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
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.
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.