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. . . AND THE REST 405

Figure 31- 23 Three typical relay
types used in robotics: single- pole,
single- throw; single- pole, double-
throw; and double- pole, double-
SPST SPDT DPDT
relay relay relay throw.

SIZES OF RELAYS
Relays are “rated” by the amount of current that can be passed through their switch contacts.
The more current, the larger the relay. For the typical desktop robot application, the smallest
of relays are ideal: these are about the same size and shape as an integrated circuit.
Larger currents (loads) require bigger contact points, so if you’re operating a robot with a
huge motor, you’ll need something more. As the relay gets bigger, properly operating it
becomes much harder. For one thing, you need more drive current from your circuit to acti-
vate the relay. The standard setup of using a pea- sized signal transistor between your micro-
controller and relay won’t cut it.
High currents can cause arcing and other unpleasant artifacts whenever the relay is ener-
gized and deenergized. Careless use of a heavy- duty relay can even result in the electrical
contacts becoming welded together! These issues aren’t a problem as long as you stick with
small relays and small motors— the stuff of the average desktop robot. Operating the motors
of a big- brute combat robot is beyond the scope of this book, but there are guides and online
sites that cover this topic. Do a Web search and visit your local library.

RELIABILITY OF RELAYS

Relays are electromechanical, and, because of this, over time they could wear out and fail.
But, in truth, for the average small relay used on the average bot, you’d have to operate your
little robotic pet for several years before the relay would give out.
Most relays are rated for 100,000 or more switches. The trick is to make sure your circuit
doesn’t exceed the current rating of the relay contacts. The less complicated the construction
of the relay, the longer it’ll last (and the cheaper it is to replace). The lowly reed relay (so called
because the contact is a simple metallic reed inside a glass ampoule) can last for years and
years.

. . . And the Rest

In the first season of the old TV show Gilligan’s Island, the characters of The Professor and
Mary Ann didn’t even rate being mentioned in the opening theme song. Instead, they were
referred to as “and the rest.”
The components that follow are part of “and the rest” when building robot electronics.
These are important in their own right, to be sure, but are better left to examples in other
chapters that show them in actual use. They include:

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