This is a single zone alarm – with independently adjustable Exit, Entry and Siren Cut-Off timers. It will accommodate the usual types of normally-closed input devices – such as magnetic-reed contacts, foil tape and PIRs.
When the alarm is activated – the Siren will sound for up to 20-minutes. Then it will switch off – and remain off. The alarm will not re-activate.
If you wish – you can use a mains power supply. But the extremely low standby current makes battery power a realistic option. I’ve used a 9-volt supply in the drawing – but the circuit will work at anything from 5 to 15-volts. All you need do is select a Siren, Buzzer, and Relay to suit the voltage you’re using.
The alarm is easy to operate. Sw1 can be any type of two-way switch. If the Buzzer sounds when you switch the alarm on – the normally-closed loop is open. Switch off again – and check the building for open doors or windows. If the Buzzer does not sound – the loop is intact.
Depending on the setting of R3 – you have up to about a minute to leave the building. As you do so – the Buzzer will sound. When you close the door behind you – it should stop sounding. This confirms that the loop has been restored within the time allowed.
When you return and open the door – the Buzzer will sound. Depending on the setting of R4 – you have up to about a minute to switch the alarm off. If you fail to do so – the Siren will sound.
Depending on the setting of R5 – the Siren will sound for up to about 20-minutes. Then it will switch off – and remain off. Of course – you can stop the noise at any time by moving Sw1 to the “off” position.
For this type of device – really precise times are not necessary. If you like – you can replace the pots with fixed resistors. For example – 2M2 resistors should give you exit and entry delays of about 30-seconds – and a Siren cut-off time of about 10-minutes.
After the cut-off timer has switched the Siren off – the Buzzer will continue to sound. So when you return – if the Buzzer is sounding – you’ll know that the alarm has been activated.
The Support Material for this alarm includes a photograph of the prototype – a parts list – a detailed circuit description – a step-by-step guide to construction – and more.