Light Activated Switch is a circuit which is operated by using some basic  electronic components such as Light Dependent Resistor, Transistor, Led and Resistor. In day today life we are using several devices which operates on Electricity. Some times it results into the wastage of power. In such cases we can utilize Light Activated Switch for automatic switching of the devices using day lights intensity. It uses one very popular component LDR.

Light dependent Resistor is component whose Resistance varies according to amount of light falls on it. This resistance is varies from few Ohms to Mega Ohm. When amount of light intensity is more at that time Resistance of LDR is less and vice versa. In this circuit when light falls on LDR it makes turn ON led with the help of Transistorized circuit. When there is no light falling on LDR, LED will remain OFF. Here transistor is used to switch between two positions provided by input which is coming from the junction formed by LDR and potentiometer R1. Sensitivity of Circuit is varied by using Potentiometer.

Circuit Diagram:


Component Values:

  •  Resistor:R2 = 470 Ohm
  •  Potentiometer:R1= 100 Kilo Ohm
  •  Transistor:Q1= BC 547
  •  Light Dependent Resistor: LDR
  •  Light Emitting Diode: LED1
  •  Battery: 9 Volts

Circuit Building Steps:

Step 1:

The very first component should be mounted on the bread board is transistor BC 547. All circuit components are connected to this transistor.

Step 2:

Connect one end of LDR to VCC

Step 3:

Connect second end of LDR to base of transistor.

Step 4:

Connect variable resistor i.e Potentiometer  R1 on the Bread Board.


Step 5:

Connect Black wire of Potentiometer to base of transistor, connect remaining two wire Red and brown wire to Ground.


Step 6:

Connect Emitter of transistor to Ground.

Step 7:

Mount Resistor and LED on Bread Board. resistor R2 is of 470 Ohm. Connect one end of resistor to VCC.


Step 8:

Connect Anode of LED to second terminal of Resistor R2.


Step 9:

Connect Cathode terminal of LED to collector of transistor.


Step 10:

Connect Battery terminals to the Bread Board as shown below:

When we put shadow on LDR its resistance get increased to its maximum value and LED due to lack of supply will be turn OFF. When we remove shadow from the LDR,it will turn ON the LED.