Lights and Display Board Circuits

Street Light Schematic Circuit Diagram

The circuit design presented here is for an automatic street light system that activates when it gets dark and turns off when daylight returns. In essence, this circuit can be adapted for various types of automatic night lights.

To detect the ambient light level, this circuit incorporates a Light Dependent Resistor (LDR).

The LDR’s resistance decreases in the presence of light, resulting in a substantial voltage drop across the POT R2. This, in turn, switches on the transistor Q1. Q1’s collector, connected to Q2’s base (SL100), ensures that Q2 and the relay remain in the off state, keeping the light off.

As darkness descends, the LDR’s resistance increases, causing the voltage across POT R2 to drop below 0.6V. This deactivates transistor Q1, allowing Q2 to turn on. Consequently, the relay is energized, and the bulb illuminates.

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.

The LDR sensor is employed to detect ambient light levels, ensuring that the streetlight automatically switches off during the daytime when the sun is bright. Conversely, when night falls and there is minimal light, the LDR sends a signal to the microcontroller to activate the streetlight.

You may have observed certain streetlights that exhibit an automatic on-off pattern at night and in the morning. This functionality is achieved through the incorporation of photoresistors or light-dependent resistors (LDRs) in the streetlight system. These LDRs perceive sunlight and manage the streetlights accordingly.

It operates by automatically activating the lights when the sunlight intensity falls below the range visible to our eyes. This function is enabled by a sensor known as the Light Dependent Resistor (LDR), which responds to light in a manner akin to our visual perception. Similarly, it automatically deactivates the lights when daylight becomes visible to our eyes.


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