Automatic street light

 Automatic Street Light Controller Using LDR

Introduction

Street lights consume a lot of energy if they remain ON during the day. An Automatic Street Light Controller solves this by switching ON the light at night and OFF during the day automatically. This project uses an LDR (Light Dependent Resistor) to sense light intensity and control a 230V AC bulb through a relay.

Circuit Diagram

Block Diagram 

Input: LDR (light sensor)

Control Unit: Transistor switching + Relay driver

Output: AC Street Light

Block Diagram 

Components Required

LDR Standard Light Sensor       -1

BC547 Transistors NPN Type     -2

Resistor R1 1kΩ                           - 1

Variable Resistor RV1 100kΩ     -1

Diode D1 1N4007                          -1

Relay 9V SPDT                               -1

AC Bulb 230V                                 -1

Battery 9V DC                                -1

Working Principle

Daytime (Light Present)

The LDR has low resistance, so little voltage is applied to the transistor base.

Transistor Q1 and Q2 remain OFF.

Relay stays de-energized and bulb remains OFF.

Nighttime (Darkness)

The LDR resistance increases, providing base current to Q1.

Q1 turns ON, which drives Q2 ON.

The relay is energized, closing the circuit for the 230V bulb.

Bulb turns ON automatically.

Key Features

Fully automatic switching (no manual effort).

Low-cost and energy-efficient design.

Adjustable sensitivity via the variable resistor (RV1).

Provides isolation between low-voltage control and high-voltage load.

Applications

Automatic street lights on roads.

Garden and park lighting.

Highway and industrial lighting systems.

Energy-saving outdoor lighting.

Advantages

Saves energy by switching lights OFF during the day.

Low-cost and easy to implement.

Works reliably in varying light conditions.

Conclusion

This simple automatic street light circuit is ideal for saving energy and reducing manual work in outdoor lighting systems. By adjusting the variable resistor, you can control the exact light level at which the bulb turns ON or OFF.