Darkness Detector Circuit Using LDR + 555 IC

A tutorial on how to make a light sensor circuit plus darkness detector circuit using 555 timer IC and a few other electronics components. When this circuit is configured as a light sensor circuit, it turns ON the output whenever light around the LDR is more than a certain amount (threshold).


Components Required:

  • 555 IC
  • LDR (Light Dependent Resistor or Photo-Resistor)
  • LED, Buzzer or any Output Device of your choice
  • Resistors:  82K, 270R (For LED)
  • Breadboard
  • Few Breadboard Connectors
  • (5-12)V of Power Supply

How This Circuit Works

We used LDR (Light Dependent Resistor) to sense the amount of light surrounding it. 555 IC is used to take a call on whether to turn the output ON or OFF depending on the feedback received from LDR.

The observation is that the lesser the amount of light around the LDR, more will be its resistance. So far so good. But how do we make 555 timer IC understand the concept of change in resistance and take a decision on controlling the output?

The only solution is to convert the change in resistance of the LDR to a change in voltage and pass it to 555 IC. This conversion can be done using a voltage divider arrangement.

The voltage at the junction of LDR + 10K resistor (Voltage Divider) is passed to reset pin of the 555 IC. Whenever it gets too dark that the resistance of LDR is huge and the voltage passed on to 555 IC becomes less than 0.7V, the output is reset and is turned OFF.

When the voltage passed on to reset of 555 IC stays above 0.7V, the output is not reset. In order to keep the output ON in this state, we pulled down the voltage at Pins 2 & 6 to 0V.


  • In automatic street light circuits
  • Multiple light sensor circuits with different sensitivities can be clubbed together and used as a Light Meter
  • In Laser light security systems (Laser light can be made incident on the LDR and then the sensitivity of the light sensor can be adjusted so that the output changes if there is an obstruction to laser light)