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Monday, September 28, 2009

Light Sensor Circuit Using Op Amp

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.


  1. what part no. of the transistor that refer to the ecg128

  2. ECG128 *is* the part number. The one I found was NTE128. The only difference is in the manufacturer. Most stores will have a cross-reference available to find an equivalent, if they don't have the exact model.

    BTW, just built this circuit, and it works great. I found LDRs were available in several ranges - 1k-10k, 10k-20k and 30k-50kohm, at one store. I figured since R1 was 10k, the 1k-10k LDR was appropriate - found it gave me a great range of adjustment.

  3. Can someone please name a site that contains any simple application of an operational amplifier?I just badly need them for a project in school.The application can be any as long as its simple for the time to do it is at minimum now and it should have two supply-positive and negative!Thanks.

  4. can we use LM358N instead of LM741???
    pls reply.....needed urgently...