Automatic Street lighting System is a simple, yet powerful concept, which uses LDR sensor as a switch. By using this system we can eradicate manual work at 100%. It automatically switches the lights ON when the sunlight goes below the visible region of our eyes. This is done by a sensor called Light Dependant Resistor (LDR) which senses the light actually like our eyes. It automatically switches OFF lights whenever the sunlight comes visible to a sensor. By using this system energy consumption is also reduced because nowadays the manually operated street lights are not switched off even after the sunlight comes visible and also switching ON earlier before sunset. In this project no need of manual operation like ON time and OFF time setting. This project clearly demonstrates the working of the transistor in the saturation region and cut-off region. The working of the relay is also known. Implementation of this project encourages digital work. Light dependent resistor, a photoconductive device has been used as the transducer to convert light energy into electrical energy. The central dogma of the circuit is that the change in voltage drop across the light dependent resistor on illumination or darkness switches the transistor between the cut-off region or saturation region and switches OFF or ON the LED.