Circuit knowledge

This is the simple burglar alarm with timed shutoff. In this schema when SI (sensor) is closed, power is applied to U2, a dual timer. After a time determined by C2, CI is energized after a predetermined time determined by the value of C5, pin 9 of U2 becomes low, switching off the transistor in the opt isolator, cutting anode current of SCR1 and de-energizing Kl. The system is now reset. 

Notice that (i6x C2) is less than (R7xC$). The ON time is approximately given by:(R7xC5)-(R6xC2) = Ton

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