Figure 2-34. Charles E. Scribner invented the “jack-knife switch” to satisfy the switching needs of telephone systems in the late 1800s. Today’s audio jacks still work on the same basis.[1]
Audio connectors on guitars and amplifiers still work on the same principle, and when we speak of them as being “jacks,” the term dates back to Scribner’s invention. Switch contacts still exist inside a jack socket.
Today, of course, telephone switchboards have become as rare as telephone operators. First they were replaced with relays—electrically operated switches, which I’ll talk about later in this chapter. And then the relays were superceded by transistors, which made everything happen without any moving parts. Before the end of this chapter, you’ll be switching current using transistors.
[1] The photo on which this drawing is based first appeared in The History of the Telephone by Herbert Newton Casson in 1910 (Chicago: A. C. McClurg & Co.).
Introducing Schematics
In Figure 2-35, I’ve redrawn the circuit from Experiment 6 in a simplified style known as a “schematic.” From this point onward, I will be illustrating circuits with schematics, because they make circuits easier to understand. You just need to know a few symbols to interpret them.
Compare the schematic here with the drawing of the circuit in Figure 2-17. They both show exactly the same thing: Components, and connections between them. The gray rectangles are the switches, the zigzag thing is the resistor, and the symbol with two diagonal arrows is the LED.
The schematic LED symbol includes two arrows indicating that it emits light, because there are some kinds of diodes, which we’ll get to later, that don’t. The triangle inside the diode symbol always points from positive to negative.
Trace the path that electricity can take through the circuit and imagine the switches turning one way or the other. You should see clearly now why either switch will reverse the state of the LED from on to off or off to on.
This same circuit is used in houses where you have a switch at the bottom of a flight of stairs, and another one at the top, both controlling the same lightbulb. The wires in a house are much longer, and they snake around behind the walls, but because their connections are still the same, they could be represented with the same basic schematic. See Figure 2-36.
A schematic doesn’t tell you exactly where to put the components. It just tells you how to join them together. One problem: Different people use slightly different schematic symbols to mean the same thing. Check the upcoming section, “Fundamentals: Basic schematic symbols,” for the details.
Figure 2-35. This schematic shows the same circuit as in Figure 2-17 and makes it easier to see how the switches function.
Figure 2-36. The two-switch circuit shown in Figures 2-17 and 2-35 is often found in house wiring, especially where switches are located at the top and bottom of a flight of stairs. This sketch shows what you might find inside the walls. Wires are joined with “wire nuts” inside boxes that are hidden from everyday view.
Fundamentals
Basic schematic symbols
Schematic symbols are like words in a language: they have mutated over the years into a confusing range of variations. A simple on/off (single-pole, single-throw, or SPST) switch, for instance, can be represented by any of the symbols shown in Figure 2-37. They all mean exactly the same thing.
Figure 2-37. Variations on a theme: Just some of the different styles used to depict a single-pole, single-throw switch in schematic diagrams. The bottom version is the style used in this book.
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
