May 112020

Automatic night light with 555 and relay

This automatic night light with 555 IC turns on the light when there is no daylight. The circuit is perfect for lighting single places such as the garden, the front door of the house, the entrance to the garage, etc.

Since a relay is used to activate the load, many types of light sources can be connected. It is better to use LED light sources. They consume less power.

How does the automatic night light work?

The 555 integrated circuit can act as a comparator. When the input voltage level at pin #2 (TRI) is less than the trigger voltage level of the timer, the output at pin #3 (OUT) will go high and activate the relay that connects the lamp or bulb.

The LDR (light dependent resistor) increases its resistance as it gets darker and the cursor (arrow) on the 100 kilo ohm potentiometer will have a lower voltage level. This variation is due to the voltage scale on the pointer (arrow) of the potentiometer (R3).

Automatic night light with 555 and relay

The circuit uses a transformer, two diodes, and an electrolytic capacitor to provide the DC voltage necessary for the 555 IC to operate. This voltage is about 9 volts. The 100K potentiometer is required to adjust the light level needed for the circuit to operate as expected.

555 timer IC Pinout


The diode in parallel with the relay is used to remove the reverse polarity voltage that is generated in the inductor of the relay when it is de-energized. It is used to prevent this peak voltage that can damage other circuit components.

Relay (Electromagnetic Switch)

List of circuit components

  • 1 555 timer integrated circuit (U1)
  • 4 10K resistors  (R1, R4, R5, R6)
  • 1 LDR (Light-Dependent Resistor) (R2)
  • 1 electrolytic capacitor 1000uF, 15V or higher (C1)
  • 1 220nF capacitor (C2)
  • 2 1N4002 Diodes (D1, D2)
  • 1 1N4148 Diode (D3)
  • 1 110/220 to 12V center-tapped transformer, 300mA (milliamps) (T)
  • 1 100K Potentiometer (R3)
  • 1 4 Amps Fuse (F)
  • 1 switch
  • 1 9V relay

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