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Modified: 21:13, 26 October 2013

Circuit and PCB designed by Phil Townshend

This is a basic exit/entry alarm with a PIR module used as the trigger. As soon as the circuit is armed there is a delay before the circuit becomes active. This is needed to allow the user to exit but also to give the PIR module some time to settle down. They need a good 10 seconds or more.
After this time the alarm is fully armed. Any detection will activate a time delay before the siren actually sounds. This allows time for the alarm to be disarmed before activating the siren.
The siren will sound for a certain time then the unit resets, returning to its armed state.

Download Circuit Details and PCB, (PDF - 30K)


The PIR sensor used
(Maplin YD85G)

A view inside the case. The lid was made vacuum formed polystyrene and the base was 3mm acrylic.

When first switched on, both monostables are kept disabled by the reset circuit R1/C1. Any trigger inputs will be ignored and provides the exit delay time. This is calculated as 0.4xC1xR1 seconds.
Once the reset time has expired, any output from the PIR sensor is inverted by TR2 which is also used to drive the 'sense' LED. The inverted signal is used to trigger the entry monostable which provides the entry time, 1.1xC2xR2. The output will trigger the Alarm monostable on the low going edge (end of entry time). This trigger is converted to a brief pulse by C5/R5 otherwise the 555 would never switch off.

The Alarm monostable will remain on for 1.1xC3xR3 activating T1 Darlington transistor. This will drive up to 250mA without much heat generated.

The siren used was from Maplins (L64AC) but Rapid also do an equivalent version.

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