Automatic Street Light

Required Components:

The Automatic Street Light is a simple design which uses the following components:

  1. LDR – Light Dependent Resistor
  2. Transistor – BC 548
  3. Resistors – 100K ohm, 330 ohm
  4. 9 V Battery
  5. LED- Light Emitting Diode


Component Description:

1. LDR:

LDR is Light Dependent Resistor works on the principle of Photo conductivity. It completely depends on the intensity of the light falls upon it. When the light falls on it, the resistance value gets decreased and when there is no light i.e. if it is kept in the dark place then the resistivity increases and its resistance will be high.

2. Transistor:

Transistor is generally of two types NPN transistor and PNP transistor. The transistor type used here is NPN transistor. Transistor is for switching purpose, it acts as a switch to Turn ON and OFF the LED.

3. LED:

LED is Light Emitting diode which is the light source. It is simply a kind of diode which emits light when gets activated by any electrical signal.

4. Resistors: Resistors given in the circuit is to limit the current in the circuit.

5. Battery: 9V Battery is connected to the circuit as the power supply.


Working Principle:

This is a simple circuit. The Transistor performs the switching operation to switch ON and OFF the LED based on which the LED will glow. Here, the transistor switches ON the circuit and activates the LED to emit light when the light does not fall on the LDR.

  • It automatically switches ON lights when the sunlight goes below the visible region of our eyes. (e.g in evening after Sunset).
  • it automatically switches OFF lights when Sunlight fall on it  ( i.e on LDR ) e.g in morning, by using a sensor called LDR (Light Dependent Resistor) which senses the light just like our eyes.



By using this Automatic system for street light controlling, we can reduce energy consumption because the manually operated street lights are not switched off properly even the sunlight comes and also not switched on earlier before sunset.


Circuit Diagram: