Posted on Nov 23, 2012

The heart of the circuit is a 555 oscillator /timer, Ul, configured for monostable operation. The output of Ul at pin 3 is tied to the gate of SCRl. As long as Sl-S5, which are connected to the trigger input of Ul, are open, the circuit remains in the ready state, and does not trigger SCRl into conduction. Because the relay is not energized, battery current is routed through the relay`s normally-closed terminal and through current-limiting resistor R3 to LED2, causing it to light.

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However, when one of the switches (Sl-S5) is closed, grounding Ul pin 2, the output of Ul at pin 3 increases, artivating SCRl. That energizes the relay, pulling the wiper of Kl to the normally-open terminal, causing LEDl to light and BZl to sound. The duration of the output is determined by the RC time-constant circuit, formed by Rl and Cl. Resistor R2 regulates the output of Ul to a safe value for the gate of SCRl. Switches Sl-S5 are to doors, windows, etc. A switch can be connected in series with Bl to activate and deactivate the alarm circuit when it"s not needed.

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