Posted on Dec 18, 2012

The single-chip, burglar -alarm circuit shown uses a dual 556 timer, draws 10 mA of standby current, and generates a pulsing alarm signal that conserves battery energy. Once activated, the alarm wilLremain on, independent of the subsequent state of any of the sensors. The sensors support both deferred and immediate-action modes. You can attach this circuit to your car`s internal lighting circuitry using a single wire and a relay. To arm this circuit, you open your car door and close switch SA. The switch discharges capacitor C4 and holds the timer (one half of the 556 !C) in a reset state to prevent false triggering while you`re arming the circuit. When you close your car door, the circuit enters a standby mode.

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If the door is then reopened, the sensors apply a negative-going pulse to trigger 1. Output 1 then increases, enabling the alarm for 1.1R1C1 seconds. Output 1 "s high state triggers the multivibrator, the other half of the 556, which begins to cycle after a delay equal to 1.1 (R2 + R3) C2 seconds. As long as the timer"s output stays high, the multivibrator will continue to cycle, turning the horn off and on at 3.3-second intervals. During the interval between time that the timer"s output increases and the time that the multivibrator"s output decreases, you can disarm this circuit using switch SA. To prevent false triggering caused by switch contacts, at S1, S2, and S3, that may bounce when closing the door, make the R6C3 time constant as large as possible. In addition, capacitors C1 and C2 should be tantalum types and should exhibit leakage of less than 1 11A at room temperature.

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