Light Beam Alarm For Intrusion Detection

When the light beam that falls in the CDS photocell is interrupted, transistor (2N3904) conducts thereby triggering SCR1 (C106) and activating alarm bell. S1 resets the SCR. The alarm bell should be a self-interrupting electro-mechanical type.
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Light Beam Alarm For Intrusion Detection - Circuitos de Electronica