Dark Sensor

Dark sensor using 2 transistor BC547

Dark Sensor

When light falls on LDR, its low resistance drives the transistor T1 into conduction. This keeps transistor T2 cut-off due to low base bias. The LED does not get power as long  as ambient light falls on it.

Parts list

  1. T1 and T2 – BC 547
  2. LDR  -1 nos
  3. Resistance  R1 – 1k
  4. R2 & R3 – 330 Ohms
  5. 6 volt battery

Modifying the above circuit to use with Relay and switch on / off

Dark sensor with relay
You can also modify the above circuit in controlling AC appliances. Remove R2 and LED from the circuit and also remove the positive connection to Pole of the relay. Connect the AC bulb or any other appliance as shown




  1. why when its light is open the LED will light and when the Light is off the LED also will off its not a dark sensor its light sensor?

  2. Title might look opposite to what it is actually doing but the message that I need to convey should be clear.

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