Automatic night light using 555 and relay
This Automatic night light using 555 IC turns lights on 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. Better if they are LED light sources due to their low current consumption.
How the automatic night light works?
The 555 integrated circuit can work 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), go high and activate the relay that connects the lamp or bulb.
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.
The circuit uses a transformer, two diodes and an electrolytic capacitor to obtain the DC voltage needed for the integrated circuit 555 to work. This voltage is approximately 9 volts. The 100K potentiometer is necessary in order to adjust the light level required for the circuit to work as expected.
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 disabled. It is used to prevent this peak voltage, that can damage other circuit components.
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 more (C1)
- 1 capacitor 220nF (C2)
- 2 1N4002 diodes (D1, D2)
- 1 1N4148 diode (D3)
- 1 110/220 to 12V center tap transformer, 300mA (milliamps) (T)
- 1 100K potentiometer (R3)
- 1 4 amps fuse (F)
- 1 switch
- 1 9V relay