What kind of circuit would a burglar alarm that causes
lights to open,
bell to ring, and a trap door to open look like?
Not different from a switch that causes these things to happen, though
a burglar alarm would probably include a latch to keep the bell ringing
after the alarm had been tripped.
A latch could simply be a relay, part of whose switched current is diverted
to supplement its control current. These days, it would more likely be
implemented with a TTL flip-flop.
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Update: June 2012