This is the Simple Miniature Loop Alarm Circuit Diagram. The 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.
Simple Miniature Loop Alarm Circuit Diagram
Notes
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. Link
Author: Tomaz Lazar - Ljubljana, Slovenia
