Magnetic Reed Switch Alarm

This circuit is made to be an alarm, both for home and handbags. If it is installed for home it will placed on door or windows, and if it is installed for bags or handbags it will placed in the bag. This circuit consists of a small magnet and a reed switch. If the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm this circuit suitable for use as an anti-theft alarm.

Magnetic Reed Switch Alarm

This circuit uses A complementary transistor-pair which is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply enable a very compact construction.

Notes :

  • The loudspeaker’s dimensions are limited only by the box that contain 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.
  • Do not supply this circuit with voltages exceeding 4.5V: it will not work and Q2 could be damaged.

[Circuit diagram source:]