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A few months ago, I decided to build a
compact, yet effective alarm. My demands were:- simple
construction, reliable operation, very small power
consumption, and, most of all, small size. I started
with CMOS logic gates, but was soon forced to abandon
the concept after a few unsuccessful (and far too
complicated) attempts. Then I suddenly realized that a
simple transistor switch might do the job and I was
right.
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Circuits
As you can clearly see from the
schematics, the circuit is utterly primitive and
consists of two identical transistor switches. Each has
its own alarm LED and they're coupled to a neat 82dB
buzzer. The two 1N4148 diodes are used to prevent a
signal from one sensor from triggering both LEDs. The
sensors used are either wire loops or normally closed
reed switches or even a combination of both. You could,
for example, tie a wire loop to your suitcase and place
a reed switch to the door of your hotel room.
Since this little alarm is intended to
be kept in arms reach at all times, there aren't any
provisions for automatic shutdown after a certain period
of time. The buzzer will sound until you turn the whole
circuit off or connect the wire loop back to the
jumpers. The same goes for the two LEDs, each indicating
its own zone.
Construction is not critical and there
aren't any traps for the novice. The two 100n capacitors
aren't really necessary, I just included them to make
sure that there is no noise interference coming from the
long wire loops. For transistors, you can use any NPN
general-purpose audio amplifiers/switches (BC
107/108/109, BC 237/238, 2N2222, 2N3904...). Assemble
the circuit on perf board. Together with the buzzer and
a 9V battery, it should easily fit in a pocket-sized
plastic box smaller than a pack of cigarettes. A fresh
battery should suffice for weeks of continuous
operation.
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