A Simpler Cmos Single Zone Alarm

Description

This is a simple single-zone burglar alarm circuit. Its features include automatic Exit and Entry delays and a timed Bell/Siren Cut-Off. 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. But it can be Easily Modified to accept normally-open triggering devices - such as pressure mats.

Schematic Diagram

It's easy to use. First check that the building is secure and that the green LED is lit. Then move SW1 to the "set" position. The red LED will light. You now have about 30 seconds to leave the building. When you return and open the door - the Buzzer will sound. You then have about 30 seconds to move SW1 to the "off" position. If you fail to do so - the relay will energize and the Siren will sound.

The maximum length of time the Siren will sound - is not fixed. The 15-minute cut-off timer only starts to run when the normally-closed loop has been restored. While the loop remains open - the Siren will continue to sound.

If you want your Siren to switch off after 15 minutes - all that's required is a Simple Modification to the trigger circuit. Then the cut-off timer will start to run the moment the alarm is activated. And the Siren will stop after 15-minutes - regardless of the state of the loop.

Because of manufacturing tolerances - the precise length of any delay depends on the characteristics of the actual components you've used in your circuit. But by altering the values of R3, R6 & R9 you can adjust the Exit, Entry and Bell Cut-Off times to suit your requirements. Increasing the values increases the time - and vice-versa.

Stripboard Layout

Parts list