How to build a CMOS based single zone alarm-EEWORLD

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How to build a CMOS based single zone alarm

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The circuit features automatic entry and exit delays - clock cut-outs - and system resets. It provides for both normally open and normally closed switches - and will suffice for the usual input devices (pressure pads, reed contacts, foil tape, PIRs and inertial sensors). CMOS single zone alarm www.elecfans.com

If the green LED is not lit - an open window or door etc. is checked once, the building is secure - and the green LED is illuminated - SW1 is moved to the "set" position. At this point - the red LED will illuminate - and the exit delay will begin. The length of the channel exit delay is then determined by the value of R12.

With the 220k resistor - you have about 30 to 40 seconds left. When you do this - the buzzer will sound. It should stop sounding when you close the door behind you. This indicates that the circuit has been successfully triggered to recover within the time allowed.

When you return to the building, open the door - the buzzer sounds - and the entry delay will begin. With the 470k resistor you have about 30 to 40 seconds to turn SW1 to the "off" position. If you fail to do this - the alarm will sound. Alarm

The length of time the sound is determined by R11. With a 4M7 resistor it will sound for about 15 to 20 minutes. Then the relay will be switched off and the alarm will try to reset itself. It is simple by using Q3 to switch itself "off".

If the trigger circuit is restored - the alarm is reset. If not - the attempt will fail - and the alarm will reactivate. It will continue to try to reset itself every 15 to 20 minutes - until the trigger circuit is restored - or the alarm is turned off.

All common triggering devices may be connected to the input terminals. These include inertial sensors. The sensitivity of the sensor is adjusted by R4. Setting it to the lowest value - will activate the alarm.

If you are not utilizing inertial sensor - replace R4 with 220k fixed resistor. If you are not using normally open switch - you can omit R1, C1 - but you have to accommodate extra connection between LED green and C2.

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