Mini Alarm

Mini Alarm

Circuit Description

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. 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.

Circuit Diagram:



  • R1 = 330K
  • R2 = 100R
  • C1 = 10nF-63V
  • C2 = 100uF-25V
  • Q1 = BC547
  • Q2 = BC327
  • B1 = 3V Battery or Two AA Cells in Series
  • SW1 = Read Switch & Small Magnet
  • SPKR = 8R Loudspeaker (See Notes)


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.
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 plugs having
its internal leads shorted. The Jack plug will be connected to the tiny
cord etc.
Do not supply this circuit at voltages exceeding 4.5V: it will not work
and Q2 could be damaged. In any case a 3V supply is the best compromise.

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