Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [14]
Either the switched is closed or it's open. Either current flows or it doesn't. Either the lightbulb lights up or it doesn't. Like the binary codes invented by Morse and Braille, this simple flashlight is either on or off. There's no in-between. This similarity between binary codes and simple electrical circuits is going to prove very useful in the chapters ahead.
Chapter 5. Seeing Around Corners
You're twelve years old. One horrible day your best friend's family moves to another town. You speak to your friend on the telephone now and then, but telephone conversations just aren't the same as those late-night sessions with the flashlights blinking out Morse code. Your second-best friend, who lives in the house next door to yours, eventually becomes your new best friend. It's time to teach your new best friend some Morse code and get the late-night flashlights blinking again.
The problem is, your new best friend's bedroom window doesn't face your bedroom window. The houses are side by side, but the bedroom windows face the same direction. Unless you figure out a way to rig up a few mirrors outside, the flashlights are now inadequate for after-dark communication.
Or are they?
Maybe you have learned something about electricity by this time, so you decide to make your own flashlights out of batteries, lightbulbs, switches, and wires. In the first experiment, you wire up the batteries and switch in your bedroom. Two wires go out your window, across a fence, and into your friend's bedroom, where they're connected to a lightbulb:
Although I'm showing only one battery, you might actually be using two. In this and future diagrams, this will be an off (or open) switch:
and this will be the switch when it's on (or closed):
The flashlight in this chapter works the same way as the one illustrated in the previous chapter, although the wires connecting the components for this chapter's flashlight are a bit longer. When you close the switch at your end, the light goes on at your friend's end:
Now you can send messages using Morse code.
Once you have one flashlight working, you can wire another long-distance flashlight so that your friend can send messages to you:
Congratulations! You have just rigged up a bidirectional telegraph system. You'll notice that these are two identical circuits that are entirely independent of and unconnected to each other. In theory, you can be sending a message to your friend while your friend is sending a message to you (although it might be hard for your brain to read and send messages at the same time).
You also might be clever enough to discover that you can reduce your wire requirements by 25 percent by wiring the configuration this way:
Notice that the negative terminals of the two batteries are now connected. The two circular circuits (battery to switch to bulb to battery) still operate independently, even though they're now joined like Siamese twins.
This connection is called a common. In this circuit the common extends from the point where the leftmost lightbulb and battery are connected to the point where the rightmost lightbulb and battery are connected. These connections are indicated by dots.
Let's take a closer look to assure ourselves that nothing funny is going on. First, when you depress the switch on your side, the bulb in your friend's house lights up. The red wires show the flow of electricity in the circuit:
No electricity flows in the other part of the circuit because there's no place for the electrons to go to complete a circuit.
When you're not sending but your friend is sending, the switch in your friend's house controls the lightbulb in your house. Once again, the red wires show how electricity flows in the circuit:
When you and your friend both try to send at the same time, sometimes both