Aug 182016
 

Automatic night light using 555 and relay

This automatic night light using 555 circuit automatically turns on the lights at the time when there is no daylight. This circuit is perfect for lighting alone places like the garden, the front door of the house, etc.

How the automatic night light using 555 works?

The integrated circuit 555 can function 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 it will activate the relay that connects the lamp or bulb.

Automatic night light using 555 and relay

The LDR (light dependent resistor) increases its resistance when it gets dark and the cursor (arrow) of the 100 kilo-ohms potentiometer will have a lower voltage level. This variation is due to the voltage division on the cursor (arrow) of the potentiometer.

This circuit uses a transformer, two diodes and an electrolytic capacitor to obtain the DC voltage needed for the integrated circuit 555 to function. This voltage is approximately 16 volts. The 100K potentiometer is necessary in order to adjust the light level required for the circuit to work as expected.

Note:

The diode in parallel with the relay is used to remove voltage with reverse polarity, that is created in the relay’s inductor when it is disable. It is used to prevent this peak voltage, that can damage other circuit components.

List of the automatic night light using 555 components

  • 1 integrated circuit 555 timer (U1)
  • 4 resistors: 10 Kohms (R1, R$, R%, R6)
  • 1 LDR (light dependent resistor) (R2)
  • 1 electrolytic capacitor 1000 uF (C1)
  • 1 capacitor 220 nF (C2)
  • 2 diodes 1N4002 (D1, D2)
  • 1 diode 1N4148 (D3)
  • 1 transformer 110/220 volts (depending on the geographic location), 12V secondary winding, 300 mA. (milliamps) with a center tap (T)
  • 1 potentiometer 100K (R3)
  • 1 fuse 4 amps (F)
  • 1 switch
  • 1 9V relay.
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