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Light Activated Switch 2011-09-04

The circuit is based on National Semiconductors comparator LM311 (IC1) and an LDR. The non inverting input of IC1 is given with a reference voltage of 6V using resistors R3 and R4. The input to the inverting input will be the voltage across the LDR that is light dependent. At darkness the resistance of the LDR will be high and so do the voltage across it. In this condition the voltage at the inverting input will be higher than the reference at non inverting pin and the output of the comparator will be low. When the LDR is illuminated, its resistance drops and so does the voltage across it. Now the voltage at the inverting input will be lower than that at non inverting input and the out put of the comparator goes high. This makes transistor Q1 on and drives the relay. As a result we get a relay switching according to the intensity of the light falling on the LDR.






  • Adjust POT R1 to set the desired light intensity for switching the relay.For this illuminate the LDR with the desire intensity light.The relay will be either on or off. Adjust POT R1 slowly so that the state of the relay changes.That’s it.Now the circuit is set for the given intensity of light.
  • Assemble the circuit on a good quality PCB or common board.
  • You can use either a 9 V battery or a well regulated & filtered 9V DC mains operated power supply.
  • The pin 5&6 (Balance & Balance/Strobe ) of IC LM311 are shorted to minimize the chance of oscillations.
  • The pin out of LM311 is also given together with the circuit diagram.


Because there is no hysteresis in this circuit, they can be some chattering with the relay when the illumination is very near triggering point.
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