Light Sensor Circuit Using Op Amp 741

This is a one of the light sensor. This circuit is a dark sensor that is based on op amp 741 IC as main control. This circuit is a simple design for sensor the light at the night. This is the figure of the circuit. For sensor the light is using LDR. Operation of the circuit is under normal conditions the resistance of the LDR is high, keeping

pin 2 low. When light falls onto the LDR the resistance drops to a couple hundred ohms and triggers pin 2 high which biases the base of Q1 via pin 6 and R4 and in turn activates the relay. Trimmer pot P1 and the two 470 ohm resistors, R2 and R3, are a voltage divider to adjust for sensitivity. If you want the action reversed (make it a dark sensor), change the positions of the LDR and R1. If the relay chatters, add a bit of hysteresis by adding a 100K to 1Meg-ohm resistor (R6) over pins 6 and 2 of the 741 op-amp, but in most cases 100K to 330K will do the job. The LDR is a regular, general purpose type. D1 serves as a spark-arrestor when the relay contacts open. This circuit power supply is using 12 V DC.

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