Intruder Alarm



The alarm uses a pyrosensor to detect the presence of animals or human beings by changes in heat radiation. The contact of the relay in the pyrosensor is linked to the input of the circuit and is closed in the quiescent state. If an animal or person approaches the sensor, the relay contact opens. The input of IC1b then goes low and its output becomes high. Pin 8 of IC1c goes low, which enables the MMV (monostable multivibrator) formed by IC1c and IC1d. Owing to the feedback to pin 9 of IC1c, the MMV output remains low for about three minutes, even if the sensor is disabled. If on completion of the three-minute period the sensor is still actuated, a new period also three minutes long is started. Alarm pulses generated when the MMV is enabled are ignored. When the MMV is quiescent, its output is high and counter IC2 remains reset. The counter position is then zero and, since output Q0 (pin 3) is not linked to T1 and the buzzer via a diode, the buzzer remains inactive. When the MMV is enabled by the pyrosensor, its output goes low. The counter is then no longer reset and begins to count the clock pulses from IC1a. The buzzer is then actuated intermittently via diodes D1-D5 and T1. When the counter reaches its highest position, Q9 (pin 11), the high level at this output impedes the clock at the enable input (pin 13). The counter stops counting and retains this position. After a short while, the mono time elapses and pin 11 of IC1d goes high, whereupon the counter is reset. A 600 µs pulse is then passed to IC3b via R3 and C2. This pulse briefly disables IC3a which reenables the MMV, provided the sensor is still actuated. The mono time may be changed by altering time constant R4-C1. When the sensor contact has been closed almost continuously for a lengthy period, the mono time may be a little shorter when the MMV is enabled for the first time after this period. If this is found inconvenient, the recovery time may be lengthened by increasing the value of R2 The circuit draws a current of 1-2 mA, which increases to 13-14 mA (via R1) when the relay contact is closed, and to 15-16 mA when the buzzer is actuated.


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