How to build a scalable transistor-based burglar alarm

This is a simple transistor burglar alarm circuit. Its features include automatic entry and exit delays - bell timer cut-off reset. It is designed to be used with normally closed input device types such as - reed contacts - micro switches - foil tapes - and PIRs.

The basic alarm has an "exit/entry" zone and an "instant" zone. This will be sufficient in many cases. However - large buildings are best divided into a number of smaller zones. The modular design of the alarm means you can add as many zones to the system as you like. They are "instant zones" - and can be triggered by both normally open and normally closed input devices

Schematic diagram

It's easy to use. Make sure the green LED is illuminated - then use switch SW1 to turn 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 turn off the alarm. If you fail to do so - the Siren will sound.

While at least one of the switch triggers is still open - the Siren will continue to sound. However - if the trigger circuit is restored - the alarm will reset itself after about 10 minutes. Of course - you can always turn off the alarm by turning the siren off.

Due to manufacturing tolerances - the exact length of any delay depends on the characteristics of the components in the actual circuit you are using. However - to some extent - by changing the values ​​of R2, R7 and R9, you can adjust the exit, entry and reset times to meet your requirements. Increasing the values ​​increases the time - and vice versa.

If you do not want "instant" zone - omit the D7, D8 and D9, R12, R13 types, C8 and green LED.

Stripboard Layout

Materials List