Burglar Alarm

The heart of the circuit is a 555 oscillator/timer, U1, configured for monostable operation. The output of U1 at pin3 is tied to the gate of SCR1. As long as S1 - S5, which are connected to the trigger input of U1, are open, the circuit remains in the ready state, and does not trigger SCR1 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.

However, when one of the switches (S1 - S5) is closed, grounding U1 pin2, the output of U1 at pin 3 increases, activating SCR1. That energizes the relay, pulling the wiper of K1 to the normally-open terminal, causing LED1 to light and BZ1 to sound.

The duration of the output is determined by the RC time-constant circuit, formed by R1 and C1. Resistor R2 regulates the output of U1 to a safe value for the gate of SCR1. Switches S1 - S5 are to doors, windows, etc. A switch can be connected in series with B1 to activate and deactivate the alarm circuit when it's not needed.
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Burglar Alarm - Circuitos de Electronica