LDR Light Detector

LDR Light Detector Circuit using 741 Op-Amp

This LDR light detector 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.

PARTS LIST
R1	100kΩ
R2	100Ω
VR1	100kΩ
Q1	2N2222
RL1	9V Relay
IC1	741
LDR

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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