Page 430 - Hacking Roomba

Appendix B — Electrical Diagram Schematics 411

FIGURE B-7: Diodes: regular, LED, and photodiode

Other Components

The preceding sections describe the most common components you’ll run into when building
projects. Figure B-8 shows some other parts you may also see.

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FIGURE B-8: Battery, transistor, switch, inductor, and relay

The first symbol is for a battery. It has a positive and negative terminal, as you’d expect. A sin-
gle short-dash/long-dash pair originally indicated a single cell of a battery (approximately
1.5V). A stacked set of cells becomes a battery with the voltage indicated by the number of
cells. This has fallen out of practice and now a general battery symbol is often shown with a
voltage value given next to it.
The next symbol is a transistor. It’s used as either an amplifier or an electrically controlled
switch.
The middle symbol is for a switch or button. Sometimes switches will have multiple
contacts operated by a single push, represented as two switch symbols joined together with
a dotted line.
The next-to-last symbol is an inductor or coil. An inductor is sort of like a capacitor, but
instead of storing charge, it stores magnetic fields. The electromagnets you might have played
with in high school science classes are a special type of inductor.
The final symbol is for a relay. It’s a compound symbol made up of an inductor (the electro-
magnet) and a switch. When current flows through the electromagnet part of the relay, it cre-
ates a magnetic field to pull down the switch contacts. Relays are great for turning on and off
things that require more power than your circuit can provide, like motors.
With the above basic components you can read just about any circuit written before 1960.
There are a lot of fun circuits to build with that toolkit: alarm systems, telephones, audio
amplifiers, clocks, and even rudimentary computers. But even the most rudimentary computer
has hundreds of transistors. Imagine being required to draw (and read!) a hundred transistors.
Some manner of summarization was needed for these more complicated parts.