3 Zone Duress Alarm
Circuit : Andy Collinson
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Description:
This is a 3 zone alarm for use in high risk areas or possible duress situations. Typical examples being banking or betting booths. The alarm consists of 3 normally open push button switches which can be hidden or in plain view. Once pressed the alarm will latch and a LED will light showing which zone has pressed the alarm.



Circuit Notes
The input trigger are switches S2, S3 and S4, zone 1, zone 2 and zone 3 respectively. These are push to make normally open press switches and can be concealed under a desk for example or in plain view. The input switches will be located in the remote location. S1 is the reset switch and is located on the control unit. D1 is the alarm LED for zone 1, D2 the LED for zone 2 and D3 the LED for zone 3.

The alarm itself uses two CMOS 4011 IC's. Each IC contains quad 2 input NAND gates. These gates are wired as SR flip flops, two gates per zone. U1 uses all four gates and IC U2 uses just two gates, all unused inputs should be tied to ground. The small black numbers on the schematic refer to the IC pinout, you also need to make sure that each IC has pin 7 wired to ground and pin 14 to Vcc; (these connections are not shown for clarity).

IC Pinout for the CMOS 4011 can be viewed in the Practical section, click here.

Power is derived via an external power supply and must be 14 Volts or higher. F1 is the input fuse, the supply is regulated by U3 a 78M12 regulator. C2 helps filter regulator output. At switch on, C3 quickly charges via the parallel combination of R2, R7 and R11 providing a reset pulse. All LED's and the piezo sounder, BZ1 will be off.

If any zone is triggered, the appropriate LED will light and the sounder will continue to bleep until reset by pressing S1. Q1, Q2 and Q3 amplify the output current from the NAND gates to send about 30mA through each LED. I used 10mm LED's in the prototype but lo-current type LED's (2mA) could easily be used and transistors eliminated if desired.

D4, D5 and D6 form a 3 input OR gate allowing the operation of the piezo sounder from any input zone.

When testing the prototype an unusual fault occurred. Pressing S2 or S3 would active the sounder and both D1 and D2. My immediate thought was some type of cable contact, however testing with a multimeter revealed no partial shortcircuits whatsoever. Cabling was about 20 metres total, so I wondered if cable capacitance was a factor. Small 100n filter capacitors were tried across the SR gates input, but to no avail. Eventually I found the cure and that was to include R15 a 220 ohm resistor into the supply line. CMOS can drive you crazy! It was either charge storage on the cabling of perhaps just the fact that I had used CMOS; however the inclusion of a 220 ohm resistor cured all this and allows proper operation. Below is a picture of my prototype.



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