diodes have much lower forward voltage and generate less heat.

See Also

You can find the datasheet for the 1N4000 family of diodes here: http://bit.ly/2lOtD71.

Problem

You need to use a diode to allow voltages up to a certain voltage to pass through.

Solution

Use a Zener diode.

When forward-biased, Zener diodes behave just like regular diodes and conduct. At low voltages, when reverse-biased they have high resistance just like normal diodes. However, when the reverse-biased voltage exceeds a certain level (called the breakdown voltage), the diodes suddenly conduct as if they were forward-biased.

In fact, regular diodes do the same thing as Zener diodes but at a high voltage and not a voltage that is carefully controlled. The difference with a Zener diode is that the diode is deliberately designed for this breakdown to occur at a certain voltage (say, 5V) and for the Zener diode to be undamaged by such a “breakdown.”

Discussion

Zener diodes are useful for providing a reference voltage (see the schematic in Figure 4-5). Note the slightly different component symbol for a Zener diode with the little arms on the cathode.

Figure 4-5. Using a Zener Diode to Provide a Reference Voltage

The resistor R limits the current flowing through the Zener diode. This current is always assumed to be much greater than the current flowing into a load across the diode.

This circuit is only well suited to providing a voltage reference. A voltage reference provides a stable voltage but with hardly any load current; for example, when the circuit is used with a transistor as in Recipe 7.4. So a resistor value of, say, 1kΩ would, if Vin was 12V, allow a current of:

The output voltage will remain roughly 5V whatever Vin is as long as it’s greater than 5V. To understand how this happens, imagine that the voltage across the Zener diode is less than its 5V breakdown voltage. The Zener’s resistance will therefore be high and so the voltage across it due to the voltage divider effect of R and the Zener will be higher than the breakdown voltage. But wait, since the breakdown voltage is exceeded it will conduct, bringing the Vout down to 5V. If it falls below that, the diode will turn off and the Vout will increase again.

Zener diodes are also used to protect sensitive electronics from high-voltage spikes due to static discharge or incorrectly connected equipment. Figure 4-6 shows how an input to an amplifier not expected to exceed ±10V can be protected from both high positive and negative voltages. When the input voltage is within the allowed range the Zener will be high resistance and not interfere with the input signal, but as soon as the voltage is exceeded in either direction the Zener will conduct the excess voltage to ground.

Figure 4-6. Protecting Inputs from Over-Voltage

See Also

Although you would generally use a voltage-regulator IC (see Recipe 7.4), you can use a Zener diode combined with a transistor to act as a voltage regulator.

Problem

You need a component that can generate light without using a lot of power.

Solution

LEDs are like regular diodes in that when reverse-biased they block the flow of current, but when forward-biased they emit light.

The forward voltage of an LED is more than the usual 0.5V of a rectifier and depends on the color of the LED. Generally a standard red LED will have a forward voltage of about 1.6V.

Discussion

Figure 4-7 shows an LED in series with a resistor. The resistor is necessary to prevent too much current from flowing through the LED and damaging it.

Figure 4-7. Powering an LED

An LED used as an illuminator will generally emit some light at 1mA but usually needs about 20mA for optimum brightness. The LED’s datasheet will tell you its optimal and maximum forward currents.

As an example, if in Figure 6-5 the voltage source is a 9V battery and the LED has a forward voltage of 1.6V, you can calculate the resistor value needed using Ohm’s Law:

370Ω is not a common resistor value (see Recipe 2.2) so you could pick a 360Ω resistor, in which case the current would be:

which would be just fine.

Finding suitable resistor values for LEDs to limit the current is such a common task that there is really no need to go through these calculations all the time. In Recipe 14.1 you will find a practical recipe for rule-of-thumb selection of current-limiting resistors.

See Also

For information on driving different types of LED, see Chapter 14.

Problem

You want to take a reading of the light level.

Solution

Use a photodiode. You can also use a photoresistor (Recipe 2.8) or a phototransistor (Recipe 5.7).

A photodiode is a diode that is sensitive to light. A photodiode usually has a transparent window, but photodiodes designed for infrared use have a black plastic case. The black plastic case is transparent to IR and usefully stops the photodiode from being sensitive to visible light.

Photodiodes can be treated as little tiny photovoltaic solar cells. When illuminated they generate a small current. Figure 4-8 shows how you can use a photodiode with a resistor to produce a small voltage that you could then use in your circuits.

Figure 4-8. A Photodiode in Photovoltaic Mode

In this circuit, the voltage when illuminated brightly might only be 100mV.

The resistor is necessary so that the small current from the photodiode is converted into a voltage (V=IR). Otherwise, any voltage that you measure will be depend on the resistance (called impedance when not from a resistor) of whatever is measuring the voltage. So, for example, a multimeter with an input impedance of 10MΩ would provide a completely different (and lower) reading than a multimeter with a 100MΩ input impedance.

R1 makes the voltage consistent. The impedance of whatever is connected to the output needs to be much higher in value than R1. If this is an op-amp (see Chapter 17) then the input impedance is likely to be hundreds of MΩ and so will not alter the output voltage appreciably. The smaller you make R1, the lower the output voltage will be, so it’s a matter of striking a balance.

Better sensitivity can be achieved by using the photodiode in a photoconductive mode with a voltage source (Figure 4-9).

Figure 4-9. A Photodiode in Photoconductive Mode

Discussion

Photodiodes are quite linear, so they are often used in light meters. They also respond pretty quickly and are used in telecommunications systems to sense optical signals.

See Also

Photoresistors (Recipe 12.6) and phototransistors (Recipe 5.7) tend to be used more than photodiodes as they are more sensitive.

Problem

You want to incorporate digital switches into your project to control whether current flows or not.

Solution

Use a low-cost bipolar junction transistor (BJT).

BJTs like the 2N3904 cost just a few cents and are often used with a microcontroller output pin from an Arduino or Raspberry Pi to increase the current the pin can control.

Figure 5-1 shows the schematic symbol for a BJT alongside one of the most popular models of this kind of transistor, the 2N3904. The 2N3904 is in a black plastic package called a TO-92 and you will find that many different low-power transistor models come supplied in the TO-92 package.

The transistor is usually shown with a circle around it, but sometimes just the symbol inside the circle is used.

Figure 5-1. A Bipolar Transistor Schematic Symbol and Actual Device

The three connections to the transistor in Figure 5-1 are from top to bottom:

• The collector—the main current to be controlled flows into the collector
• The base—the control connection
• The emitter—the main current flows out through the emitter

The main current flowing into the collector and out of the emitter is controlled by a much smaller current flowing into the base and out of the emitter.  The ratio of the base current to the collector current is called the current gain of the transistor and is typically somewhere between 100 and 400. So for a transistor with a gain of 100, a 1mA current flowing from base to emitter will allow a current of up to 100mA to flow from collector to emitter.

Discussion

To get a feel for how such a transistor could be used as a switch, build the circuit shown in Figure 5-2 using the breadboard layout shown in Figure 5-3. For help getting started with breadboard tutorial, see Recipe 20.1.

Figure 5-2. A Schematic for Experimenting with a Transistor

The push switch will turn the LED on when its pressed. Although this could be achieved much more easily by just putting the switch in series with the LED and R2, the important point is that the switch is supplying current to the transistor through R1. A quick calculation shows that the maximum current that could possibly flow through R1 and the base is:

In reality the current is less than this, because we are ignoring the 0.6V between the base and emitter of the transistor. If you want to be more precise, then the current is actually:

Figure 5-3. A Breadboard Layout for Experimenting with a Transistor

This tiny current flowing through the base is controlling a much bigger current of roughly (assuming Vf of LED 1.8V):

Just like a diode, when the BJT is in use there will be an almost constant voltage drop of around 0.5V to 1V between the base and emitter connections of the transistor.

Limiting Base Current

A resistor limiting the current that can flow into the base of the transistor (R1 in Figure 5-2) is essential, because if too much current flows through the base then the transistor will overheat and eventually die in a puff of smoke.

Because the base of a transistor only requries a small current to control a much bigger current, it is tempting to think of the base as having a built-in resistance to a large current flowing. This is not the case, since it will draw a self-destructively large current as soon as its base voltage exceeds 0.6V or so. So, always use a base resistor like R1.

The BJT transistor described earlier is the most common and is an NPN type of transistor (negative positive negative). This is not a case of the transistor being indecisive, but relates to the fact that the transistor is made up like a sandwich with N-type (negative) semiconductor as the bread and P-type (positive) semiconductor as the filling. If you want to know what this means and understand the physics of semiconductors, then take a look at https://en.wikipedia.org/wiki/Bipolar_junction_transistor.

There is another less commonly used type of BJT called a PNP (positive negative positive) transistor. The filling in this semiconductor sandwich is negatively doped. This means that everything is flipped around. In Figure 5-2 the load (LED and resistor) is connected to the positive end of the power supply and switched on the negative side to ground. If you were to use a PNP transistor the circuit would look like Figure 5-4. You can find out more about using PNP transistors in Recipe 11.2.

Figure 5-4. Using a PNP Transistor

See Also

If the gain of your transistor is not enough, you may need to consider a Darlington transistor (Recipe 5.2) or MOSFET (Recipe 5.3).

If, on the other hand, you need to switch a high-power load then you should consider a power MOSFET (Recipe 5.3) or IGBT (Recipe 5.4).

You can find the datasheet for the popular low-power BJT, the 2N3904, here: http://www.farnell.com/datasheets/1686115.pdf.

Problem

You need more gain than you can get with a BJT to switch a current with a very small control current.

Solution

Use a Darlington transistor.

A regular BJT will typically only have a gain (ratio of base current to collector current) of perhaps 100. A lot of the time, this will be sufficient, but sometimes more gain is required. A convenient way of achieving this is to use a Darlington transistor, which will typically have a gain of 10,000 or more.

A Darlington transistor is actually made up of two regular BJTs in one package as shown in Figure 5-5. Two common Darlington transistors are shown next to the schematic symbol. The smaller one is the MPSA14 and the larger the TIP120. See Recipe 5.5 for more information on these transistors.

Figure 5-5. Darlington Transistors

The overall current gain of the pair of transistors in this arrangement is the gain of the first transistor multiplied by the gain of the second. It’s easy to see why this is the case, as the base of the second transistor is supplied with current from the collector of the first.

Discussion

Although you can use a Darlington transistor in designs just as if it were a single BJT, one effect of arranging the transistors like this is that there are now two voltage drops between the base and emitter. The Darlington transistor behaves like a regular NPN BJT with an exceptionally high gain but a base-emitter voltage drop of twice that of a normal BJT.

A popular and useful Darlington transistor is the TIP120. This is a high-power device that can handle a collector current of up to 5A.

See Also

You can find the datasheet for the TIP120 here: http://bit.ly/2mHBQy6 and the MPSA14 here: http://bit.ly/2mI1vXF.

Problem

You need to switch really heavy and noisy loads like those found in motors and you need to do it efficiently without generating a lot of heat.

Solution

Use a MOSFET.

MOSFETs (metal-oxide semiconductor field effect transistors) do not have an emitter, base, and collector, but rather a source, gate, and drain. Like BJTs, MOSFETs come in two flavors: N-channel and P-channel. It is the N-channel that is most used and will be described in this recipe. Figure 5-6 shows the schematic symbol for an N-channel MOSFET with a couple of common MOSFETs next to it. The larger transistor (in a package called TO-220) is an FQP30N06 transistor capable of switching 30A at 60V. The hole in the TO-220 package is used to bolt it to a heatsink, something that is only necessary when switching high currents. The small transistor on the right is the 2N7000, which is good for 500mA at 60V.

Figure 5-6. MOSFETs

Rather than multiply a current in the way that a BJT does, there is no electrical connection between the gate and other connections of the MOSFET. The gate is separated from the other connections by an insulating layer. If the gate-drain voltage exceeds the threshold voltage of the MOSFET, then the MOSFET conducts and a large current can flow between the drain and source connections of the MOSFET. The threshold voltage varies between 2V and 10V. MOSFETs designed to work with digital outputs from a microcontroller such as an Arduino or Raspberry Pi are called logic-level MOSFETs and have a gate-threshold voltage guaranteed to be below 3V.

If you look at the datasheet for a MOSFET you will see that it specifies on and off resistances for the transistor. An on resistance might be as low as a few mΩ and the off resistance many MΩ. This means that MOSFETs can switch much higher currents than BJTs before they start to get hot.

You can calculate the heat power generated by the MOSFET using the current flowing through it and its on resistance using the following formula. For more information on power, see Recipe 1.6.

Discussion

The schematic and breadboard layout used in Recipe 5.1 needs a little modification to be used to experiment with a MOSFET. The variable resistor is now used to set the gate voltage from 0V to the battery voltage. The revised schematic and breadboard layouts are shown in Figures 5-7 and 5-8, respectively.

Figure 5-7. Schematic for Experimenting with a MOSFET

Figure 5-8. Breadboard Layout for Experimenting with a MOSFET

With the trimpot’s knob at the 0V end of its track, the LED will not be lit. As the gate voltage increases to about 2V the LED will start to light and when the gate voltage gets to about 2.5V the LED should be fully on.

Try disconnecting the end of the lead going to the slider of the pot and touch it to the positive supply from the battery. This should turn the LED on, and it should stay on even after you take the lead from the gate away from the positive supply. This is because there is sufficient charge sitting on the gate of the MOSFET to keep its gate voltage above the threshold. As soon as you touch the gate to ground, the charge will be conducted away to ground and the LED will extinguish.

Since MOSFETs are voltage- rather than current-controlled devices you might be surprised to find that under some circumstances you do have to consider the current flowing into the gate. That is because the gate acts like one terminal of a capacitor. This capacitor has to charge and discharge and so when pulsed at high frequency the gate current can become significant. Using a current-limiting resistor to the gate will prevent too much current from flowing.

Another difference between using a MOSFET and a BJT is that if the gate connection is left floating, then the MOSFET can turn on when you aren’t expecting it. This can be prevented by connecting a resistor between the gate and source of the MOSFET.

See Also

To use MOSFETs with a microcontroller output, see Recipe 11.3.

To use a MOSFET for polarity protection, see Recipe 7.17.

For level shifting using a MOSFET, see Recipe 10.17.

For the use of heatsinks, see Recipe 20.7.

Problem

You need much more power.

Solution

An IGBT (insulated gate bipolar transistor) is an exotic type of transistor found in high-power, high-voltage switching applications. They are fast switching, and generally are particularly well specified when it come to operating voltage. Switching voltages of 1000V are not uncommon.

A BJT has base, emitter, and collector; a MOSFET has a gate, source, and drain; and an IGBT combines the two, having a gate, emitter, and collector. Figure 5-9 shows the schematic symbol for an IGBT alongside two IGBTs. The smaller of the two (STGF3NC120HD) is capable of switching 7A at 1.2kV and the even bigger one (IRG4PC30UPBF) 23A at 600V.

Figure 5-9. The Schematic Symbol for an IGBT and Two IGBTs

Discussion

IGBTs are voltage-controlled devices just like a MOSFET, but the switching side of the transistor behaves just like a BJT. The gate of an IGBT will have a threshold voltage just like a MOSFET.

IGBTs are sometimes used in the same role as high-power MOSFETs but have the advantage over MOSFETs of being able to switch higher voltages at equally large currents.

See Also

For information on BJTs see Recipe 5.1 and for MOSFETS see Recipe 5.3.

You can find the datasheet for the STGF3NC120HD here: http://bit.ly/2msNM6v.

The IRG4PC30UPBF datasheet is here: http://bit.ly/2msXTb9.

Problem

There are just so many transistors to choose from, how do you pick the right one?

Solution

Stick to a basic set of go-to transistors for most applications until you have more unusual requirements and then find something that fits the bill.

A good set of go-to transistors is shown in Table 5-1.

When shopping for an FQP30N06L, make sure the MOSFET is the “L” (for logic) version, with “L” on the end of the part name; otherwise, the gate-threshold voltage requirement may be too high to connect the gate of the transistor to a microcontroller output especially if it is operating at 3.3V.

The MPSA14 is actually a pretty universally useful device for currents up to 1A, although at that current there is a voltage drop of nearly 3V and the device gets up to a temperature of 120°C! At 500mA, the voltage drop is a more reasonable 1.8V and the temperature 60°C.

To summarize, if you only need to switch around 100mA then a 2N3904 will work just fine. If you need up to 500mA, use an MPSA14. Above that, the FQP30N06L is probably the best choice, unless price is a consideration, because the TIP120 is considerably cheaper.

Discussion

Reasons for straying from the transistors listed in Table 5-1 include:

• You need to switch at high frequency—look for BJTs or FETs described as RF.
• You need to switch higher voltages—BJTs and MOSFETs are readily available at quite high voltages up to 400V, but for even higher voltages look at IGBTs.
• For high currents, MOSFETs take a beating. Look for devices with the lowest possible resistance as this will be the main thing that determines how hot it gets and therefore how much current it can cope with before it fails.

See Also

For information on BJTs see Recipe 5.1, for MOSFETs Recipe 5.3, and for IGBTs Recipe 5.4.

To decide on a transistor for switching with an Arduino or Raspberry Pi, see Recipe 11.5.

Once you start to push transistors beyond a light load, you will find that they start to get hot. If they get too hot they will burn out and be permanently destroyed. To avoid this, either use a transistor with greater current-handling capability or fix a heatsink to it (Recipe 20.7).

Problem

You need a transistor that can switch AC.

Solution

TRIAC (TRIode for alternating current) is a semiconductor-switching device designed specifically to switch AC.

BJT and MOSFETs are not useful for switching AC. They can only do that if you split the positive and negative halves of the cycle and switch each with a separate transistor. It is far better to use a TRIAC that can be thought of as a switchable pair of back-to-back diodes.

Figure 5-10 shows how a TRIAC might be used to switch a high AC load using a small current switch. A circuit like this is often used so that a small low-power mechanical switch can be used to switch a large AC current.

Figure 5-10. Switching AC with a TRIAC

Discussion

When the switch is pressed a small current (tens of milliamps) flows into the gate of the TRIAC. The TRIAC conducts and will continue conducting until the AC crosses zero volts again. This has the benefit that the power is switched off at low power when the voltage is close to zero, reducing the power surges and electrical noise that would otherwise occur if switching inductive loads like motors.

However, the load could still be switched on at any point in the circuit, generating considerable noise. Zero-crossing circuits (see Recipe 5.9) are used to wait until the next zero crossing of the AC before turning the load on, reducing electrical noise further.

See Also

See Recipe 1.7 on AC.

For information on solid-state relays using TRIACs, see Recipe 11.10.

TRIACs are usually supplied in TO-220 packages. For pinouts, see Appendix A.

Problem

You want to measure light levels using something other than a photoresistor or photodiode.

Solution

A phototransistor is essentially a regular BJT with a translucent top surface that allows light to reach the actual silicon of the transistor. Figure 5-11 shows how you can use a phototransistor to produce an output voltage that varies as the level of light falling on the transistor changes.

Figure 5-11. Using a Phototransistor

When the photoresistor is brightly illuminated, it will turn on and conduct, pulling the output voltage toward 0V. In complete darkness, the transistor will be completely off and the output voltage will rise to the supply voltage of 5V.

Discussion

Some phototransistors look like regular transistors with three pins and a clear top and others (like the TEPT5600 used in Figure 5-11) come in LED-like packages with pins for the emitter and collector, but no pin for the base. For phototransistors that look like LEDs, the longer pin is usually the emitter. Phototransistors are nearly always NPN devices.

As a light-sensing element, the phototransistor is more sensitive than a photodiode and responds more quickly than a photoresistor. Phototransistors have the advantage over photoresistors that they are manufactured using cadmium sulphide and some countries have trade restrictions on devices containing this material.

See Also

The output of the circuit of Figure 5-11 can be directly connected to an Arduino’s analog input (Recipe 10.12) to take light readings as an alternative to using a photoresistor (Recipe 12.3 and Recipe 12.6).

For the TEPT5600 datasheet, see http://bit.ly/2m8vhS0.

Problem

For safety or noise immunity reasons, you want a signal to flow from one part of a circuit to another without there being any electrical connection.

Solution

Use an opto-coupler, which consists of an LED and phototransistor sealed in a single lightproof package.

Figure 5-12 shows how an opto-coupler can be used. When a voltage is applied across the + and – terminals, a current flows through the LED and it lights, illuminating the phototransistor and turning on the transistor. This brings the output down to close to 0V. When the LED is not powered, the transistor is off and R1 pulls the output up to 5V.

The important point is that the lefthand side of the circuit has no electrical connection to the righthand side. The link is purely optical.

Figure 5-12. An Opto-Coupler

Discussion

When the sensing transistor is a TRIAC (see Recipe 5.6) the device is known as an opto-isolator rather than opto-coupler and the low-power TRIAC inside the opto-isolator can be used to switch AC using a more powerful TRIAC as shown in Figure 5-13. This circuit is often called an SSR (solid-state relay) as it performs much the same role as a relay would in AC switching, but without the need for any moving parts.

Figure 5-13. An SSR Design

R1 is used to limit the current flowing into the gate of the opto-isolator and R2 keeps the TRIAC MT1 turned off until the TRIAC in the opto-isolator is turned on by light falling on it because of the LED being energized. R3 and C1 form a filter to reduce the RF noise that would otherwise be generated by the switching. Note that R3 and C1 must both be rated to cope with the peak AC voltage (use 400V devices).

The MOC3032 package also contains a zero-crossing switch that stops the TRIAC from turning on until the AC crosses 0V. This greatly reduces the interference generated by the switching circuit.

See Also

For SSRs use opto-isolators, see Recipe 11.10.

For the MOS3032 datasheet, see http://www.farnell.com/datasheets/2151740.pdf.

For relays that also isolate the control side from the power side, see Recipe 6.4.

Problem

You want to know about ICs and how to use them.

Solution

ICs will be involved in almost every project you are likely to work on. Be it a microcontroller or special-purpose chip for motor control or a radio receiver or audio amplifier, there is probably a chip for it. There is little point in building a circuit out of transistors and other components if there is a single IC that will do the job. You will find many different sorts of IC scattered throughout this book.

ICs come in many different shapes and sizes. Figure 5-14 shows a selection of them.

Figure 5-14. A Selection of ICs

Discussion

ICs can have as few as three pins or hundreds of pins. You will often find that the same basic IC is available in different packages, both SMD and through-hole packages. This can be very useful if you plan to prototype your design using a solderless breadboard before manufacturing using SMD components.

Another way of using SMDs on a solderless breadboard is to use a “breakout board” that allows you to solder the SMD onto a board and then plug that board into the breadboard. There is an example of this shown in Figure 18-5.

ICs are identified by their part number, which is often difficult to read, so it’s a good idea to label the bag the IC comes in.

ICs with more than three pins will have a little dot next to pin 1. If there is no dot, then there will be a notch that indicates the top of the IC. The pin numbering then moves down the IC to the bottom continuing with the bottom pin of the righthand side. For some example IC pinouts, see Appendix A.

See Also

You can find a list of the ICs used in this book in “Integrated Circuits”.

Problem

You want to understand how a switch works.

Solution

Switches generally work by mechanically bringing together two metal contacts. Figure 6-1 shows how this might work.

When the push button is pressed, it presses the sprung metal contact at the top to the fixed contact in the base of the switch.

Figure 6-1. The Mechanical Construction of a Push Switch

Discussion

There are several things that can make switching less straightforward than you might expect:

1. When switching high voltages there may be arcing (sparking) just before the two contacts connect and even more so when they disconnect. This causes heat and can damage the contacts.
2. When switching high currents at any voltage, there is a danger of the contacts spot-welding themselves together as the contact is made, preventing the switch from turning off again.
3. A continuous high current through a switch will cause the contacts (which have a small resistance) to heat up, shortening the life of the switch.
4. The contacts often bounce, making and releasing contact a few times in very rapid succession before settling. This can cause a problem when used with software that toggles something on and off. In Recipe 12.1 you will see how a switch press can be debounced.

For these reasons switches have a maximum voltage and current, and there are often separate maximums for switching AC and DC.

See Also

For a discussion of the various types of switch, see Recipe 6.2.

Problem

You want to understand what types of switches are available and how to use them.

Solution

Figure 6-2 shows a selection of switches.

Figure 6-2. A Selection of Switches

The switches from left to right are:

• A tactile push switch. This is probably the most commonly used switch in commercial products. They are low-cost circuit board–mountable push switches.
• A panel mount push switch. These are good for creating one-off projects as they can just be mounted into a hole drilled into an enclosure.
• A slide switch. Moving the slider connects the center contact to either the left or right pins.
• Microswitch. The hinged lever and mounting holes make it easy to attach to a mechanical assembly. Because they have a hinged lever, they do not need much force to operate. These switches are durable and reliable and often built into microwave ovens to turn off the power if the door is opened. 
• A toggle switch. These are the classic on/off switches. 

Discussion

If you are designing a project, then you are probably most likely to be using a microcontroller of some sort, in which case a simple push switch is all that is required. Even if you have a situation where you might want to use a switch to select between two options, chances are you would use a pair of push switches and some feedback as to which is selected in a display.

Slide and toggle switches have mostly been relegated to the role of switching power on and off to the project, although you will occasionally find them on circuit boards as “selector” switches.

Toggles (and other switches) are available in many different switching configurations. Some of those described include DPDT, SPDT, SPST, and SPST. These letters stand for:

• D—Double
• S—Single
• P—Pole
• T—Throw

A DPDT switch is double pole, double throw. The word pole refers to the number of separate switch elements that are controlled from the one mechanical lever. So, a double-pole switch can switch two things on and off together. A single-throw switch can only open or close a contact (or two contacts if it is a double pole). However, a double-throw switch can make the common contact be connected to one of two other contacts. Figure 6-3 shows some of these configurations.

As seen in Figure 6-3 when drawing a schematic with a double-pole switch, it is common to draw the switch as two switches (S1a and S1b) and connect them with a dotted line to show that they are linked mechanically.

The matter is further complicated because you can have three poles or more on a switch, and double-throw switches are sometimes sprung, and do not stay in one or both of these positions. They may also have a center-off position where the common contact is not connected to any other contact.

Another kind of switch that you may encounter is the rotary switch. This has a knob that can be set to a number of positions. If you have a multimeter, then this is the kind of switch that you will use to select the range. Such switches are not often used. Rotary encoders coupled with a microcontroller (Recipe 12.2) are the more modern way of making a selection using a control knob that you turn.

Figure 6-3. Switch Poles and Throws

See Also

To use switches with an Arduino or Raspberry Pi, see Recipe 12.1.

Problem

You want to turn something on when a magnet is brought near.

Solution

A reed switch is made up of a pair of contacts running parallel to each other that are close but not touching. The contacts are enclosed in a glass capsule and when a magnet is brought close to the contacts they are drawn together, closing the switch.

Figure 6-4 shows a reed switch with and without a magnet next to it.

Figure 6-4. Switch Poles and Throws

Discussion

Reed switches are often found in intruder alarms on doors and windows. A fixed magnet is attached to the door and the reed switch to the door frame. When the door is opened, the magnet moves away from the reed switch, triggering the alarm.

See Also

Reed switches are also found in reed relays but are rarely used these days.

Problem

You want to understand electromechanical relays and how to use them.

Solution

An electromechanical relay has two parts, a coil that acts as an electromagnet and switch contacts that close when the coil is energized. Figure 6-5 shows a relay, along with the pinout of the relay and a photo of an actual relay.

Figure 6-5. A Relay

Discussion

Although somewhat old-fashioned, relays are still in use today. They are equally happy switching AC or DC and are easily interfaced to a microcontroller.

If you take a relay apart, you will probably see something like Figure 6-6.

Figure 6-6. Inside a Relay

Here you can clearly see the coil that forms the electromagnet that when energized will move the contacts seen at the top.

The electromagnet of a relay is a big coil of wire and when released will produce a voltage spike that should be snubbed using a diode (see Recipe 11.9).

See Also

See Recipe 11.9 for information on using a relay with an Arduino or Raspberry Pi.

Today SSRs are often used in place of electromechanical relays (see Recipe 11.10).

Problem

You want to know how to convert AC at one voltage to another voltage.

Solution

Use a transformer (see Recipe 3.9).

When buying a transformer designed for 60Hz AC, you will often find the windings in an arrangement something like the one shown in Figure 7-1.

Figure 7-1. A Transformer Designed for AC

This has two identical primary coils and two identical secondary coils, all wound on the same former (laminated iron frame). This allows some flexibility in how the transformer is used. For example, US electricity is 110V whereas much of the world uses 220V AC. If you are designing a product and need the same low-voltage AC output whether the input is 100V or 220V, then two primary coils allow you to do this if you power the primaries in parallel for 110V and series for 220V.

The dual secondaries provide similar flexibility to wire in series for double the output voltage.

Discussion

Transformers are only rated to handle a certain amount of power. The resistance of the windings causes them to heat as current flows through them, and if things get too hot the insulation on the wire will break down and the transformer will fail.

A transformer will normally be rated as so many VA (volt amps). For most loads connected to a transformer 1VA is the same as 1Watt, but if large inductive loads like motors are being driven, then the current and voltage become out of phase and the apparent power will be lower than the VA rating.

See Also

For an introduction to transformers, see Recipe 3.9.

Reducing the AC voltage is often the first step in making an unregulated power supply, which is the topic of Recipe 7.2.

Problem

You want to reduce AC to low-voltage DC, but it’s OK if the DC voltage rises and falls a little depending on the load current.

Solution

Use an AC step-down transformer (see Recipe 7.1) and then rectify and smooth the output.

Figure 7-2 shows one schematic for the simplest method of achieving this.

Figure 7-2. A Basic Unregulated AC-to-DC Power Supply

The output voltage in Figure 7-2 is shown as 9V. This will be 1.42 (square root of 2) of the stated output voltage of the transformer.

The diode D rectifies the low-voltage AC from the transformer (see Recipe 4.1). The capacitor C then smooths this out to a constant DC that theoretically will be the peak voltage from the cathode of the diode. The diode will prevent the capacitor from discharging back through the transformer and so once it is at the peak voltage it will stay there.

Discussion

The schematic of Figure 7-2 is missing any kind of load. The capacitor is being charged to the peak voltage, but nothing is using that charge. When you add a load to the output as shown in Figure 7-3 the capacitor will still be receiving its kicks of voltage from the diode and transformer, but now it will be discharging through the load at the same time.

Figure 7-3. A Basic Unregulated AC-to-DC Power Supply

When using power supplies you will hear the use of the word “load,” which is the thing that is being powered by the power supply. When thinking about how the power supply will operate, you can just think of this load as a resistance. In Figure 7-3 the load is represented by R.

The characteristic voltage drop during each cycle due to the load discharging C is called the ripple voltage.

The size of this ripple voltage can be calculated from the load current and also from the size of the capacitor according to this formula:

where I is the current in amps, f is the AC frequency (60Hz), and C is the capacitance in Farads.

As an example, if the load current is 100mA, a 1000µF capacitor would reduce the ripple voltage to:

Notice that the ripple voltage is proportional to the load current, so in the preceding example, a current of 1A would result in a ripple voltage of 8.3V!

This state of affairs can be improved by using a full-wave rectifier as described in Recipe 7.3, which effectively doubles the frequency f in the preceding formula, halving the ripple voltage.

See Also

For full-wave rectification, see Recipe 7.3.

Problem

The half-wave rectification of Recipe 7.2 is not very efficient. You want to reduce the ripple voltage by using full-wave rectification.

Solution

There are two solutions to this:

• If you have a center-tapped or dual secondary transformer, you just need two diodes for full-wave rectification.
• If you only have a single secondary winding, you can use four diodes as a bridge rectifier.

Full-wave rectification with dual windings

Figure 7-4 shows how you can make use of both the positive and negative halves of the AC cycle.

Figure 7-4. Full-wave Rectification using Dual Secondary Windings

The secondary windings are in serial, with the central connection between the windings becoming the ground of the DC output. When the end of the winding connected to D1 is at its maximum, the end of the winding connected to D2 will be at its minimum (greatest negative value). D1 and D2 will take it in turn to provide positive voltage to the capacitor and the whole AC cycle can contribute to charging the capacitor. Figure 7-5 shows the full-wave rectified output before it is smoothed by the capacitor. It is as if the negative cycles are being reversed.

Figure 7-5. Full-wave Rectification

Using a Bridge Rectifier

If you only have a single secondary winding, then you can still achieve full-wave rectification by using an arrangement of four diodes called a bridge rectifier. Figure 7-6 shows an unregulated DC power supply using a bridge rectifier.

Figure 7-6. Using a bridge rectifier

To understand how this works, imagine that point A is positive and B is negative. This will allow current to flow from A through D2 to charge the capacitor. If there is a load on the power supply, then the current from the negative side of the DC output will be able to flow back through D4 to B.

When the AC cycle flips polarity and it is B that is positive and A negative, then the positive DC output is supplied through D3 and returns to A through D1.

Discussion

In Recipe 7.2 you saw how the ripple voltage halves if you use full-wave rectification rather than half-wave. The ripple voltage can be reduced by using very large capacitors, but in most circumstances it is better to use a regulated power supply as described in Recipe 7.4 or a switched mode power supply (SMPS), as discussed in Recipe 7.8.

You can buy a bridge rectifier as a single component that contains the four diodes wired up in a bridge configuration. This simplifies the wiring of your power supply as the bridge rectifier will have four terminals: two marked with a ~ to indicate AC input as well as + and – terminals for the rectified output.

See Also

To learn about half-wave rectification, see Recipe 4.1 and Recipe 7.2.

Problem

You need a DC power supply that does not have any significant ripple voltage when under load.

Solution

Use a linear voltage regulator IC after your unregulated DC power supply.

Figure 7-7 shows such a circuit. As you can see, the first stage of the project is just an unregulated power supply.

Figure 7-7. A Regulated DC Power Supply

A linear voltage regulator IC such as the very common 7805 regulator has three pins:

• GND
• Vin—unregulated DC input
• Vout—regulated DC output

In the case of the 7805, the output voltage is fixed at 5V, so as long as the input voltage is above about 7V the output will not have any significant ripple voltage (because the regulator chip needs about 2V more than its output voltage of 5V to successfully regulate the voltage).

At first sight, since C1 and C2 are in parallel, it is not immediately obvious why you would need both. However, the reason is that C1 will be a large electrolytic capacitor (470µF or more) and C2 will be a smaller MLC (typically 330nF) with a low ESR that should be positioned as close as possible to the voltage regulator. C2 along with C3, which is typically 100nF, are required by the voltage regulator IC to ensure that the regulation remains stable.

Discussion

In selecting values for C2 and C3, it is best to consult the datasheet for the voltage-regulator IC that you are using. This will normally specify values for the capacitors as well as tell you other important properties of the voltage regulator such as:

• Maximum current that can be drawn from the output (1A for a 7805)
• Maximum input voltage (35V for a 7805)
• Dropout voltage—the number of volts by which the input should exceed the output for normal operation (2V for a 7805)

Some linear voltage regulators are described as low-dropout (LDO). These have a much smaller dropout voltage than the 2V of the 7805. Some only need as little as 150mV more at the input than the output. If the input voltage of the regulator does fall below the dropout voltage, the output voltage will gradually decrease as the input voltage decreases. These regulators such as the LM2937, which has a dropout of 0.5V, are great if your input voltage is unavoidably close to the desired output. Perhaps you have a 6V battery and require a 5V output. Keeping the input voltage close to the output voltage also helps to reduce the heat generated by the regulator.

Voltage regulators are available in various packages from tiny surface-mount devices to larger packages such as the TO-220 package of the 7805, which are designed to be bolted to a heatsink for higher current use.

Most voltage regulators including the 7805 have protection circuitry that monitors the device’s temperature and if it gets too hot, it drops the output voltage and therefore limits the output current so that the device is not destroyed by the heat. Voltage regulators also have a mounting hole designed to be attached to a heatsink. This is not needed for low currents, but as the current gets higher a heatsink (see Recipe 20.7) will become necessary.

The 05 part of the 7805 refers to the output voltage and, as you might expect, you can also find 7806, 7809, 7812, and other voltages up to 24V. There is also a parallel 79XX range of negative voltage regulators should you need to provide regulated positive and negative power supplies as you do for some analog applications.

The 78LXX range of regulators are a lower-power (and smaller package) version of the 78XX regulators.

See Also

You can find the datasheet for the 7805 here: https://www.fairchildsemi.com/datasheets/LM/LM7805.pdf.

These days, most regulated AC-to-DC power supplies use SMPS design (Recipe 7.8).

Problem

You need a regulated DC supply, but you want to be able to control the output voltage.

Solution

Use an adjustable voltage regulator like the LM317.

Figure 7-8 shows a typical schematic for a variable output voltage regulator using the LM317.

Figure 7-8. Using an LM317 Voltage Regulator

The output voltage of the LM317 varies according to the formula:

This only holds true as long as R2 is relatively low resistance (less than 10kΩ).

So, if R1 is 270Ω and R2 is a 2kΩ pot, then at one end of the pot’s travel, the output voltage will be:

When the pot’s knob is rotated fully the other way, the output voltage will be:

Discussion

As R2 increases, the output voltage also increases, but as with a fixed-output regulator, the regulator requires some spare voltage to do the regulating with. In the case of the LM317, this is about 1.5V less than the input voltage.

Adjustable voltage regulators like the LM317 are available in a number of different-sized packages and able to handle different amounts of current before their temperature-protection circuitry kicks in to prevent damage to the IC.

See Also

You can find the datasheet for the LM317 here: http://www.ti.com/lit/ds/symlink/lm317.pdf.

To use an LM317 as a current regulator, see Recipe 7.7.

Problem

You want a regulated fixed voltage (say 5V) from a battery-based power supply (say 9V).

Solution

Use a fixed linear voltage regulator as shown in Figure 7-9.

Figure 7-9. Using a Voltage Regulator with a Battery

Discussion

New batteries have a voltage that often starts a little higher than the rated voltage (perhaps 9.5V for a 9V battery). This voltage drops as the battery is used. A 9V battery will quickly fall to 8V and will probably only provide useful power until it falls to perhaps 7.5V as it’s almost completely empty.

A voltage regulator can be used to provide constant output voltage during the lifetime of the battery and also to reduce voltage to a specific voltage needed for a microcontroller (usually 3.3V or 5V).

Unlike the transformer-based DC supply of Recipe 7.4 a battery does not suffer from ripple voltage, so the capacitor at the input to the voltage regulator can be omitted. However, the capacitor at the output is still needed in most applications as the load is likely to vary a lot and could cause instability in the regulator.

See Also

For more information on batteries, see Chapter 8.

Problem

You need to provide constant current to a load, say, to a high-power LED.

Solution

Use an LM317 voltage regulator configured in constant-current mode as shown in Figure 7-10.

Figure 7-10. Using an LM317 as a Constant-Current Source

For the LM317 chip, the output current is set by the value of R1 using the formula:

So, to limit the current to a maximum of 100mA the resistor value would be chosen using:

Discussion

This circuit will automatically raise and lower the output voltage to keep the current at the desired level.

See Also

To see this circuit limit the power to a high-power LED, see Recipe 14.2.

Problem

You want to regulate your DC power supply in an energy-efficient manner that does not generate too much heat.

Solution

Use a switching voltage-regulator IC like the one in the schematic shown in Figure 7-11.

Figure 7-11. A Switching Voltage Regulator Using the LM2596

The LM2596 can provide a regulated power supply at 3A without needing a heatsink.

The FB (feedback) pin allows the regulator to monitor the output voltage and adjust the width of the pulses to keep the voltage constant. The EN (enable) pin can be used to turn the IC on and off.

Discussion

Linear regulators as described in Recipe 7.4, Recipe 7.5, and Recipe 7.6 suffer from the disadvantage that they simply burn off the excess voltage as heat, which means they get hot and also waste energy.

Switching regulator designs such as the buck regulator shown in Figure 7-11 operate at 85% efficiency almost independently of the input voltage. By comparison, a linear regulator with a high input voltage and a low output voltage may only have an efficiency of perhaps 20 to 60%.

In a similar manner to PWM (see Recipe 10.8) a buck converter uses a transistor to switch power to an inductor that stores the energy of each pulse at a high frequency (150kHz for the LM2596). The longer the pulse the higher the output voltage. A feed-back mechanism changes the pulse length in response to the output voltage, ensuring a constant output.

It makes little sense to design a switching power supply from discrete components when there are ICs like the LM2596 that will do the whole thing so well.

There are many switching voltage-regulator ICs available and generally their datasheet will include a well-designed application circuit and sometimes even a circuit board layout.

If you are making a one-off project, then it is worth considering buying a ready-made switching-regulator module, either from eBay or suppliers like Adadruit and Sparkfun that specialize in modules.

See Also

You can find the datasheet for the LM2596 here: http://bit.ly/2lOLtHc.

Problem

You have a low-voltage supply (say a single 1.5V cell) and want to boost the voltage to a higher voltage (say 5V).

Solution

Use a boost-converter IC as the basis for a design like the one shown in Figure 7-12.

This design will produce an output voltage of 5V from an input voltage of between 0.9V and 5V at 90% efficiency.

The SW (switch) output of the IC is used to send pulses of current through the inductor L1 generating high-voltage spikes that charge C1. The output VOUT is monitored through the FB (feedback) pin via the voltage divider formed by R1 and R2 that sets the output voltage.

Figure 7-12. Using a TPS61070 Boost Converter

Discussion

The potential divider formed by R1 and R2 sets the output voltage. The value of R2 should be fixed at 180kΩ and the value of R1 calculated using the formula:

There are many similar boost-converter ICs available. In all cases, read the datasheet carefully as it will include a reference design and guides for finding the correct values for external components.

See Also

The datasheet for the TPS61070 can be found here: http://www.ti.com/lit/ds/symlink/tps61070.pdf.

Problem

You want to convert low-voltage DC into high-voltage AC.

Solution

Use an inverter. These devices use an oscillator to drive a transformer and generate high-voltage AC from low-voltage DC.

Discussion

If you really do wish to make yourself an inverter, Figure 7-13 shows a typical schematic.

Figure 7-13. An Inverter Schematic

This design uses the CD4047 timer IC as an oscillator. This IC provides a similar function to the much more famous 555 timer (see Recipe 7.13, Recipe 7.14, and several recipes in Chapter 16) but has the advantage that it has both normal and inverted outputs. These two ouputs each drive an NPN Darlington power transistor to energize one half of the primary coil in turn.

The transformer used here is the kind used in recipes such as Recipe 7.1 to step down an AC voltage, but in this case, it will be used to step the voltage up. The windings that would normally be used as the secondary side of the transformer will be driven by the transistors and the output from the transformer will be taken from the coil that would normally have been connected to AC.

To drive the circuit from 12V DC at 60Hz to produce a 110V AC output you need a transformer with a 12-0-12V secondary and a 110V primary.

The power to the CD4047 is supplied through a 100Ω resistor (R5) and a 100µF decoupling capacitor (C3; see Recipe 15.1) across the IC’s power supply. These reduce noise on the power supply to the CD4047.

Figure 7-14 shows the inverter built on a breadboard. This arrangement is fine as a first prototype, but in practice the design would be better built onto a circuit board and the power transistors attached to sizeable heatsinks (Recipe 20.7). It would also be wise to include a fuse in the 12V supply line to limit the current to a value that the transistors can handle, otherwise the transistors could overheat and die.

Figure 7-14. An Invertor Circuit Built on Breadboard

The transformer used is a toroidal type that I had available, but any type of transformer can be used.

See Also

For information on using a 4047 IC see its datasheet.

To learn about fuses, see Recipe 7.16.

To build circuits using a solderless breadboard, see Recipe 20.1.

See Recipe 3.9 for more information on transformers.

Problem

You want to power your project from 110V or 220V AC supply efficiently, without using a large transformer like the one used in Recipe 7.2.

Solution

Designing SMPSs intended for high-voltage use is a specialized and potentially dangerous activity. Aside from the always present danger of electric shock, a poorly designed SMPS can easily overheat and become a fire hazard.

For these reasons, I strongly suggest that when you need to power a project from AC, use a ready-made commercial SMPS “wall wart” power adapter and fit a DC barrel jack socket to your project. These sealed adapters are readily available and made in such quantities that they will almost certainly be cheaper to buy than to make.

Discussion

It is interesting to see how these power supplies work. Figure 7-15 shows how such a power supply can achieve the same power output of a transformer-based design (Recipe 7.2) with perhaps ⅒th of the weight.

Figure 7-15. An SMPS

The high-voltage AC input is rectified and smoothed to produce high-voltage DC in the same way as Recipe 7.2 but without using a transformer first. This DC is then “chopped” or switched at high frequency (often around 60kHz) into a series of short pulses that then feed an output transformer that steps down the high-frequency AC. The resulting low-voltage AC is then rectified and filtered into low-voltage DC. Because the transformer operates at such a high frequency, it can be made much smaller and lighter than a 60Hz transformer. 

Voltage regulation is achieved by feedback from the DC output to the controller that alters the width of the pulses being chopped that in turn alters the DC output voltage.

To keep the DC output isolated from the high-voltage AC, the feedback path to the controller uses an opto-isolator (Recipe 5.8).

This might seem like a lot of extra complexity, but the size, weight, and cost advantages of reducing the transformer size is enough to make SMPSs worthwhile.

Although there are ICs that do a lot of the work of an SMPS, they still require quite a few external components such as an opto-isolator and a high-frequency transformer.

Another advantage of SMPSs over transformer-based power supplies is that they can generally accept input voltages from 80–240V, and if the input voltage is higher, you just end up with shorter pulses to generate the same output voltage.

See Also

For the old-fashioned transformer-based approach to AC to low-voltage DC conversion, see Recipe 7.2.

For more information on transformers see Recipe 3.9 and for opto-isolators see Recipe 5.8.

Problem

You have an AC voltage that you want to step up to a higher DC voltage.

Solution

This is a job for the very elegant voltage-multiplier circuit, which uses a ladder of diodes and capacitors to increase the voltage without the use of inductors. It can be built in multiple stages to increase the voltage by different multiples. The schematic of Figure 7-16 shows a four-stage voltage multiplier that will multiply the AC voltage by 4.

Discussion

To understand how this circuit works, consider what happens at the peak positive and negative value of the AC inputs. When the input is at its negative maximum, C1 will be charged through D1. On the next positive maximum, C1 will still be charged to the peak, but now, the input will be effectively added to C1. If we stopped there this would be a voltage doubler, but now C2 will charge from the next negative peak and so on until the output is at four times the peak input voltage.

Figure 7-16. A Four-Stage Voltage Multiplier

The diodes should all be specified with a maximum voltage greater than the RMS AC input voltage times 1.4 (peak voltage). When selecting the capacitors (which should all be of the same value), you have a similar problem to that of selecting a smoothing capacitor in Recipe 7.2. If there is no load on the output then you can use very low value capacitors, but as soon as you add a load, they will discharge through it creating a ripple. Given that you will want the capacitors to be nonpolarized (no electrolytics here) and high-voltage, then you are likely to use something like 10nF capacitors rated for the same maximum voltage as the diodes for a low-current load such as a Geiger–Müller tube.

This kind of circuit is often used in high-voltage applications to multiply an initially high voltage up to even higher voltages.

See Also

For an example of using this kind of voltage multiplier in a power supply for a Geiger–Müller tube, see Recipe 7.14.

For background information on diodes and capacitors, see Recipe 4.1 and Recipe 3.1, respectively, and for background on AC see Recipe 1.7.

Problem

You need a 450V DC low-current voltage source from a battery to supply power to a Geiger–Müller tube in a radiation meter.

Solution

Use a version of the inverter circuit of Recipe 7.10 to drive a small high-frequency transformer. Figure 7-17 shows the schematic for the power supply. Note that D1 and C4 should both be rated at 1000V.

Figure 7-17. Schematic for High-Voltage DC Supply

The 555 timer is designed to provide a frequency of between 7kHz and 48kHz controlled using the variable resistor R3. The Darlington transistor provides pulsed current to the transformer, the output of which is rectified and smoothed.

Altering the frequency will alter the output voltage. With a high-voltage multimeter (1000V DC) adjust the variable resistor until the maximum voltage is found. For my transformer this took place at a frequency of 35kHz.

Discussion

Figure 7-18 shows the project built on two pieces of breadboard. The left breadboard has the 555 oscillator and the righthand board the transistor and high-voltage part of the design.

Figure 7-18. High-Voltage DC Supply on Breadboard

As you can see, the high-frequency transformer is tiny. The device I used is marked DT5A and is scavenged from the flash unit of an old disposable camera.

To check the output voltage, you will need a voltmeter with a 1000V DC range. Modern digital multimeters generally have an input impedance of around 10MΩ. At 1000V that means that a load current of 1kV / 10MΩ = 100nA. This does not sound like much, but for a low-current circuit like Figure 7-17, this loading may well reduce the voltage being measured. Increasing the value of C4, perhaps by putting a couple of capacitors of the same value in parallel with the original C4, should tell you if the multimeter is having an effect.

To measure high voltages reliably, you can use a specialist high-voltage meter with a very high input impedance, but these are expensive.

See Also

See Recipe 7.14 for a version of this design that adds a three-stage voltage multiplier to increase the output voltage.

For information on using the 555 timer, see Recipe 16.6.

For information on measuring high voltages, see Recipe 21.8.

To build circuits using a solderless breadboard, see Recipe 20.1.

Problem

The 450V output of Recipe 7.13 is not enough—you need 1.2–1.6kV for a Geiger–Müller tube that detects alpha radiation.

Solution

Add a three-stage voltage multiplier to the schematic of Recipe 7.13. The result of this is shown in Figure 7-19.

Figure 7-19. Adding a Voltage Trippler to the High-Voltage Supply

The diodes, C4, C5, and C6, can all still be rated at 1kV since the voltage across each should still be under 1kV.

Discussion

As with the design of Recipe 7.13 the output of this design is likely to contain a few volts of unwanted noise.

See Also

For a version of this design without the voltage multiplier, see Recipe 7.13.

Problem

You want to make a Tesla coil.

Solution

One common and safe design for a Tesla coil uses a homemade transformer. Figure 7-20 shows the completed Tesla coil illuminating an LED from a distance of half an inch and Figure 7-21 shows the power supply built on the breadboard.

Figure 7-20. A Tesla Coil Lighting an LED

Figure 7-21. A Tesla Coil on Breadboard

Figure 7-22 shows the schematic for the project that uses a self-oscillating design based around a transformer and transistor.

Figure 7-22. A Tesla Coil Schematic

The transformer for this design is made from a length of 2-inch plastic pipe onto which around 300 turns of 34 SWG (30 AWG) enameled copper wire are wound next to each other. The primary is made up of just three turns of plastic insulated multicore wire.

This circuit is called a solid-state flyback converter (sometimes called a slayer exciter). The top end of the transformer secondary is not directly connected to anything, but with the top end of the coil connected to something with a large area (such as the copper ball I used) there is a small stray capacitance to ground. The top surface of the large area could be a metal plate or various other arrangements, as long as it’s made from a large conducting area without too many sharp points to encourage discharge.

When power is first applied to the circuit, the transistor will turn on through R1, resulting in a large current flowing through the three turns of the transformer primary. This induces current to flow through the secondary, creating a potential difference due to the stray capacitance. Although small, this capacitance is sufficient to reduce the voltage on the bottom end of the secondary (and base of the transistor) to such an extent that the transistor turns off. The LED ensures that the base never falls more than 1.8V (the forward voltage of the LED) below ground. Because the transistor turns off, the magnetic field collapses and R1 reasserts its influence over the transistor’s base to turn the transistor back on and so the cycle continues.

If you get the connections of the primary the wrong way around, or even just the geometry of the secondary wrong, there is a chance the transformer will turn on but oscillation fail to start, which will rapidly destroy the transistor. If all is well, LED1 will light. A lab power supply with current limiting is very useful in this situation (see Recipe 21.1).

Discussion

Holding a second LED close to the high-voltage end of the secondary will cause it to light. By holding one lead of the LED, you will be providing a weak path to ground.

A quick way to increase the power of the circuit is to place several transistors in parallel (all three pins). By using four 2N3904s I was able to generate enough voltage to cause the gas in a compact florescent lamp to ionize, lighting the lamp from a range of almost a foot.

See Also

For information on prototyping with a solderless breadboard, see Recipe 20.1.

You can see this Tesla coil in action at https://youtu.be/-DEpQH7KMj4.

For other types of high-voltage power supply, see Recipe 7.13 and Recipe 7.14. The “joule thief” circuit (Recipe 8.7) operates in a similar way to this design.

To see a miniature Tesla coil design in action using a ferrite core rather than an open tube, visit https://www.youtube.com/watch?v=iMoDAspGPPc. The design here uses the same schematic as in Figure 7-22.

If you search the internet for Tesla coil, you will find some incredible builds.

Problem

You want to protect a circuit from too much current flowing, since too much current could destroy expensive components or cause a fire.

Solution

Use a fuse. Figure 7-23 shows a selection of fuses.

Figure 7-23. From Left to Right: SMD Polyfuse, Polyfuse, Thermal Fuse, 20mm Fuse, and 25mm Fuse

Fuses can be broadly classified as one-shot traditional fuses or polyfuses. As the name suggests, one-shot fuses can only be used once. When their current or temperature limits have been exceeded and they have blown, that’s it, they must be discarded. For this reason they are generally attached to electronics using a fuse holder, so they can be replaced easily.

Polyfuses (a.k.a. resettable fuses) do not blow, but when the current through them increases above their limit value, their resistance increases, only resetting when the current returns to zero and they have cooled down. This means they can be permanently attached to a circuit. You will often find them used in places like USB ports, where you need to protect computer ports against overcurrent by misbehaving peripherals. Polyfuses use the same technology as PTC thermistors (see Recipe 2.9).

Discussion

In modern circuit design, and especially for low-voltage and relatively low-current use, polyfuses are the best approach. However, for AC applications and other high-current situations, where overcurrent will only happen if something is seriously wrong, then traditional single-use fuses are the safest option because they require manual intervention before the power can be reapplied to the downstream circuit.

If a single-use fuse blows, then work out why it blew before replacing it. The most likely cause of overcurrent is a short circuit, either from wires touching that should not be touching, or a component failing that causes some part of the circuit to draw more current than it should.

There are a number of different single-use fuses:

• Slow-blow—survives short periods of overcurrent. For example, for use with a motor that draws a lot of current when starting.
• Fast-blow—blows as soon as the current is exceeded.
• Thermal—designed to blow on overcurrent, but also if the fuse gets hot because of external influences such as fire.

See Also

To test a fuse, see Recipe 21.5.

For a circuit to produce a constant current, see Recipe 7.7.

To protect a circuit against accidental polarity reversal, see Recipe 7.17.

Problem

You are building a project that you want to be idiot-proof so that if an idiot puts the batteries in the wrong way, the project won’t be harmed.

Solution

Many ICs and circuits designed with discrete transistors will be destroyed by overcurrent and hence heating if the power is applied with the wrong polarity.

If you don’t care about a small voltage drop of 0.5V to 1V, then use a single diode from the positive battery terminal to the positive supply of your circuit (see Figure 7-24).

Figure 7-24. Protecting Against Reverse Polarity with a Diode

Remember to select a diode that can handle the current that you expect the circuit to draw.

If you can’t afford to lose a volt through the diode but are losing 0.2–0.3V, then use a Schottky diode in place of the regular diode.

If even 0.3V is too much to lose, then use an P-channel MOSFET as shown in the circuit in Figure 7-25.

Figure 7-25. Reverse Polarity Protection Using a MOSFET

With the correct polarity, the P-channel MOSFET’s gate-drain voltage will be high enough to keep the MOSFET on. Since MOSFETs with extremely low on-resistances are available, there will be very little voltage drop across the MOSFET to the load. The circuit only works for battery-supply voltages over the gate-threshold voltage, or the MOSFET will not turn on.

When the polarity is reversed, the MOSFET will be firmly off, preventing any significant current from flowing into the load circuit.

Remember that the link between the gate and other connections of the MOSFET is capacitative and so no current can leak through the gate to the negative terminal of the battery.

Although almost by tradition people tend to prefer to switch and use polarity protection on the positive supply, you can also do the same trick with an N-channel MOSFET on the negative battery supply as shown in Figure 7-26.

Figure 7-26. Negative Side Polarity Protection

Discussion

It’s a good idea to include polarity protection in any battery-powered project, even with a 9V battery that has a clip that only goes one way around, because it is all too easy to touch the wrong contacts together.

Using a MOSFET is probably the cheapest way to protect a design, as it can cope with several amps and will generally be smaller and cheaper than a diode with the same properties, especially in surface-mount technology.

See Also

To use fuses to protect a circuit against overcurrent, see Recipe 7.16.

For background on diodes, see Chapter 4.

For more information on MOSFETs, see Recipe 5.3.

Problem

You want to know how long your battery is going to last.

Solution

The capacity of a battery is specified in Ah (ampere hours) or mAh (milliampere hours). To calculate how many hours your battery will last, you need to divide this number by the current consumption of your project in A or mA.

For example, a 9V PP3 rechargeable battery typically has a capacity of about 200mAh. If you were to connect an LED with a suitable series resistor that limited the current to 20mA, the battery should be good for about:

Discussion

The estimate of time you get using the preceding calculation is very much an estimate and all sorts of factors such as the age of the battery, the temperature, and the current will affect the actual battery life.

If you are using a number of batteries in serial (e.g., in a battery holder that contains 4xAA batteries), then you do not get to multiply the battery life by 4, because the same current will be flowing out of each battery (Figure 8-1).

Figure 8-1. Batteries in Series

From Kirchhoff’s Current Law (Recipe 1.4) we know the current flowing past any point in the circuit will be the same. Assuming an LED with a forward voltage of 1.8V the current will be around 9mA. Even though there is more than one battery cell, each cell will have 9mA flowing through it. A typical AA battery has a capacity of around 2000mAh and so we could expect this circuit to run for:

It is best to avoid putting batteries in parallel because none of the cells in the battery will be at exactly the same voltage and when you connect them together, the first thing that will happen is their voltages will equalize with the higher voltage batteries discharging into the lower voltage batteries. If the batteries are rechargeable, you may be okay if the voltage difference is very small, but for nonrechargeable batteries, this charging process can heat the batteries and be dangerous.

Batteries all have an internal resistance, very similar to the ESR of a capacitor (Recipe 3.2). This will cause heating as the battery discharges, which is why you cannot use a small 200mAh battery to supply 10A for 72 seconds. The internal resistance of the battery will both limit the current and cause the battery to heat up. Small batteries tend to have higher internal resistance than big batteries.

Most nonrechargeable batteries bought from a component supplier will specify the maximum continuous discharge current.

See Also

For information on choosing between different types of rechargeable batteries see Recipe 8.3 and for nonrechargeable batteries see Recipe 8.2.

To measure the current used by your project using a multimeter, see Recipe 21.4.

Problem

You need a nonrechargeable battery for your project, but you are not sure what type to use.

Solution

Decide what your minimum battery life for the product should be and calculate the capacity of the battery you need in mAh, then use Table 8-1 to select a battery.

If you need more volts than a single battery cell provides, then place several of the cells in series in a battery holder.

Discussion

Higher capacity nonrechargeable batteries are expensive and so C and D cells are found less and less often in favor of rechargeable LiPo battery packs (see Recipe 8.3).

You should also be wary of using exotic or unusual batteries. It’s generally a good idea to stick to universally available batteries like AA cells unless you need something smaller in which case AAA batteries will do.

Battery holders such as the 4 x AA battery holder shown in Figure 8-2 are readily available to hold 1, 2, 3, 4, 6, and 8 AA cells to make a battery pack capable of supplying 1.5, 3, 4.5, 6, 9, and 12V, respectively.

Figure 8-2. A 4 x AA Battery Pack

Perfect batteries would allow you to draw as large a current as you want. However, in reality batteries have a small resistance that will cause heating as the battery discharges. The higher the current the greater the heat produced. Smaller capacity batteries have a lower maximum discharge rate than larger batteries.

See Also

For information on rechargeable batteries, see Recipe 8.2.

Problem

You want to select a rechargeable battery for your project, but are not sure what type to use.

Solution

Decide how long you would like your project to operate between charges and use Table 8-2 to choose a battery.

Discussion

LiPo (lithium polymer) and the closely related lithium ion cells are the lightest and also now as cheap per mAh as older technologies such as MiMh, but they require care when charging and discharging or they can catch fire (see Recipe 8.6).

You will still find NiMh batteries in products like electric toothbrushes. They are heavier than LiPos of the same capacity but are easy to charge (see Recipe 8.4).

SLA batteries (sealed lead acid) are even heavier than NiMh cells but cope well with trickle charging and last for years. However, their use is on the decline in favor of lighter-weight LiPo batteries.

Many rechargeable batteries will specify safe charge and discharge rates in units of C. This refers to a ration of the battery’s nominal capacity. So, a 1Ah battery that says it can be discharged at 5C is capable of supplying 5A (5 x 1A). If the maximum charge current is specified as 2C the battery can be charged at 2A.

See Also

For a similar recipe but for nonrechargeable batteries, see Recipe 8.2.

Problem

You want to charge a NiMh or SLA battery while it’s connected to your circuit and you don’t need to charge it quickly.

Solution

Trickle charge the battery from a power supply using a resistor to limit the current. If you don’t know how the power supply works, then also include a diode in the circuit to prevent damage to the power supply when the power is turned off. Figure 8-3 shows the schematic for charging a 6V battery from a 12V supply.

Figure 8-3. Trickle Charging

Discussion

A battery’s datasheet will specify the charging current to use for fast charging and in this case slow trickle charging in terms of C, where in this case, C is not capacitance but rather the capacity of the battery in mAh. So the datasheet for an AA rechargeable battery might say that it should be trickle charged at C/10 mA, meaning that if the battery is a 2000mAh AA cell, it can be trickle charged at 2000/10 = 200mA indefinitely without damaging the battery. Charging below 200mA will still result in the battery being charged, and in situations like battery backup (Recipe 8.5) charging at C/50 (40mA) is probably a good compromise between charging speed and power consumption of the circuit.

To charge at 40mA we just need to set the value of R. The voltage across R will be 12V – 6V – 0.5V = 5.5V. We know that the current is 40mA, so we can calculate R using Ohm’s Law as:

Using the standard resistor value of 120Ω will result in a slightly higher current of:

We also need to make sure the resistor has a sufficient power rating, which we can calculate as:

So, a ¼W resistor is probably OK, but ½W would not get as hot.

An alternative to using a resistor to limit the current is to use Recipe 7.7 to provide a constant charging current. This design is more complex and expensive, but will cope better with variations in the input voltage and the battery’s voltage changing as it charges.

See Also

Charging from a photovoltaic solar cell is very similar to trickle charging (see Recipe 9.1).

Problem

You have a project that is powered through an AC power adapter but you want it to automatically change over to a battery if the AC power fails.

Solution

Use a battery of slightly lower voltage than the power adapter–supplied voltage and a pair of diodes in the arrangement shown in Figure 8-4.

The diodes ensure that the output is supplied from either the battery or the power supply, whichever has the higher voltage. Since you want the power supply to be providing the voltage most of the time, arrange for it to be a volt or more higher than the battery voltage (remember the battery voltage will drop as it discharges). In Figure 8-4, when the power supply is producing 10V, D1 will be forward-biased (conducting) and D2 reverse-biased. This reverse-biasing of D2 prevents any accidental charging of the battery (which may not be rechargeable). If the power supply were to turn off, D2 becomes forward-biased, providing the output voltage and D1 would become reverse-biased preventing any possible flow of current into the power supply that could damage it, especially if it is an SMPS design (Recipe 7.8).

Figure 8-4. Automatic Battery Backup

Discussion

When selecting a diode type for D1 and D2, you need to make sure it can handle the current that the output needs to provide. Although in most cases regular diodes like the 1N4xxx series are just fine, they will result in a forward-voltage drop of at least 0.5V. You can reduce this voltage drop by using Schottky diodes (Recipe 4.2).

The circuit of Figure 8-4 will work just fine for rechargeable or nonrechargeable batteries, but it will not charge the batteries. If using rechargeable batteries, you can modify the circuit slightly so that the power supply trickle charges the battery and supplies the output. Figure 8-5 shows the schematic for this. See Recipe 8.4 for the calculation for the charging resistor.

Figure 8-5. Trickle Charge and Battery Backup

Now when the power supply is in charge some of the current from D1 will flow through R1 and charge the battery (D2 is reverse-biased and can effectively be ignored). When the power supply is off (0V) D2 becomes forward-biased and supplies power to the output. At this time, R1 becomes irrelevant as D2 will be conducting.

See Also

See Recipe 4.2 for help selecting the right diode.

For the basics of what diodes do, see Recipe 4.1.

Problem

You want to charge a single LiPo cell.

Solution

Use a LiPo-charging IC designed specifically for the purpose of charging LiPo batteries. The schematic of Figure 8-6 is an example design using the MCP73831 IC.

Figure 8-6. Charging a LiPo Battery

Many consumer electronics use LiPo batteries (often charged from a USB connection) with chips like the MCP73831 and many are low cost (less than $1) and include all sorts of advanced features that make sure the LiPo cell is charged safely.

The MCP73831 only needs two capacitors and a resistor to make a complete LiPo-charging circuit. The LED and R2 provide a status indicator that shows that the battery is charging and can be omitted if you don’t need it.

The chip will automatically control the charge current, turning it to a safe trickle charge when the cell is fully charged. However, you can set the maximum charge current in amps (consult the datasheet for the LiPo cell that you plan to use) by means of R1, using the formula:

So, to charge at 500mA (the maximum allowed by the chip anyway) set R1 to 2kΩ.

As an alternative to using a design of your own to charge a LiPo cell, you can also buy ready-made LiPo charge modules such as the Adafruit 1905 and Sparkfun PRT-10217 (both use the MCP73831).

Discussion

LiPo batteries have made the news in recent years because they will catch fire if abused. Using a charger chip like the MCP73831 in your circuits will help ensure your design is safe and reliable. For extra safety, you can add a thermal fuse (Recipe 7.16) to the supply to the charging circuit.

If you are looking for a lower-tech solution then consider using a NiMh or SLA battery instead.

Unlike NiMh or SLA batteries, LiPo batteries are not suited for charging in parallel. Each cell effectively has to be charged independently using special hardware. If your project cannot operate directly from the 3.7V of a LiPo cell and needs a bit more voltage, then it is generally easier to use a boost converter (Recipe 7.9) to generate the voltage you need.

See Also

You can find the datasheet for the MCP73831 here: http://bit.ly/2n2XUPP.

Problem

You want to squeeze the last drop of energy from a battery.

Solution

The novelty circuit known as a joule thief allows you to power an LED from an alkaline AA cell even when its voltage drops to around 0.6V.

A joule thief is actually a small self-oscillating boost converter (see Recipe 7.9) that requires just a transistor, resistor, and homemade transformer to drive an LED from a single 1.5V battery, even when the battery voltage drops as low as 0.6V. The circuit is shown in Figure 8-7.

The circuit is very similar in operation to the self-oscillating boost converter used in Recipe 7.15 except that the secondary winding has the same number of turns as the primary and is connected back to the base via a resistor rather than using parasitic (from the winding) capacitance.

Figure 8-7. Joule Thief Schematic

Discussion

Figure 8-8 shows a joule thief. This circuit is interesting because it is fun to make one and see just how long an AA battery can power an LED.

Figure 8-8. A Joule Thief on a Breadboard

I made my transformer by winding 12 turns of 30 AWG (34 SWG) enameled copper wire, for each winding, around a tiny toroidal ferrite core. In reality, more or less of any ferrite core and even plastic insulated wire will work just fine.

The supply generates pulsed DC at a frequency of about 50kHz (depending on the transformer).

See Also

For more practical boost converters, see Recipe 7.9.

Recipe 7.15 uses a very similar circuit to this recipe.

For information on wire gauges, see Recipe 2.10.

Problem

You want to understand how to power an electronics project using solar power, so that you are not dependent on power or the need to regularly replace batteries.

Solution

Use a photovoltaic solar panel (Figure 9-1) to generate electricity to trickle charge a battery that then supplies your project. The leftmost panel in Figure 9-1 is reclaimed from a low-power solar LED on a stick that cost about $1.50.

Figure 9-2 shows the schematic for solar charging a battery.

Solar panels are made up of a number of solar cells wired together in series to increase the output voltage. You will need a solar panel with an output voltage that is greater than the battery voltage.

The diode D1 is essential to prevent current from flowing back through the solar panel and damaging it when the solar panel is not illuminated enough for its output voltage to exceed the battery voltage.

R1 sets the charging current in just the same way as trickle charging (Recipe 8.4). As with trickle charging, you can also use a current regulator such as the one described in Recipe 7.7.

Figure 9-1. A Selection of Solar Panels—Left to Right Unknown Wattage, 1W, and 20W Panels

Figure 9-2. Solar Charging a Battery

Discussion

The performance of the solar panel varies enormously with the amount of sunlight falling on it. A solar panel will produce useful amounts of power when in direct sunlight, but in shadow or on a cloudy day it is unlikely to produce any more than 1/20 of its direct sunlight performance.

If you are thinking of using a solar panel to power a project indoors, unless you plan to keep the project on a sunny window sill or your project uses microamps, then forget it. The size of panel that you will need will make the project huge.

Two key parameters specified for a solar panel are its nominal output voltage and its power output. Both these values require some explanation.

The power output of the panel in mW for a small panel and W for a larger panel, as stated in the specification for the panel, is the power output under ideal conditions. That is, pointing directly at the sun without clouds, haziness, or any other impairments. In Table 9-1 you see the results of practical tests on the solar panels shown in Figure 9-1. These tests were carried out in the early afternoon at my home (53° latitude). The “no direct sunlight” readings were made with the panel pointing at clear sky but away from direct sunlight. For details on how to take such measurements yourself, see Recipe 9.3.

Two things emerge from these measurements. First, the actual power output, even for the highest quality panel (the 20W panel), was actually almost ⅓ of its rated value. Secondly, when out of direct sunlight, the power output fell to almost 1/100 of the full-sunlight reading in the worst case. Now, while I’m sure the panels would do better at noon near the equator, that’s not where most of us live.

Similarly, the output voltage at suboptimal lighting levels is also not the same as the specified output voltage. Table 9-2 shows the no-load output voltage for the same solar panels.

The output voltage is higher than the nominal voltage so it can be used to charge a battery of nominal voltage.

See Also

To work out the power output of the solar panel that you need, see Recipe 9.3.

For the basics of trickle charging, see Recipe 8.4.

Problem

You want to be able to decide on the output power of a solar panel to suit your project.

Solution

This solution will not give you an exact solar-panel power requirement and accompanying battery capacity, but it will at least give you a rough estimate to get you started with field testing.

Start by calculating the energy use of your project in Ws (joules) for a 24-hour period because a 24-hour day will include both the daylight and nighttime parts of the daily cycle. Calculating this current consumption is easy if the device is on all the time and the current consumption is constant.  Let’s say we have a garden temperature sensor that uses WiFi and consumes a constant 70mA from its 5V power supply. The power consumption is therefore 70mA x 5V = 350mW. The energy consumption is 350mW times the number of seconds in 24 hours. That is, 350mW x 24 x 60 x 60 = 30,240J.

This means that we need a solar panel that can during a 24-hour period provide 30,240J of energy. You can calculate the required solar-panel power using the formula:

where E is the energy needed (30,240 in this example) and H is the average number of hours that the solar panel will be in direct sunlight. Let’s assume that you live in the tropics, have more or less no seasonal variation in day length, and 10 hours of sunlight a day. In that case you can calculate the power of the panel you need as:

So, in theory a 1W solar panel is fine for this particular situation.

If you think you might need the thermostat to keep working for three days without any solar charging (perhaps during a tropical storm) you will need a battery that can deliver 3 x 30,240 = 90,720J of energy.

The storage capacity in mAh (C) can be calculated using this formula:

Discussion

This equation has made a number of optimistic assumptions. First, the tropical daylight pattern is likely to be markedly different from a wet maritime climate at high altitude. In that case, you may only be able to assume a much lower average. For example, according to US Climate Data, December in Seattle only averages 62 hours of sunshine in the entire month (2 hours a day). This would require a solar panel of five times the power of the tropical equivalent.

The equation also assumes that any voltage conversion that is needed between the output of the solar panel down to the battery voltage is 100% efficient. If you are using a switching regulator like Recipe 7.8 then you might get 80%, but a linear regulator will not be as good, maybe only 50% or lower.

Also, the battery you are charging or the circuit charging it may limit the maximum power that you are using from the solar panel to prevent damage to the battery by charging it with too high a current. This all needs to be taken into account. Using a higher power solar panel than your battery can keep up with will have the advantage that under lower light conditions you will still get some charging.

All in all, using the worst-case sunshine hours scenario for your location, you should probably use double the calculated solar-panel output power requirement as your starting point, before ordering any solar panels to start experimenting with.

In addition to assumptions about the generation side of things, the load side is also often a lot more complicated than a constant load current. It may be that a device has a constant standby current most of the time, but then is turned on by a user or as a result of a timed operation and uses considerably more current for a while, before going back to standby mode. In addition to manually logging the current consumption and making estimates of the energy usage over 24 hours, you can also use a logging multimeter that will make and store periodic readings of current consumption. The resulting data can be downloaded into a spreadsheet for further analysis.

A logging multimeter can also be a great way of assessing the real performance of a solar panel (see Recipe 9.3).

If you are making a solar-powered project, then it is always worth working out how to keep the power consumption to a minimum. For an Arduino or other microcontroller project this may mean making the microcontroller go into a low-power sleep most of the time, just waking periodically to check if anything needs to happen.

Of course, some projects (maybe a solar-powered water pump) don’t need a battery backup because they only need to operate when it’s sunny.

See Also

For information on measuring the actual power output of a solar panel, see Recipe 9.3.

For examples of this approach for powering an Arduino see Recipe 9.4 and for a Raspberry Pi see Recipe 9.5.

Problem

Your solar panel has nominal output power, but you want to measure the actual output in your location.

Solution

Using a load resistor attached to the solar panel, measure the voltage across the resistor and from this calculate the power output. Figure 9-3 shows this arrangement.

The output power of the solar panel P is calculated as:

So, if you use a 100Ω load resistance and the voltage is 5V, the output power of the solar panel is:

Figure 9-3. Measuring the Voltage Across a Load Resistor

The value of the load resistor needs to be low enough that the maximum output power of the solar panel is not exceeded, otherwise the solar panel will not generate as much as it could. In other words, if the solar panel is a 12V panel, you will probably find that the no-load output voltage is up to 18V in bright sunlight. So, you really want a load resistor value that gives an output of about 12V. You can calculate the maximum value of resistor to use using:

So, if your solar panel claims to be 20W at 12V, an ideal resistor value would be:

Remember that the resistor will have to have a power rating sufficient to handle the full power output of the panel—in this case 20W.

Discussion

In addition to simply taking readings under different circumstances (full sunlight, shade, etc.) you can also use a logging multimeter such as the one shown in Figure 9-4 to record the voltage across the load resistor at regular intervals.

Figure 9-4. A Logging Multimeter with Serial Output Connected to a Computer

This way, you can create a picture of what your panel is likely to generate in a day.

See Also

In Recipe 9.2 this measurement technique was used to assess the performance of some of my solar panels.

For information on using a voltmeter, see Recipe 21.2.

Problem

You have an Arduino project that you want to power from a solar panel.

Solution

Use a 5V solar cellphone charger, like the one shown in Figure 9-5.

Figure 9-5. A Solar Charger/Battery

Discussion

Unless you are designing a product, using a ready-made charger circuit is by far the easiest way of solar powering your Arduino.

An Arduino Uno uses around 50mA, so a 4Ah charger/battery like the one shown in Figure 9-5 will power the Arduino for around:

The Arduino Uno is not the most efficient of boards, when it comes to trying to minimize current consumption. It is more efficient to use an Arduino Pro Mini that has an external USB interface that you only need to connect when programming the Arduino. This can reduce your current consumption to 16mA. Using software to put the Arduino to sleep periodically can also greatly reduce current consumption.

See Also

For an introduction to Arduino, see Recipe 10.1.

To power your Raspberry Pi from solar, you will need a bigger solar panel than provided with ready-made phone boosters, as the Raspberry Pi is considerably more power hungry (see Recipe 9.5).

Problem

You want to power your Raspberry Pi from solar.

Solution

Use a 12V solar cell, charge controller, and SLA battery with a 12V to 5V power adapter.

This may sound excessive, but a Raspberry Pi with a WiFi adapter (WiFi is power hungry) can easily consume 600mA. If you add in a small 12V HDMI monitor, then you are likely to be drawing well over 1A.

Figure 9-6 shows a typical setup.

Figure 9-6. Solar Powering a Raspberry Pi

A ready-made charge controller generally has three pairs of screw terminals. One is attached to the solar panel, one to the battery, and one pair for the load, which in this case is a 12V to USB (5V) adapter commonly used as a phone car charger. Be sure to use one that can supply at least 1A.

Discussion

To run the Raspberry Pi continuously, you will need a 20W or greater solar panel. The choice of battery size depends on how long you want your Pi to keep running after it goes dark, or is dull for a few days. See Recipe 9.2 for information on how to make that choice.

If you are planning to use your Raspberry Pi as a controller, you may want to consider whether it would be better to use an Arduino, which uses only ⅒ of the power, or if you need network connectivity, a Particle Photon or an ESP8266 module (see Recipe 10.6), both of which use less than 100mA.

See Also

To power an Arduino using solar energy, see Recipe 9.4.

Problem

You want to understand just what an “Arduino” is and why it finds its way into so many electronics projects.

Solution

Figure 10-1 shows the most popular flavor of Arduino, the Arduino Uno R3.

Figure 10-1. An Arduino Uno

An Arduino is not a microcontroller but instead a microcontroller interface board. It has a microcontroller chip on the board, but it also has a whole load of other components that provide:

• Regulated power to the microcontroller
• A USB interface to program the Arduino from your computer
• A “power” LED
• A “user” LED connected to one of its pins that can be turned on and off programatically
• A 16MHz quartz crystal, necessary for the microcontroller’s operation
• GPIO sockets for connecting external electronics

An Arduino does not do anything until it has been programmed. That is, you have to use your computer to write some instructions in the programming language C that can control and read the GPIO pins. Special software, called the Arduino IDE, is provided to both write the programs and then upload them onto the Arduino using a USB cable. Figure 10-2 shows the Arduino IDE with the “Blink” program loaded and ready to transfer to your Arduino. As its name suggests, the Blink program simply makes the Arduino’s built-in LED blink on and off and is the starting point for most people’s Arduino adventures.

Figure 10-2. The Arduino IDE

You can download the Arduino IDE from http://arduino.cc.

In this book, it will be assumed that you have experimented with Arduino and at least got as far as uploading a program onto it to make an LED blink. If you are new to programming or need a slower-paced introduction to Arduino, then see some of the book suggestions at the end of this recipe.

Discussion

In addition to the GPIO pins that you will learn more about in Recipe 10.7 the Arduino also has peripheral interfaces for I2C (Recipe 14.9 and Recipe 14.10) and SPI devices (Recipe 19.4).

The Arduino is an extremely useful and reliable component to use in your designs, even if it is just while you are prototyping the project and will later build your own design with a microcontroller.

In addition to the Arduino Uno shown in Figure 10-1, there are many other types of Arduinos that are all programmed the same way. This means you can select an Arduino based on its price, size, and number of GPIO connections.

See Also

There are many excellent books written about Arduino that will help you fill in the background. Two I particularly recommend are:

• If you are new to programming: Programming Arduino: Getting Started with Sketches by Simon Monk, TAB DIY, 2016.
• For an encyclopedic Arduino reference: The Arduino Cookbook by Michael Margolis, O’Reilly, 2011.

The following is a list of some of the Arduino-related recipes in this book, outside of this chapter:

• Recipe 11.6
• Most of Chapter 12
• Most of Chapter 13
• Most of Chapter 14
• Recipe 18.1
• Recipe 19.3
• Recipe 19.4

Problem

You don’t want to have to type in all the example code in the book; you want to be able to download and use it.

Solution

All the Arduino sketches and Python programs for Raspberry Pi for this book are downloadable from GitHub.

To use the Arduino sketches, download them from GitHub by cloning the directory if you are a git user, or use the Download Zip option behind the Clone or Download button on the GitHub page. You do not need to have a GitHub login to download the files.

Once extracted, the directory contains a directory called arduino with each of the Arduino sketches inside its own directory. Double-clicking on it will open the sketch in the Arduino IDE.

Discussion

An alternative way to access the sketches is to copy the contents of the arduino directory that you downloaded from GitHub into your Arduino IDE’s sketches directory, which is found in your operating system’s Documents or My Documents (Windows) folder in a directory called Arduino. Then you will be able to open any of the sketches from the File→Sketchbook menu of the Arduino IDE.

See Also

To download the Python files for Raspberry Pi, see Recipe 10.4.

If you are looking for a primer to teach you Arduino C, then my book Programming Arduino: Getting Started with Sketches (TAB DIY, 2016) should help you out.

Problem

You want to understand just what a “Raspberry Pi” is and why it finds its way into so many electronics projects.

Solution

A Raspberry Pi (Figure 10-3) is an SBC running a version of Debian Linux (called Raspbian) as an operating system. You can plug in a keyboard, mouse, and monitor and use it to browse the internet just like a “normal” computer.

Figure 10-3. From Left to Right: Raspberry Pi Zero Model A and Pi 2 Model B

The Raspberry Pi is available in a number of different sizes, from the very low-cost Pi Zero up to the Model 3, which includes built-in WiFi.

The reason the Raspberry Pi has become popular for electronics projects is that, like the Arduino, it also has GPIO pins that you can connect to external electronics. What’s more, because the Raspberry Pi can be easily connected to the internet, you can also use it for all sorts of Internet of Things (IoT) projects.

When it comes to programming the Raspberry Pi, there are lots of options. In fact, all major programming languages are available to run on the Raspberry Pi, but the most popular language for electronics projects is probably Python, which is used in conjunction with the RPi.GPIO library.

While you need a computer to program an Arduino, to write programs for Raspberry Pi, you will use the Raspberry Pi itself.

Discussion

To use a Raspberry Pi unattended, you will want to be able to make your program automatically run when the Raspberry Pi Boots up (Recipe 10.3).

See Also

My book The Raspberry Pi Cookbook by O’Reilly (2016) is a cookbook like this, but dedicated entirely to the Raspberry Pi.

If you are new to programming and need a gentle introduction to Python programming on the Raspberry Pi, you might want to consider my book Programming the Raspberry Pi: Getting Started with Python (TAB DIY, 2015).

Here is a list of some of the Raspberry Pi-related recipes in this book outside of this chapter:

• Recipe 11.7
• Most of Chapter 12
• Most of Chapter 13
• Much of Chapter 14
• Recipe 18.2
• Recipe 19.2

Problem

You want to get the Python programs for the Raspberry Pi in this book onto your Raspberry Pi so that you can run them.

Solution

All the Python programs for Raspberry Pi for this book can be found here: https://github.com/simonmonk/electronics_cookbook.

To use the programs, you can fetch them from GitHub directly onto your Raspberry Pi using the command:

$ git clone https://github.com/simonmonk/electronics_cookbook

This will also fetch all the Arduino sketches, but these can be ignored. You will find the Python programs in a directory called pi.

To run a program (e.g., blink.py) use the following command:

$ sudo python blink.py

Note that the sudo command is not required on the latest version of the Raspberry Pi’s operating system (Raspbian) so you may be able to just type:

$ python blink.py

Discussion

Although I suggested ignoring the Arduino sketches earlier in this recipe, in reality you can download and install the Arduino IDE onto a Raspberry Pi and then use the Raspberry Pi to program the Arduino (see http://spellfoundry.com/sleepy-pi/setting-arduino-ide-raspbian/).

See Also

To access the Arduino code, see Recipe 10.2.

To set a Raspberry Pi Python program to automatically run on startup, see Recipe 10.5.

If you are looking for a primer to teach you Python with Raspberry Pi, my book Programming Raspberry Pi: Getting Started with Python (TAB DIY, 2015) should help you out.

Problem

You want a program or script to start automatically as your Raspberry Pi boots.

Solution

Modify your rc.local file to run the program you want.

Edit the file /etc/rc.local by using the command:

$ sudo nano /etc/rc.local

Add the following line after the first block of comment lines that begin with #:

/usr/bin/python /home/pi/my_program.py &

It is important to include the & on the end of the command line so that it runs in the background; otherwise, your Raspberry Pi will not boot.

Discussion

Be careful when you edit rc.local, or you may stop your Raspberry Pi from booting.

See Also

For general background on Raspberry Pi, see Recipe 10.3.

Problem

Neither an Arduino nor Raspberry Pi are quite what you are looking for and you want to know what the alternatives are.

Solution

Table 10-1 lists some other popular boards (MC, microcontroller; SBC, single-board computer).

Some of these boards are shown in Figure 10-4.

Figure 10-4. From Left to Right: Digispark, Photon, NodeMCU, and BeagleBone Black

Discussion

The Arduino IDE is a very flexible piece of software that can be easily extended to operate with unofficial Arduino-type boards. This means that you can stick to the Arduino IDE and the Arduino C programming language to program Arduino-like boards that may use a different processor or include useful addons such as WiFi, Bluetooth, or battery-charging hardware.

The Particle Photon deserves special mention, since although it uses a language that is based on Arduino C, you do not program it with the Arduino IDE, but rather with a web-based IDE. Deployment is then over the internet, allowing remote updating of applications. It also includes a very easy-to-use software framework for building IoT projects.

Other good boards to use where WiFi is required are any of the boards based on the ESP8266 modules, such as the NodeMCU and smaller modules like the ESP01. These can be programmed from the Arduino IDE. The ESP8266 boards are not as simple to use as the Photon but are extremely cheap.

See Also

For information on the Arduino see Recipe 10.1 and for the Raspberry Pi, see Recipe 10.3.

Problem

You want to use the Arduino, Raspberry Pi, and other microcontrollers and SBCs to control external electronic components.

Solution

Microcontrollers and SBCs have GPIO pins that allow you to connect external electronics to them. Figure 10-5 shows the schematic for a typical GPIO pin within a microcontroller chip or SoC of a Raspberry Pi. The pin can function as either a digital input or digital output, under software control.

Figure 10-5. Schematic for a GPIO Pin

Referring to Figure 10-5, if the output enable control of the GPIO is set using software to be enabled, the pin functions as a digital output and the push-pull driver (Recipe 11.8) allows the GPIO pin to source or sink a few tens of milliamps.

If the push-pull driver is disabled, then the pin can function as a digital input. If the pull-up resistor enable control is set, then the transistor Q1 is enabled to allow the resistor to pull up the input to a default high. This is commonly used when connecting a switch to a digital input to prevent a floating input from oscillating between high and low (see Recipe 10.7).

Discussion

The GPIO schematic of Figure 10-5 is typical of most Arduino pins. Some of the Arduino pins (marked A0 to A5) can be used as analog inputs and for those pins, the GPIO is also connected to ADC hardware inside the microcontroller chip.

Some GPIO pins have pull-down resistors as well as pull-up resistors, which can be turned on and off in the same way as pull-down resistors.

Table 10-1 compares the GPIO features of an Arduino and Raspberry Pi 3.

Figures 10-6 and 10-7 describe each of the accessible GPIO and power pins available on an Arduino and Raspberry Pi 3, respectively.

Figure 10-6. Arduino Uno R3 Pinout

Some of the pins require a little more explanation:

• IO reference voltage is the output voltage of the Arduino (5V for an Uno) but some other types of Arduinos operate at 3.3V. Intended for use by plug-in shields but rarely used.
• Vin is the supply voltage, which might be 9V if an external power supply is connected to the barrel jack, or 5V if the board is USB powered.
• The two I2Cs (IC to IC buses) are used for connecting to I2C devices (see Recipe 14.9), but are also connected to A4 and A5 of an Arduino Uno. On some other Arduino boards such as the Leonardo, these are separate.
• Analog reference voltage can be connected to a reference voltage below 5V to narrow the analog input range for greater precision at low voltages. If no connection is made, the analog inputs will be relative to 5V on an Arduino Uno.
• Pins 0 and 1 can in a pinch be used as extra GPIO pins, but if so you will need to disconnect anything attached to them to allow the USB connection to work. So generally it’s best not to use them.

Figure 10-7. Raspberry Pi GPIO Connector Pinout

Unlike an Arduino, the GPIO pins on a Raspberry Pi are not labeled. If you are going to be connecting things to the GPIO pins, it’s a good idea to get a hold of a GPIO template, such as the Raspberry Leaf (Adafruit 2196), that fits over the GPIO pins allowing you to identify them easily.

Most of the pins on a Raspberry Pi can be used as GPIO, but some have second functions:

• Pins 2 and 3 can also be used to connect I2C devices.
• GPIO pins 9 to 11 can also be used as an SPI for devices that support that connection type.
• ID_SD and ID_SC are dedicated to an interface for any HAT (hardware attached to top) that fits over the GPIO connector and allows software to identify the HAT.
• Pins 14 and 15 can be used to provide TTL serial interfaces for devices such as GPS modules that often use that interface standard.

If you have an older Raspberry Pi (before the model B+) it will only have 26 pins on the GPIO connector. These are the same as the top 26 pins (above the dashed line) of the 40-pin layout of newer Raspberry Pis shown in Figure 10-7.

See Also

For an introduction to Arduino see Recipe 10.1 and for Raspberry Pi see Recipe 10.3.

Many of the recipes in Chapters 11, 12, 13, and 14 use GPIO pins.

Problem

You want to configure an Arduino GPIO pin to be an output and then turn the output on and off using software.

Solution

Use the pinMode function to set the pin to be an output and then use digitalWrite to turn the pin on and off. The following example program will make digital pin 13 (attached to the Arduino’s built-in LED) blink on and off:

const int ledPin = 13;

void setup() 
{
  pinMode(ledPin, OUTPUT);
}

void loop() {
  digitalWrite(ledPin, HIGH);  // turn the LED on 
  delay(1000);                 // wait for a second
  digitalWrite(ledPin, LOW);   // turn the LED off 
  delay(1000);                 // wait for a second
}

The code for this sketch is available as part of the book downloads (Recipe 10.2). The sketch is called blink.

Discussion

The program (or “sketch” as they are known in the Arduino world) starts by defining a constant for the pin connected to the LED called ledPin, which is given the value 13. If you change your mind and decide to make another pin blink, you only need to change 13 to that pin number in one place in the code.

The setup function is run just once after the Arduino is reset. The pinMode function then specifies that ledPin is to be an OUTPUT. It is perfectly possible to change a pin’s mode while the sketch is running. You can find a good example of this with Charlieplexing (see Recipe 14.6).

The loop function will be run repeatedly over and over again, and each time it is run, it will first set ledPin (pin 13) high, wait a second (1000 milliseconds) then set it low, then wait another second, and so on.

See Also

For digital outputs on a Raspberry Pi, see Recipe 10.9 and for digital inputs on an Arduino see Recipe 10.10.

For the current-handling capabilities of an Arduino digital output, see Recipe 10.7.

Problem

You want to configure a Raspberry Pi GPIO pin to be an output and then turn the output on and off using software.

Solution

Use the RPi.GPIO library (included in Raspbian) with Python. The following example program will make GPIO pin 18 turn on and off once a second:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

led_pin = 18

GPIO.setup(led_pin, GPIO.OUT)


try:         
    while True:
        GPIO.output(led_pin, True)  # LED on
        time.sleep(1)               # delay 1 second
        GPIO.output(led_pin, False) # LED off
        time.sleep(1)               # delay 1 second
finally:  
    print("Cleaning up")
    GPIO.cleanup()

The code for this program is available as part of the book downloads (see Recipe 10.4). The file is called blink.py.

Unlike the Arduino, the Raspberry Pi does not have any user-controllable LEDs built in so to see this program in action see Recipe 14.1.

Discussion

The code starts by importing the GPIO and time libraries. It then sets the GPIO pin identification mode to be BCM (Broadcom). This is a hangover from the early days of Raspberry Pi when two ways of identifying the pins were almost equally used. However, for historical reasons, you still need to include this line at the top of all your Python programs. Some internet and book resources still exist that use the pin positions on the connector rather than the pins’ name. If you find such a resource, it will tell you to use a pin mode of BOARD instead of BCM.

The variable led_pin refers to the GPIO pin to be blinked and the pin set to be an output.

The main program loop is contained inside a try/finally block. This is not strictly speaking essential and the program will run just fine without it, but by calling GPIO.cleanup() whenever the program exits, all the GPIO pins are put back into a safe input state so that accidental shorts of the pins will not cause any damage.

Inside the while loop, the pin is first turned on, then there is a delay of a second, then it is turned off, etc. The sleep function takes a time in seconds as a parameter that can also be less than a second using a decimal notation. For example, to delay for half a second you would use time.sleep(0.5).

See Also

The Arduino equivalent to this program can be found in Recipe 10.8.

For digital inputs on a Raspberry Pi, see Recipe 10.11.

Problem

You want to read an Arduino digital input in an Arduino sketch.

Solution

Use the Arduino C digitalRead function. To see the result of the read, use the Arduino Serial Monitor. The following sketch illustrates this:

const int inputPin = 7;

void setup()
{
  pinMode(inputPin, INPUT);
  Serial.begin(9600);
}

void loop()
{
  int reading = digitalRead(inputPin);
  Serial.println(reading);
  delay(500);
}

You can find this sketch in the downloads for the book (see Recipe 10.2). It is called ch_10_digital_input.

The constant inputPin is defined as pin 7 and initialized to be an INPUT in the setup function.

The loop function first assigns the value resulting from carrying out a digitalRead on inputPin to the variable reading and then sends this value over the Arduino’s USB interface to the Serial Monitor. Finally, a delay of 500 milliseconds is added to slow things down to a manageable rate.

To open the Serial Monitor (Figure 10-8), click the rightmost icon on the Arduino IDE’s toolbar. This looks like a magnifying glass.

Figure 10-8. The Arduino Serial Monitor

You should see a steady stream of numbers appear as shown in Figure 10-8. These should be predominantly 0s, but there may be occasional 1s. Try attaching a male-to-male jumper wire or short length of solid core wire to pin 7 of the Arduino (Figure 10-9). You should find that you get a selection of 1s and 0s. This is because you are acting as an antenna and the digital input is picking up electromagnetic noise (most likely “hum”). The digital input is “floating” and since it has a very high input impedance, it is very sensitive to noise.

Figure 10-9. A Floating Digital Input

Next, try attaching the floating end of the lead to one of the GND connections on the Arduino as shown in Figure 10-10. You should see that the output in the Serial Monitor is all 0s.

Figure 10-10. Connecting a Digital Input to GND

Finally, move the end of the wire that goes to GND to 5V and the Serial Monitor output should now be all 1s.

Discussion

The Arduino Serial Monitor is one of the few ways to see what is going on inside your Arduino and is often used to see what’s going wrong in a sketch.

See Also

For digital inputs on a Raspberry Pi, see Recipe 10.11.

Problem

You want to be able to read a GPIO pin as a digital input from your Python program.

Solution

Use the RPi.GPIO library. The following example program reads GPIO23 every half second and prints out the result:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

input_pin = 23

GPIO.setup(input_pin, GPIO.IN)

try:         
    while True:
        reading = GPIO.input(input_pin) 
        print(reading)
        time.sleep(0.5)         
finally:  
    print("Cleaning up")
    GPIO.cleanup()

The code for this program is available as part of the book downloads (see Recipe 10.4). The file is called ch_10_digital_input.py.

Discussion

When you run the preceding program, you will see mostly 0s in the terminal:

$ sudo python ch_10_digital_input.py 
0
0
0
0
0
0
0

With both this program and Recipe 10.10 you can connect a switch to the digital input (Recipe 12.1) or use a female-to-female jumper wire to connect GPIO23 to either GND or 3.3V.

See Also

For digital inputs on the Arduino, see Recipe 10.10.

Problem

You want to read the voltage at a GPIO pin from your Arduino sketch.

Solution

Use the Arduino analogRead function and one of the Arduino pins A0 to A5 that can be used as analog inputs. The following example sketch writes the analog reading as a voltage to the Serial Monitor every half a second:

void setup()
{
  pinMode(inputPin, INPUT);
  Serial.begin(9600);
}

const int inputPin A0;

void loop()
{
  int reading = analogRead(inputPin);
  float volts = reading / 204.6;
  Serial.println(volts);
  delay(500);
}

You can find this sketch, which is called ch_10_analog_input, with the downloads (Recipe 10.2).

The sketch defines the inputPin as A0. When referring to one of the A0 to A5 pins on an Arduino, you have to use the letter and the number (A and 0), whereas for the other pins you just use the number.

The analogRead function returns a number between 0 and 1023, where 0 means 0V and 1023 means 5V. This number is converted to a voltage by dividing it by 204.6 (1023 / 5). Note that the analog range can be changed by connecting the Arduino AREF (analog reference) pin to a different voltage.

When you open the Serial Monitor you will see a series of values displayed. As with the digital inputs of Recipe 10.10 the analog input is floating and so the numbers (voltage on A0) will probably fluctuate something like this:

2.42
2.36
2.27
2.13
1.99
1.86
1.74
1.62
1.40
0.70

Discussion

Try the experiments in Recipe 10.11 and put a wire into A0. Touching the end of the wire with your fingers so that you act as a radio antenna should make the analog readings even more wild. Connecting A0 to the 5V connector of the Arduino should display a voltage reading of 5.00 on the Serial Monitor and GND will give you 0.00; connecting A0 to the 3.3V socket on the Arduino should give you a reading of around 3.30.

If the maximum voltage you want to measure is below 5V, then you can use the Arduino AREF pin to keep the 0–1023 range of readings but for a smaller voltage range and thus achieve higher precision.

For example, if you tie the AREF pin to the 3.3V pin on the Arduino you will get a full range of readings over the 3.3V range. The voltage at AREF needs to be stable and regulated or your readings will be inaccurate.

See Also

The Raspberry Pi does not have analog inputs, but you can use an ADC IC (Recipe 12.4).

Problem

You want your Arduino to control the output power from a GPIO pin, say to control the brightness of an LED or the speed of a motor.

Solution

Use the analogWrite function of the Arduino on one of the PWM-capable pins.

The following example sketch will set the brightness of an LED wired to pin 11. The brightness is set by sending a number between 0 and 255 from the Arduino Serial Monitor to the Arduino board. For the curious, 255 is 1 less than 2 to the power of 8.

const int outputPin = 11;

void setup()
{
  pinMode(outputPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("Enter brightness 0 to 255");
}

void loop()
{
  if (Serial.available())
  {
    int brightness = Serial.parseInt();
    if (brightness >= 0 && brightness <= 255)
    {
      analogWrite(outputPin, brightness);
      Serial.println("Changed.");
    }
    else 
    {
      Serial.println("0 to 255");
    }
  }
}

You can find this sketch, which is called ch_10_analog_output, with the downloads for the book (Recipe 10.2).

In addition to illustrating PWM output from a GPIO pin, this example sketch also illustrates how you can send data from your computer to the Arduino via the Serial Monitor.

The setup function sets outputPin to be an OUTPUT and then starts serial communication. Finally, the setup function sends a message to the Serial Monitor instructing you how to use the sketch by sending a number between 0 and 255.

Inside the loop function, the call to Serial.available() tests to see if anything has been sent from the Serial Monitor, and if it has, it is converted into an int and assigned to the variable brightness. The analogWrite command is then used to set the output of outputPin.

To see this sketch in operation, you will first need to follow Recipe 14.1 and attach an LED to pin 11.

Open the Arduino IDE’s Serial Monitor (see Recipe 10.10) and then try some different values between 0 and 255 to see how the brightness changes (Figure 10-11).

Figure 10-11. Setting the PWM Output from the Serial Monitor

Discussion

Not all the pins on an Arduino can be used for PWM like this. In fact, only the pins on the Arduino Uno marker with a ~ can. On an Arduino Uno, that’s pins 3, 5, 6, 9, 10, and 11. Other Arduino models have different sets of pins that are PWM-capable, but you will need to refer to the Arduino documentation to know which ones.

Figure 10-12. Pulse-Width Modulation

See Also

For the Raspberry Pi version of this recipe, see Recipe 10.14.

Problem

You want your Raspberry Pi to control the output power from a GPIO pin, say to control the brightness of an LED or the speed of a motor.

Solution

Use the PWM feature of the RPi.GPIO library to control the output power of a GPIO pin. The following Python example program illustrates this:

import RPi.GPIO as GPIO

led_pin = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(led_pin, GPIO.OUT)

pwm_led = GPIO.PWM(led_pin, 500)
pwm_led.start(100)

try:
  while True:
    duty_s = raw_input("Enter Brightness (0 to 100):")
    duty = int(duty_s)
    pwm_led.ChangeDutyCycle(duty)

finally:  
  print("Cleaning up")
  GPIO.cleanup()

You can find this program in the downloads for the book (see Recipe 10.4). If you are using Python 3 rather than Python 2, change the command raw_input to just input.

To see this program in action, you will need to attach an LED to GPIO pin 18 (see Recipe 14.1).

The RPi.GPIO library is a little more complex to use than its Arduino counterpart. After defining the pin as an output, you then have to create a PWM channel using the line:

pwm_led = GPIO.PWM(led_pin, 500)

The number 500 is the frequency of the PWM pulses in Hz. This PWM channel is then started using the following line:

pwm_led.start(100)

Here, the value of 100 is the initial duty cycle (percentage of time the pin is high) of the PWM signal (in this case, 100% of the time).

The rest of the script interacts with the user requesting a value of the duty cycle between 0 and 100. Run the program and try different versions of brightness as shown here:

$ sudo python led_brightness.py
Enter Brightness (0 to 100):0
Enter Brightness (0 to 100):20
Enter Brightness (0 to 100):10
Enter Brightness (0 to 100):5
Enter Brightness (0 to 100):1
Enter Brightness (0 to 100):90

When you want to exit the program, press CTRL-C.

Discussion

The Raspberry Pi does not use a real-time operating system. That is, at any one time, there are many differnt processes running. So if you are trying to generate pulses of an exact length such as with PWM, you will find that you get a certain amount of jitter in the LED brightness as the pulse generation is interrupted.

See Also

For the Arduino counterpart to this recipe, see Recipe 10.13.

Problem

You want to enable the I2C bus on your Raspberry Pi to connect I2C peripherals to it such as the displays used in Recipe 14.9 and Recipe 14.10.

Solution

In the latest versions of Raspbian, enabling I2C (and for that matter SPI—Recipe 10.16) is simply a matter of using the Raspberry Pi configuration tool that you will find on the main menu under Preferences (Figure 10-13). Just check the box for I2C and click OK. You will be prompted to restart.

On older versions of Raspbian, the raspi-config tool does the same job.

Figure 10-13. Pi Configuration Before Enabling I2C Using the Pi Configuration Tool

Start raspi-config  using the following command:

$ sudo raspi-config

Then select Advanced from the menu and scroll down to I2C (Figure 10-14).

When you are asked “Would you like the ARM I2C interface to be enabled?” say “Yes.” You will also be asked if you want the I2C module to load at startup, to which you should also say “Yes.”

Figure 10-14. Enabling I2C Using raspi-config

Discussion

I2C is a commonly used standard for connecting devices together. It uses two data pins SDA (data) and SCL (clock) to transfer data bidirectionally between devices. Usually one of the devices on the bus is a microcontroller or in this case, the SoC of the Raspberry Pi. Multiple devices can be connected to the same I2C bus pins, so, for example, you could have both a display and a sensor connected to the same two bus pins, each device having its own unique address.

To use I2C devices from Python, install the Python I2C library by using the commands:

$ sudo apt-get update
$ sudo apt-get install python-smbus

You will then need to reboot the Raspberry Pi for the changes to take effect.

When using I2C hardware the i2c-tools software can be a great help in debugging and making sure devices are properly connected to the Raspberry Pi. This can be installed using the command:

$ sudo apt-get install i2c-tools

When you have a device attached to the I2C bus, running the i2cdetect utility will tell you if it’s connected and what I2C address it’s using, as shown in Figure 10-15.

Figure 10-15. Using i2cdetect

See Also

To set up the Raspberry Pi’s SPI, see Recipe 10.16.

For recipes that connect I2C peripherals to a Raspberry Pi, see Recipe 14.9, Recipe 14.10, and Recipe 19.3.

Problem

You want to enable the SPI bus on your Raspberry Pi to connect peripherals to it.

Solution

By default, Raspbian is not configured for the Raspberry Pi’s SPI. To enable it use the Raspberry Pi configuration tool found in the main menu under Preferences (as in Recipe 10.15), or on older versions of Raspbian, use raspi-config using the command:

$ sudo raspi-config

Then select Advanced, followed by SPI, and then “Yes” before rebooting your Raspberry Pi. After the reboot, SPI will be available.

Discussion

The SPI allows serial transfer of data between the Raspberry Pi and peripheral devices, such as ADCs and port expander chips, among other devices. It is similar in concept to I2C but uses four pins instead of two. As with I2C SPI data is synchronized with a clock signal (SCLK) but separate lines are used for each direction of communication: MOSI (master out slave in) and MISO (master in slave out) and a separate “Enable” pin is needed from the master for each of the devices connected to the bus. It is an older and less elegant standard than I2C but still widely used.

You may come across examples of interfacing with SPI that use an approach called bit banging, where the RPi.GPIO library is used to interface with the four GPIO pins used by the SPI.

See Also

Recipe 12.4 uses an SPI ADC converter chip.

The SPI is used in Recipe 12.4 and Recipe 19.4.

Problem

You want to connect a 5V device to a Raspberry Pi or one of the 3.3V Arduino models.

Solution

When you are connecting a 3.3V-level output to a 5V input, with very few exceptions (see discussion), no level conversion is needed—you can connect the two directly.

However, if you want to connect a 5V output to a 3.3V input, then that’s a different matter. Direct connection will wall damage the 3.3V device. If the 3.3V device is specified as having 5V-tolerant inputs, then you can just connect the two together directly. Note that the inputs of a Raspberry Pi are not 5V tolerant, so you should connect them together using a voltage divider as shown in Figure 10-16.

Figure 10-16. Reducing a 5V Signal to 3V

Discussion

Occasionally you will find a 5V device that specifies that the logic level for an input should be above 3.3V. For example, WS2812 LED ICs (Recipe 14.8) theoretically need an input greater than 4V to be considered as logic high according to their datasheet. In practice, I have always found them to work without level conversion, but if you are designing a product to sell, then you would not take that chance and the kind of level shifter shown in Figure 10-17 should be used.

Figure 10-17. Bidirectional Level Shifting with a MOSFET

This circuit will actually convert levels in both directions; that is, 5V outputs will be reduced to 3.3V and 3.3V outputs increased to 5V. It makes use of the fact that MOSFETs have a “substrate” protection diode that prevents current from flowing from drain to source.

To understand how this circuit works, first consider the case where the 3.3V side is an output and the 5V side an input; that is, the level is being shifted up.

In this case, if the 3.3V GPIO is high, then the gate-source voltage will be 0V, the MOSFET will be off, and R2 will pull-up the 5V input HIGH. If the 3.3V GPIO output is low, the gate-source voltage will be 3.3V, the MOSFET will turn on, and the 5V input will be effectively connected to the LOW signal from the 3.3V side.

If we swap the direction so that the 5V side is now the output and the 3.3V side the input, then when the 5V output is HIGH the MOSFET source and gate will be at 3.3V, the MOSFET will be off, and the 3.3V input pulled up to 3.3V by R1. If the 5V output is LOW the MOSFET’s built-in protection diode will conduct, pulling the 3.3V input down to the diode’s forward voltage (about 0.6V). This turns the MOSFET on, pulling the 3.3V input fully to ground.

See Also

The simplest level conversion using just two resistors is a voltage divider as described in Recipe 2.6.

For a general background on MOSFETs see Recipe 5.3.

For an interesting discussion of the necessity of level shifting for Raspberry Pi inputs, see http://tansi.info/rp/interfacing5v.html.

If you have a lot of signals all requiring level shifting, then it makes sense to use a level-shifting IC or module such as these products from Adafruit: http://bit.ly/2lLHmuG (four signals) and http://bit.ly/2msMgku (eight signals).

Problem

You want to allow a pin, such as a microcontroller’s GPIO pin, to control more power than it otherwise could.

Solution

Use a transistor in a common-emitter arrangement as a “low-side” switch with a resistor to limit the base current. Figure 11-1 shows the schematic for this circuit, that, along with Recipe 11.2, you will use time and time again in your designs.

This type of switching is called “low-side” switching because the transistor acts as a switch between the low voltage of GND and the load.

Figure 11-1. Using a Transistor as a Switch

Discussion

The resistor R makes sure that the current drawn from the GPIO pin does not exceed the current limit of the pin (40mA for Arduino and 16mA for Raspberry Pi; see also Recipe 10.7). It also protects the transistor from too much base current flowing. This protective role must be balanced against the limited gain of a bipolar transistor, which may only be 100 or less. So if you are planning to use the transistor to switch a current of 1A, you could reasonably expect a base current of 10mA. This means that for this 1A load, you need to choose a value of R that gives a base current somewhere between the available current of the GPIO and 10mA.

Aiming for a 10mA base current (if you are using a 5V GPIO pin and assume that the base-emitter voltage is a fairly constant 0.6V) you can calculate the value of R as:

Applying the same calculation for a 3.3V GPIO pin you get:

The two connections from this circuit to an Arduino or Raspberry Pi are the GND connection and the GPIO pin. The positive voltage supply to the load is entirely separate from the Raspberry Pi or Arduino. This means that you are not restricted to switching the logic level of the Arduino or Pi (3.3V or 5V) but can switch up to the maximum voltage that the transistor is capable of switching.

Having said that, you will often use the positive voltage supply of an Arduino or Raspberry Pi for the convenience of having a single voltage source.

See Also

For a discussion of bipolar transistors, see Recipe 5.1.

GPIO ports and their output logic are described in Recipe 10.7.

You will find an example of switching with a transistor using an Arduino in Recipe 11.6 and for a Raspberry Pi in Recipe 11.7.

For switching using a MOSFET, see Recipe 11.3.

Problem

You want to switch using a BJT, but you need one end of the load to be connected to ground.

Solution

The arrangement of Figure 11-1 is called low-side switching because the switching takes place on the lower voltage side; that is, to GND rather than from the positive supply. This means that whatever is being switched has to have a connection to the positive supply voltage.

One approach to high-side switching is to simply rearrange Figure 11-1 a little as shown in Figure 11-2.

Figure 11-2. High-side Switching with an NPN BJT (Restricted Switching Range)

The circuit of Figure 11-2 cannot switch voltages (+V) greater than the GPIO pin’s voltage less half a volt.

A transistor in this arrangement is called an emitter follower (Recipe 16.4) because the emitter will generally be about 0.6V less than the base. This means that we can use this arrangement for high-side switching but only so long as the voltage at the GPIO pin is less than +V.

In short, the circuit of Figure 11-2 is only useful for switch loads where one end of the load must be connected to ground and the other does not need a supply voltage that is greater than the control voltage of the GPIO pin.

Discussion

If you need to switch a higher voltage (perhaps 12V) you might be tempted to consider an alternative to Figure 11-2 that uses a PNP transistor rather than an NPN transistor as shown in Figure 11-3.

Figure 11-3. High-side Switching with a PNP BJT (Defective)

This circuit is defective because Q2 will turn on with a base current when its base is about 0.6V lower than +V. So, if you were trying to switch a 12V load from 5V logic, the base voltage would always be over 0.6V and enough base current would flow to keep the transistor on whether the GPIO pin was at 5V or GND.

If you need to be able to switch voltages of V+ that are greater than the 5V of your Arduino or 3.3V of your Raspberry Pi and you don’t want to use the tristate logic trick, then you will need to use the arrangement shown in Figure 11-4.

This uses a separate NPN transistor Q1 to control the base current to Q2 giving a full switching range.

Figure 11-4. High-side Switching with an NPN BJT Driving a PNP BJT

Low-side switching (Recipe 11.1) is the most common and simplest arrangement, and unless you have a good reason such as the need for one end of the load to be connected to ground, you should use low-side switching.

See Also

For a discussion of NPN and PNP bipolar transistors, see Recipe 5.1.

GPIO ports and their output logic are described in Recipe 10.7.

You will find an example of switching with a transistor using an Arduino in Recipe 11.6 and for a Raspberry Pi in Recipe 11.7.

For switching using a MOSFET, see Recipe 11.3.

Problem

You want to allow a GPIO pin to control more power than it otherwise could, but a BJT isn’t enough.

Solution

You can use a MOSFET as an electronic switch. Use the transistor in a common-source arrangement. Figure 11-5 shows the schematic for this circuit. Along with Recipe 11.1, you will find yourself using this circuit a lot.

This type of switching is called “low-side” switching because the transistor acts as a switch between the low voltage of GND and the load.

If the GPIO pin is high (3.3V or 5V) and exceeds the gate-threshold voltage of the MOSFET, the MOSFET will turn on, allowing current to flow from +V through the load to GND.

Figure 11-5. Switching with an N-Channel Enhancement-mode MOSFET

Discussion

You may be wondering why the gate resistor R is needed. In reality for most MOSFETS in most relatively low-current applications, it isn’t needed at all and you can just connect the GPIO pin directly to the gate.

However, the MOSFET’s gate appears to the GPIO pin as a capacitor that will be charged and discharged as the GPIO pin goes high and low. So, if you are switching the MOSFET at high speed (say PWM at a high frequency) or the MOSFETs’ gate capacitance is high (it increases with load current) then the maximum rated output current of the GPIO pin may be exceeded and the transistors inside the GPIO output could overheat. In any case, resistors are cheap and it’s good practice to include one.

In Figure 11-5 the gate of the MOSFET is floating. If it is connected to a GPIO set to be an output, then the MOSFET will be in a stable state, but there are several reasons why this might not be the case:

• The program to control the GPIO might not be running yet on your Raspberry Pi, in which case the GPIO will be a floating input.
• The control hardware may be detachable from the Arduino or Raspberry Pi GPIO pin while still retaining its positive supply.
• If the gate is left floating the load is likely to turn itself on and off in response to electrical noise. This is just like the experience of a floating input in an Arduino as described in Recipe 10.10.

To prevent this unwanted behavior, you can just add a pull-down resistor to keep the gate low unless the GPIO pin is actively driving the gate (see Figure 11-6). This resistor should have a resistance of perhaps 10 times that of the gate-protection resistor (R1) to prevent the effective voltage divider (Recipe 2.6) formed by R1 and R2 from reducing the gate voltage by any significant amount.

Figure 11-6. Preventing a Floating Gate from Causing Problems

See Also

For a discussion of MOSFETs, see Recipe 5.3.

GPIO ports and their output logic are described in Recipe 10.7.

You will find an example of switching with a MOSFET using an Arduino in Recipe 11.6 and for a Raspberry Pi in Recipe 11.7.

For switching using a BJT, see Recipe 11.1.

Problem

You want to switch using a MOSFET, but you need to share a ground connection between the load being switched and the Arduino or Raspberry Pi doing the switching.

Solution

The problems with high-side switching using a MOSFET are similar to those of Recipe 11.2. However, whereas in Recipe 11.2 it was okay to use an emitter-follower configuration or PNP transistor on its own, the need for a gate-source voltage above the threshold voltage to turn the MOSFET on means that the switchable range is reduced by at least the threshold voltage, making the circuit unsuitable for most applications.

So, for high-side switching there is little alternative to using the circuit of Figure 11-7.

Figure 11-7. High-side Switching with a P-Channel MOSFET and BJT

Discussion

By using a BJT rather than a MOSFET for Q1, there will always be enough leakage current through Q1 to keep the gate of Q2 low when the control pin is left floating.

See Also

For a discussion of MOSFETs, see Recipe 5.3.

GPIO ports and their output logic are described in Recipe 10.7.

You will find an example of switching with a MOSFET using an Arduino in Recipe 11.6 and for a Raspberry Pi in Recipe 11.7.

Problem

You can’t decide whether to use a BJT or a MOSFET for GPIO-controlled switching.

Solution

Use Table 11-1 to help you decide which NPN BJT or N-Channel MOSFET to use in low-side switching.

While any of these transistors would technically work, over time you’ll find that you return to a few of your favorite transistors.

Discussion

When it comes to switching small loads (say under 100mA) a small BJT (like the 2N3904) and a small MOSFET (like the 2N7000) are pretty much interchangeable. In fact, their pinouts mean that they are in the same transistor package and are pin-compatible.

Above 100mA but below 200mA I would usually select a 2N7000 because its low on-resistance means that it will not get as hot as a BJT.

Between 200mA and 500mA, I would either use a Darlington MPSA14 to save space over a FQP30N06L MOSFET (in a larger package), but only if I could afford the 1.5V or more voltage drop of the MPSA14.

Between 500mA and 3A you are definitely in large-transistor territory in a TO-220 package. If you can afford the voltage drop of a Darlington then the TIP120 is a good choice; otherwise, a FQP30N06L will serve you well.

For powers over 3A use a FQP30N06L with a heatsink.

See Also

See also Recipe 5.5.

2N3904 datasheet: https://www.sparkfun.com/datasheets/Components/2N3904.pdf

2N7000 datasheet: https://www.fairchildsemi.com/datasheets/2N/2N7000.pdf

MPSA14 datasheet: http://www.farnell.com/datasheets/43685.pdf

FQP30N06L datasheet: https://www.fairchildsemi.com/datasheets/FQ/FQP30N06L.pdf

TIP120 datasheet: https://www.fairchildsemi.com/datasheets/TI/TIP122.pdf

Problem

You want to use an Arduino to switch a load on and off; that is, more current or a higher voltage than the Arduino GPIO pin can handle.

Solution

If you have a 5V Arduino like the Arduino Uno, then each GPIO pin can switch a maximum of 5V at 40mA. To increase either the voltage or the current, you need to use a transistor. The principles and even the breadboard layouts are much the same whether you use a BJT or a MOSFET. See Recipe 11.5 for the relative merits of each technology.

Figure 11-8 shows the schematic for this.

Figure 11-8. Schematic for an Arduino Controlling a 12V Load

Discussion

You can build this circuit on a breadboard to experiment. Figure 11-9 shows the breadboard layout for the project using a 2N7000, which is good for 200mA (2.4W). If you prefer, you could use a 2N3904 or an MPSA14 with exactly the same breadboard layout.

Figure 11-9. Breadboard Layout for Arduino Control Using 2N7000

If you have a higher-power LED lamp, you should use a FQP30N06L as shown in Figure 11-10. Again, you could if you prefer use a TIP120.

Figure 11-10. Breadboard Layout for Arduino Control Using FQP30N06L

The Arduino test program for this project is listed here and is available in the book downloads (see Recipe 10.2) where the sketch is called ch_11_on_off:

const int outputPin = 11;

void setup()
{
  pinMode(outputPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("Enter 0 for off and 1 for on");
}

void loop()
{
  if (Serial.available())
  {
    char onOff = Serial.read();
    if (onOff == '1')
    {
      digitalWrite(outputPin, HIGH);
      Serial.println("Output ON.");
    }
    else if (onOff == '0')
    {
      digitalWrite(outputPin, LOW);
      Serial.println("Output OFF.");
    }
  }
}

This sketch follows a similar pattern to that of Recipe 10.13. Pin 11 is set to be an output and when a single-command character “1” or “0” is sent from the Serial Monitor (Figure 11-11) digitalWrite is used to turn pin 11 on or off.

Figure 11-11. Using the Serial Monitor to Turn a Lamp On and Off

You can see the circuit in action in Figure 11-12, where a 12V SLA battery is used to power the lamp.

Figure 11-12. Switching a 12V DC Lamp from an Arduino

Although intended as a demonstration circuit, this recipe is actually quite powerful, allowing you to control the lamp from your computer.

See Also

For information on getting started with breadboard, see Recipe 20.1.

For transistor pinouts, see Appendix A.

The basics of an Arduino Uno are described in Recipe 10.1.

To use this same design with Raspberry Pi, see Recipe 11.7.

Problem

You want to use a Raspberry Pi to switch a load on and off; that is, more current or a higher voltage than the GPIO pin can handle.

Solution

The circuits used in Recipe 11.6 will also work just fine if you wire the control connection and GND to a Raspberry Pi rather than an Arduino. Figure 11-13 shows the breadboard layout for using a 2N7000 MOSFET with a Raspberry Pi to switch a 12V LED lamp module.

Figure 11-13. Breadboard Layout for Raspberry Pi Control Using 2N7000

To control the lamp, you can use the following program, which is called ch_11_on_off.py and can be found with the downloads for the book (see Recipe 10.4):

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)

led_pin = 18

GPIO.setup(led_pin, GPIO.OUT)


try:         
    while True:
        answer = input("1 for on 0 for off: ")
        if answer == 1:
            GPIO.output(led_pin, True)  # LED on
        elif answer == 0:
            GPIO.output(led_pin, False) # LED off
finally:  
    print("Cleaning up")
    GPIO.cleanup()

When you run the program you should see this in the terminal session:

$ sudo python ch_11_on_off.py 
1 for on 0 for off: 1
1 for on 0 for off: 0

When you enter 1, the lamp should light (pressing 0 will turn it off again).

Discussion

As with Recipe 11.6 you can also use other transistors than the 2N7000 or FQP20N06L MOSFETs.

See Also

For information on getting started with breadboards, see Recipe 20.1.

For transistor pinouts, see Appendix A.

The basics of a Raspbery Pi are described in Recipe 10.3.

To use this same design with an Arduino, see Recipe 11.6.

Problem

You want to be able to switch a load on both the high and low sides because you want to be able to reverse the flow of current through your load, perhaps to control the direction of a DC motor.

Solution

Use transistors in the half-bridge configuration shown in Figure 11-14.

Figure 11-14. A Half-Bridge

This circuit combines elements of Recipe 11.3 and Recipe 11.4 and relies on their being three power lines: +V, –V, and GND in the middle. Thus, the three power lines might be +6V, –6V, and 0V.

The control signals A and B turn on Q2 and Q3, respectively. If A and B are both low with respect to –V no current will flow through the load. If A is high, Q2 will conduct and current will flow through Q2 to the load and then to GND. Conversely, if A is low and B is high, Q3 will conduct and current will flow from GND through the load to –V.

Under no circumstances should A and B both be high, since if both Q2 and Q3 are conducting there will effectively be a short-circuit between +V and –V and a damagingly large current will flow. In fact, if you are connecting A and B to GPIO pins and using software to set them high or low, you should put a small delay in between settings to allow A and B to be high at the same time. For example, to change from A high, B low to A low, B high, you should follow these steps:

1. Set A low
2. Delay
3. Set B high

Discussion

The half-bridge and its relative the full-bridge are often used to control DC motors, where their ability to reverse the direction of the flow of current allows them to reverse the direction in which the motor rotates.

Although it is perfectly possible to build half-bridge circuits from discrete components, it is more common to use an IC. You will meet several of these in Chapter 13.

See Also

For a full-bridge circuit, see Recipe 13.3.

For the FQP27P06 P-Channel MOSFET’s datasheet, see https://www.sparkfun.com/datasheets/Components/General/FQP27P06.pdf.

Problem

You want to switch a relay on and off using an Arduino or Raspberry Pi GPIO pin.

Solution

Use a low-power BJT or MOSFET to switch the power to the relay’s coil as shown in Figure 11-15.

Figure 11-15. Controlling a Relay from a GPIO Pin

A typical relay coil will need about 50mA to activate the relay contacts, which is slightly too much to use an Arduino pin directly and far too much for a Raspberry Pi GPIO pin. A relay with a nominal 5V coil will generally work with a little over 4V if you use a BJT, but supplying the full 5V by way of a MOSFET is generally better. So a good choice for Q1 would be a 2N7000. R is not at all critical, but 1kΩ would be fine.

The diode D1 is called a “freewheeling” or “flyback” diode and provides a discharge path for high voltages generated as the relay coil is switched off, which protects Q1.

Discussion

Building a design like this on a breadboard can be a little tricky, as most common relay packages don’t have pins that will fit into the breadboard, so you may have to solder short extension wires to the relay coil or use a prototyping setup like the MonkMakes Protoboard that has a place to solder a relay. Figure 11-16 shows the breadboard layout for using a relay with an Arduino and Figure 11-17 for a Raspberry Pi.

To test out the relay on an Arduino and Raspberry Pi, you can use the programs described in Recipe 11.6 and Recipe 11.7, respectively.

Although it can be tempting to try to get away with connecting the relay coil directly to a GPIO pin, this may damage the Arduino or Pi and is most certainly operating outside of the specifications for the Arduino, Raspberry Pi, or relay and cannot be guaranteed to work correctly.

Figure 11-16. Breadboard Layout for Controlling a Relay from an Arduino

Figure 11-17. Breadboard Layout for Controlling a Relay from a Raspberry Pi

See Also

For information on relays see Recipe 6.4.

Problem

You want to connect a SSR with an GPIO pin and control the SSR from an Arduino or Raspberry Pi.

Solution

SSRs are generally opto-isolated, which means that controlling them is as simple as using an LED. In fact, it’s usually easier because in an SSR a series resistor is generally included. Figure 11-18 shows an SSR module connected to a Raspberry Pi. The SSR’s negative input terminal is connected to GND and the positive terminal to a GPIO pin.

Figure 11-18. Controlling an SSR Using a Raspberry Pi GPIO Pin

Discussion

To test out the relay on an Arduino and Raspberry Pi, you can use the programs described in Recipe 11.6 and Recipe 11.7, respectively.

See Also

A safe and easy way to control AC using an SSR is to use a PowerSwitch Tail.

Problem

You need to know how to use a module (such as a motion detector) or other design that has open-collector outputs.

Solution

As the name suggests, a circuit that has an open-collector output uses an NPN BJT as an output stage (Figure 11-19) with the implication that the emitter is connected to GND, with no internal connections to the collector except to the output.

Figure 11-19. A Module with an Open-Collector Output

At first glance this may seem strange, and a common mistake is to treat an open-collector output as a logic output and connect it straight to a GPIO pin. Unless the internal pull-up resistor on the GPIO pin (Recipe 10.7) is enabled, this will not work.

Leaving the collector floating like this has the advantage that by simply using a resistor from the collector to a positive supply, the output voltage of the open collector can be set to match that of the GPIO input you are connecting it to. Figure 11-20 shows how an open-collector output can be used with 3.3V logic (left) and 5V logic (right).

The value of R1 is not critical and needs to lie somewhere in the large range of not exceeding the transistor’s collector current limit and not being succeptable to electrical noise. So, anything between 1kΩ and 1MΩ is fine. Common resistor values for this are 1kΩ and 10kΩ.

Figure 11-20. Pulling Up an Open-Collector Output

If your GPIO pin has the ability to enable an internal pull-up resistor you can use this instead of an external resistor.

Discussion

If the circuit or module you are using specifies the current rating of the open-collector output, you can use it to directly drive a load (e.g., a relay). The load should be in place of the resistor between the output and a positive voltage supply.

The MOSFET equivalent of an open-collector output is the open-drain output (see Figure 11-21). This can be used in exactly the same way as its BJT equivalent. In fact, it’s not uncommon to find device outputs labeled as open collector that actually use a MOSFET.

Figure 11-21. An Open-Drain Output

See Also

For more information on BJTs, see Recipe 5.1.

To learn about GPIO pins, see Recipe 10.7.

Problem

You want to be able to convert a mechanical movement to a digital on/off signal that you can use with an Arduino or Raspberry Pi.

Solution

Connect the switch between GND and a GPIO pin set to be a digital input as shown in Figure 12-1 and enable the GPIO pin’s internal pull-up resistor.

Figure 12-1. Connecting a Switch to a GPIO Pin

Your software will probably also need to “debounce” the signal from the switch.

Figure 12-2. Switch Contacts Bouncing

Arduino

Here is an Arduino sketch that writes a message to the Serial Monitor whenever the switch is pressed. You can find this sketch (ch_12_switch) with the downloads for the book (see Recipe 7.2):

const int inputPin = 12;

void setup()
{
  pinMode(inputPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop()
{
  if (digitalRead(inputPin) == LOW)
  {
    Serial.println("Button Pressed!");
    while (digitalRead(inputPin) == LOW) {};
    delay(10);
  }
}

The internal pull-up resistor is turned on by specifying INPUT_PULLUP in the pinMode function.

Inside the loop the inputPin is continually read until it is pressed (becomes LOW). At this point, a message is displayed in the Serial Monitor and the while loop ensures there are no further triggerings until the switch has been released. The debouncing is achieved by the 10-millisecond delay that is triggered after each button press is detected. This gives the switch contacts time to settle before they are tested again.

You can try out the example by poking the pins of a tactile push switch between GND and pin 12 on the Arduino as shown in Figure 12-3.

Figure 12-3. Connecting a Tactile Push Switch to an Arduino

Raspberry Pi

The equivalent code for a Raspberry Pi is listed here (ch_12_switch.py):

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

input_pin = 23

GPIO.setup(input_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

try:         
try:         
    while True:
        if GPIO.input(input_pin) == False: 
            print("Button Pressed!")
            while GPIO.input(input_pin) == False: 
                time.sleep(0.01)         
finally:  
    print("Cleaning up")
    GPIO.cleanup()

The logic of this program follows exactly the same pattern as for the Arduino.

Having male GPIO pins on a Raspberry Pi makes it a little more difficult to attach a switch. One way to attach the switch is to use a microswitch pushed into a pair of female-to-female jumper wires (Figure 12-4) or use a Squid Button (Figure 12-5).

Figure 12-4. Connecting a Tactile Push Switch to a Raspberry Pi

Figure 12-5. Connecting a Squid Button to a Raspberry Pi

If you plan to use a number of push switches, then whether or not you are using a Squid Button, you can use the Squid library to simplify the process of debouncing. Download and install the Squid library from https://github.com/simonmonk/squid where you will also find installation instructions. Once installed, the test program can be simplified to the following example (ch_12_switch_squid.py):

from button import *
b = Button(23)

while True:
    if b.is_pressed(): 
        print("Button Pressed!")

Discussion

Microswitches (Figure 12-6) are push switches that are not designed to be directly pressed by a person, but rather have a lever attached and are used to sense physical movement such as when a linear actuator has reached its end position or as a safety interlock to make sure a microwave door is closed.

The microswitch is an SPDT device generally with closed, normally open, and common terminals (see Recipe 6.2).

Although switch using an Arduino or Raspberry Pi generally uses a pull-up resistor and switches to ground, you can also switch to the positive supply.

Figure 12-6. A Microswitch

To switch to the positive supply on an Arduino, you have to use an external pull-down resistor to prevent the input from floating because the Arduino does not have internal pull-down resistors on its GPIO pins (pin mode INPUT).

On the other hand, the Raspberry Pi does have internal pull-down resistors that can be enabled for a particular pin using the command:

GPIO.setup(input_pin, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

See Also

For information on the Squid Button, see https://www.monkmakes.com/squid_combo/.

Digital inputs are also described in Recipe 10.10 and Recipe 10.11.

GPIO pins are described in Recipe 10.7.

Problem

You want to use a rotating knob with your Arduino or Raspberry Pi.

Solution

You’ll want a type of rotary encoder called a quadrature encoder, which behaves like a pair of switches (Figure 12-7). The sequence in which they open and close as the rotary encoder’s shaft is turned determines the direction of rotation.

Figure 12-7. Schematic of a Rotary Encoder

A basic rotary encoder will have three pins. One for A, one for B, and a common pin. The rotary encoder shown in Figure 12-7 has two extra pins that are used for a push switch that is activated when the knob is pushed rather than turned.

When using a rotary encoder with a microcontroller or SBC, the common contact is connected to GND and the switch contacts connected to digital inputs with pull-up resistors enabled.

const int aPin = 6;
const int bPin = 7;

int x = 0;

void setup()
{
  pinMode(aPin, INPUT_PULLUP);  
  pinMode(bPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop()
{
  int change = getEncoderTurn();
  if (change != 0)
  {
    x += change;
    Serial.println(x);
  }
}

int getEncoderTurn()
{
  // return -1, 0, or +1
  static int oldA = 0;
  static int oldB = 0;
  int result = 0;
  int newA = digitalRead(aPin);
  int newB = digitalRead(bPin);
  if (newA != oldA || newB != oldB)
  {
    // something has changed
    if (oldA == 0 && newA == 1)
    {
      result = (oldB * 2 - 1);
    }
    else if (oldB == 0 && newB == 1)
    {
      result = -(oldA * 2 - 1);
    }
  }
  oldA = newA;
  oldB = newB;
  return result;
} 

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

input_A = 18
input_B = 23

GPIO.setup(input_A, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.setup(input_B, GPIO.IN, pull_up_down=GPIO.PUD_UP)

old_a = 1
old_b = 1

def get_encoder_turn():
    # return -1, 0, or +1
    global old_a, old_b
    result = 0
    new_a = GPIO.input(input_A)
    new_b = GPIO.input(input_B)
    if new_a != old_a or new_b != old_b :
        if old_a == 0 and new_a == 1 :
            result = (old_b * 2 - 1)
        elif old_b == 0 and new_b == 1 :
            result = -(old_a * 2 - 1)
    old_a, old_b = new_a, new_b
    time.sleep(0.001)
    return result

x = 0

while True:
    change = get_encoder_turn()
    if change != 0 :
        x = x + change
        print(x)

pi@raspberrypi ~ $ sudo python rotary_encoder.py
1
2
3
4
5
6
7
8
9
10
9
8
7
6
5
4

Discussion

Both the Arduino and Raspberry Pi versions of the software work in just the same way.

Figure 12-8 shows the sequence of pulses that you will get from the two contacts, A and B. You can see that the pattern repeats itself after four steps (hence the name quadrature encoder).

Figure 12-8. How Quadrature Encoders Work

When rotating clockwise (left to right in Figure 12-8), the sequence will be:

When rotating in the opposite direction, the sequence of phases will be reversed:

The Python program listed previously implements the algorithm for determining the rotation direction in the function get_encoder_turn. The function will return 0 (if there has been no movement), 1 for a rotation clockwise, or -1 for a rotation counterclockwise. It uses two global variables, old_a and old_b, to store the previous states of the switches A and B. By comparing them with the newly read values, it can determine (using a bit of clever logic) which direction the encoder is turning.

The sleep period of 1 millisecond is used to ensure that the next new sample does not occur too soon after the previous sample; otherwise, the transitions can give false readings (see “Contact Bounce”).

The test program should work reliably no matter how fast you twiddle the knob on the rotary encoder; however, try to avoid doing anything time consuming in the loop, or you may find that turn steps are missed.

See Also

To use a pot to sense rotation, use Recipe 12.3.

Problem

You want to use a sensor whose resistance varies with some property with an analog input. For example, a photoresistor or thermistor to sense light and temperature, respectively.

Solution

Put the resistive element in a voltage divider with a fixed-value resistor to convert the resistance measurement into a voltage as shown in Figure 12-9.

Figure 12-9. Using a Photoresistor with an Analog Input

Discussion

When it comes to choosing the value of a fixed resistor, the aim should be to try and maximize the range of output voltages that will be presented to the analog input. With an Arduino Uno’s analog input range of 0 to 5V you ideally want an analog input voltage of 0 to 5V. Because a voltage divider “divides” the input voltage, you are never going to get the full voltage range as an output unless you use a higher input voltage. This introduces the potential for overvoltage at the analog input and so generally it is better to accept a slightly narrowed range rather than risk damaging your board.

Let’s take as an example a photoresistor that has a minimum resistance of 1kΩ under bright light and a resistance that rises to 1MΩ in total darkness. Assuming that you are using a 5V Arduino:

If we pick a value for the fixed resistor R2 of about 10 times of the minimum value of the photoresistor R1 then R2 would have a value of 10kΩ, so at maximum brightness:

For a resistance of 10k for the photoresistor, the Vout would be 2.5V and when the photoresistor is in complete darkness (1MΩ), the Vout would fall to 0.05V.

This would give pretty good coverage of the whole range of the photoresistor’s readings.

The following Arduino sketch assumes that R1 is a photoresistor with 1kΩ light resistance and R2 is a fixed 10kΩ resistor. The sketch calculates the resistance of R1 and displays it in the Arduino IDE’s Serial Monitor every half-second. You can find the sketch here and with the downloads for the book (see Recipe 10.2). It is called ch_12_r_adc.

const int inputPin = A0;
const float r2 = 1000.0;
const float vin = 5.0;

void setup()
{
  pinMode(inputPin, INPUT);
  Serial.begin(9600);
}

void loop()
{
  int reading = analogRead(inputPin);
  float vout = reading / 204.6;
  float r1 = (r2 * (vin - vout)) / vout;
  Serial.print(r1); Serial.println(" Ohms");
  delay(500);
}

See Also

For an introduction to photoresistors, see Recipe 2.8.

For Arduino code to display such analog readings, see Recipe 10.12.

For information on voltage dividers, see Recipe 2.6.

To add analog inputs to a Raspberry Pi, see Recipe 12.4.

Problem

You want to measure an analog voltage with a Raspberry Pi.

Solution

The Raspberry Pi does not have analog inputs so connect an ADC IC to the Raspberry Pi.

Use the MCP3008 8-channel ADC chip. This chip actually has eight analog inputs, so you can connect up to eight sensors to one of these and interface to the chip using the Raspberry Pi SPI. Figure 12-10 shows the schematic for using an MCP3008 with a Raspberry Pi.

Figure 12-10. Adding Analog Inputs to a Raspberry Pi Using the MCP3008

Note that these analog inputs have a maximum input voltage of 3.3V. You will also need to make sure the SPI of the Raspberry Pi is enabled and the py-spidev Python library (included in new releases of Raspbian) is installed (Recipe 10.16).

Discussion

You can find the Raspberry Pi program to access and display the analog readings from analog channel 0 in the program ch_12_mcp3008.py (see Recipe 10.4):

import spidev, time

spi = spidev.SpiDev()
spi.open(0, 0)

def analog_read(channel):
    r = spi.xfer2([1, (8 + channel) << 4, 0])
    adc_out = ((r[1]&3) << 8) + r[2]
    return adc_out

while True:
    reading = analog_read(0)
    voltage = reading * 3.3 / 1024
    print("Reading=%d\tVoltage=%f" % (reading, voltage))
    time.sleep(1)

See Also

You can use this recipe with Recipe 12.7, Recipe 12.9, and Recipe 12.10 to read analog sensors with a Raspberry Pi.

You can use resistive sensors with a Raspberry Pi directly, without the need for an ADC IC (see Recipe 12.5).

Problem

You want to use a resistive sensor with a Raspberry Pi, without the need for an ADC IC.

Solution

See how long it takes for a capacitor to charge the resistive sensor and use this time to calculate the resistance of the sensor.

Figure 12-11 shows the schematic for measuring resistance using nothing more than two resistors and a capacitor.

A Python library to simply read values of resistance can be downloaded from GitHub where you will also find installation instructions and documentation.

Figure 12-11. Schematic for Measuring Resistance Using the Step-response Method

The following example shows how you can use this library to measure the value of the sensor resistance when the schematic of Figure 12-11 is wired up to a Raspberry Pi with pin A connected to GPIO 18 and pin B to GPIO 23. The program is located in the examples folder of the pi_analog library and is called resistance_meter.py:

from PiAnalog import *
import time

p = PiAnalog()

while True:
    print(p.read_resistance())
    time.sleep(1)

To test out the method, try using a few different values of fixed resistance. You can also use different values of C1 and R1 by specifying the value of capacitance in µF and R1 in Ω in the constructor. For example, for high resistance values you might want to use a smaller capacitor (10nF) to speed up the conversion. You could use the following code:

from PiAnalog import *
import time

p = PiAnalog(0.01, 1000)

while True:
    print(p.read_resistance())
    time.sleep(1)

Discussion

This technique relies on the ability of GPIO pins to switch between being an input and an output while the controlling program is running.

The basic steps for taking a measurement are as follows:

1. Make pin A an input. Make pin B an output and LOW and wait until the capacitor is discharged.
2. Make a note of the time. Make pin B an input and pin A a HIGH output. C1 will now start to charge.
3. When the voltage across C1 reaches about 1.35V it will stop being a LOW input and be measured as HIGH by the GPIO pin connected to B. The time taken for this to happen is a measurement of the resistance of the sensor and R1.

See Also

The Monk Makes Electronics Starter Kit for Raspberry Pi has several projects that use this step-response method to measure temperature and light.

Problem

You want to measure light intensity with a Raspberry Pi or Arduino.

Solution

If you have an Arduino or board with analog inputs then use Recipe 12.3. If your board does not have analog inputs (say a Raspberry Pi) then use Recipe 12.4 or Recipe 12.5.

Discussion

All the preceding methods will work just fine for determining if the sensor is illuminated more brightly in one reading than in another, but obtaining a measurement in common units of light measurement is a lot trickier. Questions that make getting the light measurement difficult are:

• What frequencies of light are you talking about? All light frequencies, or just the ones the photoresistor is sensitive to?
• Do you want to measure light from a particular direction? If so, the angle of view becomes important.

Even if you answer these questions, you get into problems with the linearity of the photoresistor and will likely need to manually calibrate your meter to account for manufacturing differences in the photoresistors.

In other words, despite its apparrant simplicity making a light meter that will give you a reading in Lux or Watts per square meter is a specialist task.

See Also

For a discussion on light measurement, see https://en.wikipedia.org/wiki/Lux.

Problem

You want to measure temperature with an Arduino or device with analog inputs.

Solution

Use a thermistor in a voltage-divider arrangement (see Recipe 12.3), calculate the resistance, and then use the Steinhart–Hart equation to calculate the temperature.

Figure 12-12 shows the schematic for connecting a thermistor to an Arduino analog input. You should use an NTC thermistor, which has a specified nominal resistance (its resistance at 25° C) and a value of B (sometimes called beta) that determines its resistance characteristics.

Figure 12-12. Connecting a Thermistor to an Analog Input

You can calculate the resistance of the thermistor R1 using the voltage-divider formula (see Recipe 2.6):

This can be rearranged to:

With a 5V Arduino and a fixed 1k resistor for R2, this gives:

The Steinhart–Hart equation states that:

where:

• t is the temperature in Kelvin (subtract 273.15 for degrees C)
• t0 is 25° C (the standard temperature for the thermistor’s base resistance)
• B is a parameter of the thermistor that determines its sensitivity
• R is the resistance of the thermistor at temperature t
• R0 is the resistance of the thermistor at 25° C

If you plug all this math into an Arduino sketch and wire up a thermistor as shown in Figure 12-12, you end up with the following code, which can also be found with the downloads for the book in the sketch ch_12_thermistor:

const int inputPin = A0;

// r2 is the bottom fixed resistor in the voltage divider
const float r2 = 1000.0;

// thermistor properties
const float B = 3800.0;
const float r0 = 1000.0;

// other constants
const float vin = 5.0;
const float t0k = 273.15;
const float t0 = t0k + 25;

void setup()
{
  pinMode(inputPin, INPUT);
  Serial.begin(9600);
}

void loop()
{
  int reading = analogRead(inputPin);
  float vout = reading / 204.6;
  float r = (r2 * (vin - vout)) / vout;
  float inv_t = 1.0/t0 + (1.0/B) * log(r/r0);
  float t = (1.0 / inv_t) - t0k;
  
  Serial.print(t); Serial.println(" deg C");
  delay(500);
}