LDR Light Detector Circuit using 741 Op-Amp
he circuit’s threshold is set by resistor VR1. When the intensity of the light falling on the LDR is lowered, resistance of that unit increases, and lowers the voltage applied to the inverting input of the 741.
The reference voltage at the noninverting input of the 741 is set (via Vr1) so that the comparator switch from low to high when the light falling on the LDR is reduced. That high activates transistor Q1, Which causes the relay contacts to close.
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