An automatic dark sensor can be made using a PNP transistor(BC557).

As the ambient light falling on LDR decreases, the LED glows. The sensitivity of the circuit can be adjusted using the variable resistor VR1.

If this circuit does not work with 10K, use 100K.


  1. Your website looks very informative, but why dont you incorporate the Joule Thief into your circuits? Any particular reason?

  2. There is one problem in this circuit during the ligth transtion it gives flickring ( i am operating a relay). Please suggets if someone has done something to remove this.

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  4. Can you provide these by switching the LDR and VR1, so that as light is on the LDR the LED turns on? Can I accomplish the same thing by switching to an NPN transistor, such as a 2N2222A?

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