Field-strength meters

Posted on Feb 4, 2014

This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts osc

Field-strength meters
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illating and the loudspeaker emits a loud alarm sound. This device can be very useful in signalling the opening of a door or window: place the box on the frame and the magnet on the movable part in a way that magnet and reed switch are very close when the door or window is closed. A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply enable a very compact construction. If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3. 5mm mono Jack socket and the magnet by a 3. 5mm. mono Jack plug with its internal leads shorted. The Jack plug will be connected with the tiny cord etc.

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