@▷ SCR Alarm Circuit Schematic Diagram | Diagram for Schematic

SCR Alarm Circuit Schematic Diagram

This is a simple SCR based burglar alarm circuit. Its features include automatic Exit and Entry delays – together with a timed Bell cut-off and Reset. It’s designed to be used with the usual types of normally-closed input devices such as – magnetic-reed contacts – micro switches – foil tape – and PIRs.


The basic alarm has a single zone with “Exit/Entry” delays. This will be adequate in many situations. However – larger buildings are better divided into zones.  These “Instant” zones may be triggered by normally-open as well as normally-closed input devices.

It’s easy to use. When you switch on the alarm – you have about 30 seconds to leave the building. When you return and open the door – the Buzzer will sound. You have about 30 seconds to switch off the alarm. If you fail to do so – the Siren will sound.

After about 10 minutes – the alarm will attempt to reset itself. If the trigger circuit has been restored – the attempt will be successful. But – if the loop is still open – the attempt will fail – and the alarm will re-activate. Of course – you can turn the Siren off at any time by switching off the alarm.

A conventional bell uses up to about 400ma. An electronic siren generally uses less. If you intend to draw a heavier current from either the Buzzer or Siren terminals – the SCR in question will need to be bolted to a metal heatsink – and the relay contacts may need upgrading.

If you don’t want the timed “cut-off and reset” feature – leave out D5, D6, R11, R12, Q3, Q4, C6 and the Relay.

source : zen’s circuit collections