741 Op-Amp Sensor

Parts List:
R1,R4 = 10K
R2,R3 = 470 ohm
   R5 = 1K
   R6 = 100K - 1M (optional--see text)
   P1 - 10K, trimmer pot
 LDR1 = Light Dependent Resistor
   U1 = LM741, opamp
   D1 = 1N4001, General Purpose diode
   Q1 = NTE128, or any other replacement
   Ry = Relay
The circuit as shown act as a light sensor. 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.

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Copyright © 1995 - Tony van Roon