Make: Electronics

by Charles Platt

  Figure 2-38 shows double-pole, double-throw switches. A dotted line indicates a mechanical connection inside the switch, so that when you flip it, you affect both the poles simultaneously. Remember, the poles are electrically isolated from each other.

  Figure 2-38. More variations: some different styles for depicting a double-pole, double-throw switch. The style at bottom-right is used in this book.

  Once in a while, you may find a schematic in which switches seem to be scattered around, but the way they are identified (such as S1A, S1B, S1C, and so on) tells you that this is really all one switch with multiple poles.

  In the schematics in this book, I’ll place a gray rectangle behind each switch. This gray rectangle is not a standard symbol; you won’t find it in other books. I’m just including it to remind you that the parts inside are all contained in one package.

  A very important stylistic variation in schematics is the way they show whether wires make a connection with each other. Old schematics used to show a little semicircular bump in a wire if it crossed another wire without making a connection. Because modern circuit-drawing software doesn’t create this style of schematic, it is no longer often used. The modern style, which you are likely to find if you browse through schematics online, can be summarized like this:

  A dot joining two wires indicates an electrical connection.

  No dot indicates no connection.

  Fundamentals

  Basic schematic symbols (continued)

  The problem is that this is not very intuitive, especially when you’re just beginning to use schematics. When you see two wires crossing, you can easily imagine that they are making a connection, even though there’s no dot at the intersection. Therefore, in the interests of clarity, I’ve chosen to use the old “semicircular bump” style of schematic in this book (see Figure 2-39). It can be summarized like this:

  A dot joining two wires indicates an electrical connection.

  A bump in a wire that crosses another wire indicates no connection.

  In this book, you won’t find wires crossing each other without either a dot or a bump.

  Figure 2-39. In wiring schematics, a dot always indicates an electrical connection. However, the cross-shaped intersection of wires at top-right is considered bad style because if the dot is accidentally omitted or poorly printed, the intersection can be mistaken for the type shown at bottom left, in which the wires do not make a connection. All three of the configurations in the bottom row indicate no connection, the first example being the most common style, the center example being least common, and the third being the most old-fashioned—although for reasons of clarity, it is used in this book.

  In a battery-powered circuit, you may find a battery symbol, but more often you will find a little note indicating where positive voltage enters the system, while negative is indicated by a “ground” symbol. In fact there may be ground symbols all over the place. You have to remember that when you build a circuit, all the wires leading to grounds must actually be connected together, to the negative side of the voltage supply.

  The idea of the ground symbol dates back to the time when electronic gadgets were mounted on a metal chassis, which was connected to the negative side of the power supply. The ground symbol really meant “connect to the chassis.” Some variants in the ground symbol are shown in Figure 2-40.

  Figure 2-40. All of these symbols are used to mean the same thing: connect the wire to “ground” or “chassis” or the negative side of the power source. The far-right symbol is used in this book.

  In this book, we have color throughout, so I’ll show a red positive and blue negative to clarify where the power is connected, and I won’t use ground symbols. Once again, my purpose is to minimize the risk of misunderstandings, because I know how frustrating it is to build a circuit that doesn’t work.

  A big inconsistency in schematics is the way in which they show resistors. The traditional zigzag symbol has been abandoned in Europe. Instead they use a rectangle with a number inside indicating the number of ohms. See Figure 2-41. The Europeans also changed the way in which decimal points are represented: they omit them as much as possible, because in badly printed schematics, the little dots tend to get lost (or can be confused with dust and dirt). So, a 4.7KΩ resistor will be listed as 4K7, and a 1.2MΩ resistor will be 1M2. I like this notation, so I’m going to be using it myself, but I’ll be keeping the zigzag resistor symbol, which remains widely used in the United States.

  Figure 2-41. Two styles for depicting a 220Ω resistor. The upper version is traditional, and still used in the United States. The lower version is European.

  Fundamentals

  Basic schematic symbols (continued)

  Potentiometers suffer from the same inconsistent style between the United States and Europe, but either way, you’ll find an arrow showing where the wiper (usually, the center terminal) touches the resistance. See Figure 2-42. And sometimes LEDs are shown inside circles, and sometimes not. I prefer circles, myself. See Figure 2-46.

  Figure 2-42. Potentiometer symbols: the left is traditional and used in the United States, the right is European. In both cases the arrow indicates the wiper (usually the center terminal).

  Figure 2-43. Three ways of indicating a pushbutton switch.

  Figure 2-44. The battery symbol is usually shown without + and – symbols. I’ve added them for clarity.

  Figure 2-45. Symbol for an incandescent lightbulb.

  I’ll explore other symbol variants later in the book. Meanwhile, the most important things to remember are:

  The positions of components in a schematic are not important.

  The styles of symbols used in a schematic are not important.

  The connections between the components are extremely important.

  Figure 2-46. Sometimes an LED is shown with a circle around it; sometimes not. In this book, I will include the circle. The arrows indicate emitted light.

  Fundamentals

  Basic schematic symbols (continued)

  For example, the three LED circuits that I have included in Figure 2-47 show components in different positions, using different symbols, but all three circuits function exactly the same way, because their connections are the same. In fact, they all depict the circuit that you built in Experiment 4, shown in Figure 1-50.

  Often the symbols in a schematic are placed so that the circuit is most intuitively easy to understand, regardless of how you may build it with actual components. Compare the example in Figure 2-48, showing the two DPDT switches, with the version shown back in Figure 2-35. The previous one looked more like your bench-top version of it, but Figure 2-48 shows the flow of electricity more clearly.

  Figure 2-47. These three schematics all depict the same basic circuit. It’s the circuit that you built with the potentiometer in Experiment 4.

  In many schematics, the positive side of the power supply is shown at the top of the diagram, and negative or ground at the bottom. Many people also tend to draw a schematic with an input (such as an audio input, in an amplifier circuit) at the left side, and the output at the right. So, “positive voltage” flows from top to bottom while a signal tends to pass from left to right.

  When I was planning this book, initially I drew the schematics to conform with this top-to-bottom, left-to-right convention, but as I started building and testing the circuits, I changed my mind. We use a device known as a “breadboard” to create circuits, and its internal connections require us to lay out components very differently from a typical schematic. When you’re starting to learn electronics, it’s very confusing to try to rearrange components from a schematic in the configuration that you need for a breadboard.

  Therefore, throughout this book, you�
��ll find that I have drawn the schematics to imitate the way you’ll wire them on a breadboard. I believe the advantages of doing things this way outweigh the disadvantages of being a little different from the schematic styles that are used elsewhere.

  Figure 2-48. This schematic is just another, clearer, simpler way of showing the circuit that appeared in Figure 2-35.

  Experiment 7: Relay-Driven LEDs

  You will need:

  AC adapter, wire cutters and strippers.

  DPDT relay. Quantity: 2.

  LEDs. Quantity: 2.

  Resistor, 680Ω approx. Quantity: 1.

  Pushbutton, SPST. Quantity: 1.

  Hookup wire, 22 gauge, or patch cords.

  Alligator clips. Quantity: 8.

  Utility knife.

  The next step in our exploration of switching is to use a remote-controlled switch. By “remote-controlled,” I mean one to which you can send a signal to turn it on or off. This kind of switch is known as a relay, because it relays an instruction from one part of a circuit to another. Often a relay is controlled by a low voltage or small current, and switches a larger voltage or higher current.

  This setup can be cost-effective. When you start your car, for instance, a relatively small, cheap switch sends a small signal down a relatively long, thin, inexpensive piece of wire, to a relay that is near the starter motor. The relay activates the motor through a shorter, thicker, more expensive piece of wire, capable of carrying as much as 100 amps.

  Similarly, when you raise the lid on an a top-loading washing machine during its spin cycle, you close a lightweight switch that sends a small signal down a thin wire to a relay. The relay handles the bigger task of switching off the large motor spinning the drum full of wet clothes.

  Before you begin this experiment, you need to upgrade your power supply. We’re not going to use batteries anymore, because most relays require more than 6 volts, and in any case you should have a power supply that can give you a variety of voltages on demand. The simplest way to achieve this is by using an AC adapter.

  First you’ll set up the AC adapter. After you have it running, you’ll use it to power the relay. Initially the relay will just switch between two LEDs, but then you’ll modify the circuit to make the LEDs flash automatically. Finally you’ll rebuild the circuit on a breadboard, and say goodbye to alligator clips, for most of the time at least.

  Preparing Your AC Adapter

  An AC adapter plugs into the wall and converts the high-voltage AC supply in your home into a safe, low DC voltage for electronic devices. Any charger that you use with your cell phone, or iPod, or laptop computer is a special-purpose AC adapter that delivers only one voltage via a specific type of plug. I’ve asked you to buy a general-purpose adapter that delivers many different voltages, and we’re going to begin by getting rid of its plug.

  1. It’s important to make sure that your AC adapter is not plugged into the wall!

  2. Chop off the little plug at the end of its wire. See Figure 2-49.

  Figure 2-49. Preparing an AC adapter. First, cut off the little low-voltage plug and throw it away.

  3. Use a box cutter or utility knife or scissors to make a half-inch cut between the two conductors, and then pull the conductors apart a couple of inches.

  4. Use wire cutters to trim one of the conductors shorter than the other, so that after you strip away a little of the insulation, the exposed copper wires cannot easily touch each other. This is a precaution against short-circuiting your AC adapter and burning it out.

  5. Strip the two conductors using your wire strippers. Twist the copper strands between finger and thumb so that there are no loose strands sticking out. See Figure 2-50.

  Figure 2-50. Second, strip the wires, making one shorter than the other to reduce the risk of them touching. Color one of the adapter wires red with a marker, to identify it as the positive one.

  6. Make sure that the two wires are not touching each other, and plug your AC adapter into a wall outlet. Set your meter to DC volts and apply the meter probes to the wires from the adapter. If the voltage is preceded with a minus sign, you have the probes the wrong way around. Reverse them and the minus sign should go away. This tells you which wire is positive.

  7. Mark the positive wire from the adapter. If the wire has white insulation, you can mark it with a red marker. If it has black insulation, you can tag it with a label. The positive wire will remain positive regardless of which way up you plug the AC adapter into a wall outlet.

  The Relay

  The type of relay that I want you to use has little spiky legs on the bottom, in a standardized layout. If you buy some other kind of relay, you will have to figure out for yourself which pins are connected to the coil inside, which pins go to the poles of the switch inside it, and which go to the normally closed and normally open contacts. You can check the manufacturer’s data sheet for this purpose, but I strongly suggest you use one of the relays mentioned in the shopping list, so that you can follow the instructions here more easily.

  I asked you to buy two relays so that you can use one for investigational purposes—meaning that you can break it open and take a look inside. If you do this very, very carefully, the relay should still be usable afterward. If not, well, you still have a spare.

  The easiest way to open the relay is with a box cutter or utility knife. Figures 2-52, 2-53, and 2-54 show the technique. Shave the edges of the plastic shell containing the relay, beveling them until you see just a hair-thin opening. Don’t go any farther; the parts inside are very, very close to its housing. Now pop the top off. You can use needle-nosed pliers to nibble the rest of the shell away. Read the following section, “Fundamentals: Inside a relay,” and then apply power to the relay to see how it works.

  Figure 2-51. This is one way that the parts inside a relay can be arranged. The coil, A, generates a magnetic attraction pulling lever B downward. A plastic extension, C, pushes outward against flexible metal strips and moves the poles of the relay, D, between the contacts.

  Figure 2-52. To look inside a sealed relay, shave the top edges of the plastic package with a utility knife til you open a thin crack.

  Figure 2-54. If you are really, really careful, the relay should still work after you open it.

  Figure 2-53. Insert the blade of your knife to pry open the top, then repeat the procedure for the sides.

  Figure 2-55.

  Figure 2-56. Patience is essential when carving the edges of a relay package in order to open it. Faster methods such as a tomahawk or a flamethrower will satisfy the emotional needs of those with a short attention span, but results may be unpredictable.

  Figure 2-57. Four assorted 12-volt relays, shown with and without their packages. The automotive relay (far left) is the simplest and easiest to understand, because it is designed without much concern for the size of the package. Smaller relays are more ingeniously designed, more complex, and more difficult to figure out. Usually, but not always, a smaller relay is designed to switch less current than a larger one.

  Fundamentals

  Inside a relay

  A relay contains a coil of wire wrapped around an iron core. When electricity runs through the coil, the iron core exerts a magnetic force, which pulls a lever, which pushes or pulls a springy strip of metal, closing two contacts. So as long as electricity runs through the coil, the relay is “energized” and its contacts remain closed.

  When the power stops passing through the coil, the relay lets go and the springy strip of metal snaps back into its original position, opening the contacts. (The exception to this rule is a latching relay, which requires a second pulse through a separate coil to flip it back to its original position; but we won’t be using latching relays until later in the book.)

  Relays are categorized like switches. Thus, you have
SPST relays, DPST, SPDT, and so on.

  Compare the schematics in Figure 2-58 with the schematics of switches in Figure 2-38. The main difference is that the relay has a coil that activates the switch. The switch is shown in its “relaxed” mode, when no power flows through the coil.

  Figure 2-58. Various ways to show a relay in a schematic. Top left: SPST. Top right and bottom left: SPDT. Bottom right: DPDT. The styles at bottom-left and bottom-right will be used in this book.

  The contacts are shown as little triangles. When there are two poles instead of one, the coil activates both switches simultaneously.

  Most relays are nonpolarized, meaning that you can run electricity through the coil in either direction, and the relay doesn’t care. You should check the data sheet to make sure, though. Some relay coils work on AC voltage, but almost all low-voltage relays use direct current—a steady flow of electricity, such as you would get from a battery. We’ll be using DC relays in this book.

  Relays suffer from the same limitations as switches: their contacts will be eroded by sparking if you try to switch too much voltage. It’s not worth saving a few dollars by using a relay that is rated for less current or voltage than your application requires. The relay will fail you when you need it most, and may be inconvenient to replace.

  Because there are so many different types of relays, read the specifications carefully before you buy one. Look for these basics:

  Coil voltage

  The voltage that the relay is supposed to receive when you energize it.