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Beam blocking alarm

2017-01-08 07:13  
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Beam blocking alarm
As shown,when the photosensitive resistor R4 is under light, its resistance is small, so the transistor VT1 and unidirectional thyristor SCR are closed; when the beam is blocked, base voltage of VT1rises, and VT1, SCR is conductive, and buzzer BZ will have the power and send an alarm. S1 is the reset switch. When S1 is pressed, the circuit stops the alarm.