This is a simple transistor-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.

transistor-alarm

The basic alarm has an “Exit/Entry” zone and an “Instant” zone. This will be adequate in many situations. However – larger buildings are best divided into a number of smaller zones. They are “Instant Zones” – and may be triggered by both normally-open and normally-closed input devices.

It’s easy to use. Make sure that the green LED is lighting – then switch the alarm on using Sw1. 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.

While at least one of the trigger switches remains open – the Siren will continue to sound. However – if the trigger circuits have been restored – the alarm will reset itself after about 10 minutes. Of course – you can turn the Siren off at any time by switching off the alarm.

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 – to some degree – by altering the values of R2, R7 & R9 you can adjust the Exit, Entry and Reset times to suit your requirements. Increasing the values increases the time – and vice-versa.

If you don’t want the “Instant” zone – leave out D7, D8, D9, R12, R13, C8 and the Green LED.

source : zen’s circuit collection

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