Description:
This is a simple SCR based burglar alarm circuit. Its features include
automatic Exit and Entry delays - together with a timed
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. The modular design means that you can Add Any Number Of Zones to the system. These
"Instant" zones may be triggered by normally-open as well as
normally-closed input devices.
Schematic Diagram:
Notes:
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.
Return to Alarm Circuits