Electronics All-in-One For Dummies. Doug Lowe
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that represent every painful uphill step I’ve carried my 50-pound backpack up.
Without maps, we’d be lost. We’d never get to our destinations because we wouldn’t know where the roads are. Think of all the sights we’d miss along the way!
Electronics has its own form of maps. They’re called schematic diagrams. They show how all the different parts that make up an electronic circuit are connected.
Just as maps use symbols to represent features like cities, bridges, and railroads, schematic diagrams use special symbols to represent the different parts of a circuit, such as batteries, resistors, and diodes, and like maps, schematic diagrams have conventions that almost always are used. For example, positive voltages are almost always shown at the top of a schematic diagram, just as north is almost always shown at the top of a map.
In this chapter, you learn about the symbols used in schematic diagrams and the conventions used to draw them.
Introducing a Simple Schematic Diagram
I’ve read a lot of computer programming books in my day, and I’ve written a few too. In a computer programming book, the first complete computer program usually shown is a program called Hello World, a program that simply displays the text “Hello World!” on a screen, and then quits. It’s pretty much the simplest possible computer program that can be written. It doesn’t do anything useful, but it’s a great starting point for learning how to write computer programs.
Figure 5-1 shows a schematic diagram that is the electronic equivalent of the Hello World program. This diagram is about the simplest schematic diagram possible that actually does something: It lights a lamp, thus announcing to the world that a circuit is indeed working.
FIGURE 5-1: A simple schematic of a circuit that lights a lamp.
This diagram contains two symbols representing the two components in the circuit: a 1.5 V battery and an incandescent lamp. The lines that connect the two components represent conductors, which could be actual wires or traces of copper in a printed circuit board.
In the circuit depicted in this schematic, the positive side of the battery is connected to one lead from the lamp, and the other lead from the lamp is connected to the negative side of the battery. Once these connections are made, current will flow from the battery to the lamp, through the lamp’s filament to produce light, and then back to the battery.
Schematic diagrams always depict conventional current flow, which, as you learn in Chapter 2 of this minibook, means that current flows from positive to negative. Thus, the current flows from the positive terminal of the battery through the lamp and then back to the negative terminal of the battery.
As it passes through the lamp, the resistance of the lamp’s filament causes the current to heat the filament, which in turn causes the filament to emit visible light.
Laying Out a Circuit
For example, the circuit shown in Figure 5-1 shows the battery on the left side of the circuit and the lamp on the right. It also shows the battery oriented so that the positive terminal is at the top and the negative terminal is at the bottom. However, that doesn’t mean the circuit would actually have to be built that way. If you want, you could put the lamp on the left and the battery on the right, or you could put the battery at the top and the lamp on the bottom.
The physical arrangement of the circuit doesn’t matter as long as the component connections remain the same as shown in the schematic. Thus, in this example, no matter how you physically arrange the components, you must connect the positive terminal of the battery to one lead of the lamp and the negative terminal to the other lead.
Because there are only two components and two conductors in the circuit shown in Figure 5-1, it would be pretty hard to mess up the connections. However, in a more complicated circuit with perhaps dozens of components and dozens of connections, laying out the circuit and making sure that all the connections exactly match the connections indicated in the schematic can be a challenge. Each connection must be checked carefully to make sure it’s correct.
To Connect or Not to Connect
One of the goals when laying out a schematic circuit diagram is to keep the diagram as simple as possible. However, the lines in all but the simplest of schematic diagrams will at some places need to cross over each other. When they do, it’s vital that you can tell whether the lines that cross represent actual connections (also called junctions) between the conductors or whether the lines cross over each other but don’t actually connect.
Unfortunately, there isn’t one clear and universally used standard that dictates how to indicate whether crossed lines represent a junction. Figure 5-2 shows some of the ways for showing crossed wires with or without junctions.
FIGURE 5-2: Wires that cross may or may not actually be connected.
The three examples on the left side of Figure 5-2 show how junctions are indicated. The example at the top left shows the most common way to indicate a junction: by placing a conspicuous dot at the point where the wires cross. Any time you see a dot where two lines intersect, you know that the two lines form a junction.
In the two junction styles shown in the middle-left and bottom-left examples in Figure 5-2, the vertical lines are angled to avoid coming together at the same spot on the horizontal line. With or without the dot, junctions are clearly indicated in both of these examples.
The three examples on the right side of Figure 5-2 show how lines that cross but don’t connect to form junctions are most commonly shown. In the top two examples, one line “hops” over the other, and one of the lines is broken at the spot where it crosses the other.
The example in the bottom-right corner of Figure 5-2 is a bit ambiguous. Here, the lines cross each other. However, there’s no hop or break to indicate that no junction is present, nor is there a dot to indicate that a junction should be present. So is there a junction here or not? The answer is, in most cases, no. You can usually assume that a junction is not present when lines cross but there’s no dot. However, you should examine the rest of the diagram to make sure. If you find other places in the diagram where nonjunctions are indicated by a hop or a break, the crossed lines without the hop or break may indeed indicate a junction.