Adjustable light detection switch

  
R2 sets the circuit`s threshold. When the light intensity at PCI`s surface is decreased, the resistance of PCI a cadmium-sulfide photo-resistor is increased. This decreases the voltage at the inverting input of the 741. When the reference voltage at the 741`s noninverting input is properly adjusted via R2, the comparator will switch from low to high when PCI is darkened
Adjustable light detection switch - schematic

This turns on Q1 which, in turn, pulls in relay Kl.




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