Burglar Alarm With Timed Shutoff

When S1 (sensor) is closed, power is applied to U2, a dual timer. After a time determined by C2, C1 is energized after a predetermined time determined by the value of C5, pin 9 of U2 becomes low, switching off the transistor in the optoisolater, cutting anode current of SCR1 and de-energizing K1. The system is now reset. Notice that (R6xC2) is less than (R7xC5). The on time is approximately given by:

(R7xC5)-(R6xC2)=Ton

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