Make: Electronics (17 page)

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.

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.
  

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.