Jan 222021

Dark detector circuit using LDR and relay

This dark detector circuit uses as a main component an LDR. An LDR changes its resistance value depending on the amount of light it receives. The more light means less resistance, the less light means more resistance. To activate the output (for example a lamp), a relay is used

How the Dark detector circuit works?

Note: LDR = Light Dependent Resistor = Photoresistor.

The set R2, VR1 (potentiometer) and R4 (LDR), form a voltage divider. The output voltage of this voltage divider is taken at the junction of the LDR and the potentiometer.

Dark Detector Circuit usind LDR and Relay

  • When the photoresistor receives light, there is a low voltage at the base pin of the transistor. The transistor is in the cut-off region, there is no collector-emitter current and the relay will not activate.
  • When the LDR receives no light, the voltage at the base pin of the transistor rises high enough to put the transistor into the saturation region, there is collector-emitter current and the relay is activated.

As the level of illumination on the LDR gradually changes, a potentiometer is used to set the appropriate voltage level of activation of the relay.

LED D1 indicates that the circuit is in operation, and LED D3 lights when the light level decreases. The semiconductor diode D2 is used to protect the transistor when the relay is switched off.

LDR - Light Dependent Resistor - Photoresistor

LDR – Light Dependent Resistor

Maybe you want to know why is there a diode connected in parallel to a relay coil?

Dark detector circuit list of components

  • 1 2N2222A NPN transistor (Q1)
  • 2 green and red LEDs, (D1, D3)
  • 1 1N4001 semiconductor diode (D2)
  • 2 1K, 1/4W resistors (R1, R3)
  • 1 10K, 1/4W resistor (R2)
  • 1 LDR / photoresistor (R4)
  • 1 47K potentiometer / variable resistor (VR1)
  • 1 10uF / 25 volts or more electrolytic Capacitor (C1)
  • 1 12 volts relay (RL1)

 Leave a Reply