Posted on Mar 5, 2013

False alarms produced by semiconductor failure are impossible with this burglar-alarm circuit equipped with relays. What`s more, the circuit is virtually immune to false triggering. With a standby current of less than 0.1 mA, the circuit offers all the features an alarm needs: entry and exit delays, a timed alarm period, and automatic reset after an intrusion. One CMOS CD4093B quad NAND gate, IC1, supplies both logic and analog timing functions with the aid of Schmitt-trigger switching action. Relays make the circuit fail-safe in the alarm-active mode, even when the semiconductors fail. The relays are 12-V, with coil resistances of 250 0 or more. Closing switch S1 initiates circuit operation.

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Capacitor C2 begins charging through resistor R2 and arming indicator LEDllights. When pin 2 of IC1a reaches its switching point, its output decreases, extinguishing LEDl and indicating that the exit delay has ended. That output also drives the base of Qllow, so that if the emitter circuit completes to the V00 line, Ql conducts. The circuit is now armed, and current drain drops to less than 0.1 mA. When the vehicle is entered, relay RY1 contacts close momentarily, completing the emitter circuit of Ql and causing the RY2 contacts to close. Charging C4 through R7 determines the entry-ilelay period. U the system isn"t turned off by opening Sl during this period, the oscillator circuit of IC1c and IC1d activates, and a rapid on/off hom-honking cycle kicks on with the aid of Q2 and RY3. The alarm cycle ends after about a minute, when C2 charges through R3 to the threshold voltage of IC1a at pin 1. This voltage resets the timing circuit, readying it for another entry/alarm cycle. RYl is connected for vehicles that use door switches connected to + 12 V. For vehicles that use groundiog door switches, the bottom of the RYl coil should connect to + 12 V instead of ground. In the latter case, the polarity of C7 should be reversed. For home use, the R3C3 time constant should be increased to give a longer alarm.

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