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This circuit switches a relay ON or OFF according the brightness of light shining on the photo sensitive resistor CDS.
CDS (cadmium sulphide) or so called LDR (light dependent resistor) has resistance that changes inversely proportional
to light intensity.   i.e. the brighter the light, the lower the resistance

The circuit can operate in 2 different modes :   (you can select the mode by setting the slide switch S1)

1)   LIGHT detection mode ("L") : In this mode, relay actuates (LED lights up) when the light shining on the CDS
     is sufficiently BRIGHT (sensitivity set by trim pot (VR1).

2)   DARK detection mode ("D") : In this mode, relay turns on (LED lights up) when the light shining on the CDS

     is sufficiently DARK (sensitivity set by trim pot (VR2))
 

Applications :  automatic corridor or porch light control,  "chop-beam" intrusion alarm,  . . .