Automatic Street Light using BC 547 Transistor project done with the help of LDR and Transistor. In this project, the transistor acts as a switch and LDR is used to sense darkness and brightness of the surrounding. LDR is a Light-dependent resistor which works on the principle, variation in resistivity changes with light intensity. Mostly, they are inversely proportional to each other.
During the daytime, the light intensity is high so that resistance is low that is it exhibits photoconductivity. The resistance varies from a few ohms to Megaohms. LDR contains Calcium sulfide as the main material. LDR connected with a potentiometer parallelly i.e) act as a voltage divider circuit. This resistance acts as an input for the transistor BC 547. If the light intensity is low then, led will glow indicating automatic Turn ON during the night. As the voltage for the transistor is based on the LDR, the transistor acts as a switch accordingly. When the intensity of light is high means base of the first transistor activated and the circuit closed and no input supplied to the second transistor which indicates, Street Light is OFF. If the light intensity is low, resistance for the first transistor is high and act as an open circuit. The second transistor gets supply and turns the LED ON i.e) Street Light is ON.