Page 425 - Hacking Roomba

406 Appendix B — Electrical Diagram Schematics

Conventions

When drawing schematics you should follow a few conventions. These aren’t hard rules and
will be broken in the interest of making a schematic easier to understand.

Signal flow goes from top-left to bottom-right.
Positive voltages are on top; negative voltages are at the bottom.
Each component is labeled with the part’s value, for example, 220 for 220 ohms, or 7805
for the 7805 voltage regulator.
Each component is labeled with a unique identifier to distinguish it from other parts
of the same type, for example R2 (for resistor #2) or IC4 (for integrated circuit #4).
Sometimes integrated circuit chips (ICs) are labeled with identifiers starting with U
(for U #4) instead of IC.
No diagonal wires, only up-down and left-right.
Minimize the crossing of wires.

To read a schematic, note the above conventions and just dive in. You’ll often find little sub-
circuits you understand, like the power supply and LED light circuits that keep popping up in
this book. When you understand a sub-circuit, you can focus on the other parts of the circuit
you don’t understand. Most circuits are built by connecting sub-circuits and they should be
fairly intuitive when you understand the basics presented in this appendix.
If you come across a symbol you don’t recognize, don’t worry. It’s probably a new symbol cre-
ated with a special purpose (or perhaps a symbol from a different idiom, as mentioned earlier in
this appendix). Usually you can tell by context and similarity with previous symbols as to what
it does. Otherwise there will be some explanatory text to go with the circuit diagram.
Similarly, when you’re drawing a schematic and you don’t have a symbol for a part, make one
up based on what you think it should look like. Then, if it’s not obvious, label your new part
so others understand. It is generally a good idea to familiarize yourself with some of the com-
monly used idioms. That way, you can make sure that you don’t create a symbol that already
exists, and you can also make sure that you make a symbol that will clearly imply what type of
component it is, when possible. For example, a resistor should look like a resistor. By creating
your symbols this way, you can make it easier for other people to understand your schematics
later.

It’s All About the Connections

Schematics describe the connectivity between components. They are wiring diagrams. It’s not
important where the parts are placed on the page but rather how the parts are connected to
each other. There is no one right way of drawing a schematic; in fact, there’s an infinite number
of ways. You can see throughout the book that I predominantly use the U.S. convention as
drawn by the Eagle schematic capture software. Sometimes other conventions are used to
match the style in which a circuit is normally seen. For example, the Basic Stamp circuits use a