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Personal Alarm

Personal Alarm

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 oscillating and the loudspeaker emits a loud alarm sound.The device can be reverse connected, i.e. the box can be placed in a pocket and the cord connected to the bag.

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 allow a very compact construction.

The loudspeaker can be any type, its dimensions are limited only by the box that will enclose it.
An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
Current consumption when the alarm is sounding is about 100mA

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