Lights and Display Board Circuits

Street Light Schematic Circuit Diagram

The circuit design shown here is for a street light that comes on automatically when darkness falls and goes off automatically when the sun rises. In reality, this circuit can be used to create any form of automatic night light.

To detect light, the circuit employs a Light Dependent Resistor (LDR).

The resistance of LDR will be low when there is light. As a result, the voltage loss over POT R2 will be significant. This turns on the transistor Q1. Q1’s collector (BC107) is connected to Q2’s base (SL100). As a result, Q2 will be turned off, as will the relay. The light will not be turned on.

When darkness falls, the resistance of the LDR rises, lowering the voltage across POT R2 below 0.6V. This turns transistor Q1 off, allowing Q2 to turn on. The relay will turn on and the bulb will light up.

Street Light Schematic Circuit diagram with Parts list:


  • POT R2 can be used to modify the circuit’s sensitivity.
  • Any wattage bulb can be used, as long as the relay is rated appropriately.
  • A controlled 9V DC power supply can be used to power the circuit.
  • To download the power supply circuit for this project, go here!
  • K1 is a 9V SPDT relay that can be used.
LDR sensor is used for detecting the presence of surrounding light so that during the day time when the sun is bright, the street light is switched off automatically. During the night time when there is no light, the LDR sends a signal to the microcontroller to turn on the street light.
You might have noticed some street lights that automatically get turned on at night and turned off in the morning. This happens because the lights have photoresistors or light-dependent resistors (LDR) that sense the sunlight and control the street lights accordingly.
It automatically switches ON lights when the sunlight goes below the visible region of our eyes. This is done by a sensor called Light Dependent Resistor (LDR) which senses the light actually like our eyes. It automatically switches OFF lights whenever the sunlight comes, visible to our eyes.

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