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Beam Blocking Alarm Circuit

2017-01-06 21:23  
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Beam Blocking Alarm Circuit
As shown in the figure, when the photosensitive resistor R4 is irradiated, the resistance is low, thus, transistor VT1 and one-way thyristor SCR are closed; when the beam is blocked, the base voltage of VT1 increase, VT1, SCR is conducted, and audible alarm buzzer BZ is energized. S1 is the reset switch. Press S1, the circuit stops the alarm.