Door
Alarms work
on one of two types of systems: a closed circuit or an open circuit
system. Effectively, in a closed circuit system, the closed door
completes the circuit allowing electricity to flow through it. When
the circuit is broken, an alarm is triggered. Conversely, in an
open circuit system, when the door is opened, the open circuit closes
also triggering an alarm. This same idea is also used in window
alarms.
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