In this experiment, we will build an useful burglar alarm circuit for normally-closed and normally-open alarm systems. 1. Build the Circuit. Using the schematic along with the pictorial diagram, place the components on a solderless breadboard as shown. Verify that your wiring is correct. 2. Do the Experiment. Theory: Closing switch S1 will put a positive voltage on the gate of the SCR, causing it to conduct and to sound an alarm. Or, by opening the normally-closed switch S2, capacitor C1 will discharge, putting a positive pulse on the gate of the SCR which will cause it to conduct. The only way to stop the SCR from conducting once it has started to conduct is to remove power from the circuit. Thus, even if the sensor switches S1 or S2 are opened or closed again, the alarm will not turn off. The only way to reset the alarm is to disconnect the batterv.
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