Circuit : Ron J
This is a simple single-zone burglar alarm circuit. Its features include automatic Exit and Entry delays. 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.
It has an extremely small standby current - making it ideal for battery-powered operation. I've used a 9-volt battery in the diagram - but the circuit will work at anything from 5 to 15-volts. Just choose a relay and Siren suitable for the voltage you want to use.
It's easy to use. To set the alarm move SW1 to the "set" position. You now have about 10 to 15 seconds to leave the building. When you return and open the door - the green LED will light. You then have about 10 to 15 seconds to move SW1 to the "off" position. If you fail to do so - the relay will energize and the Siren will sound. Of course - you can turn the Siren off at any time by moving SW1 to the "off" position.
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 R5 & R6 you can adjust the Exit and Entry times to suit your requirements. Increasing the values increases the time - and vice-versa.
The Support Material
for this alarm includes a complete circuit description - a parts list - a step-by-step guide to construction - and more.