Automatic Street Light Science Kit

Automatic Street Light using LDR Automatic street light system is very common nowadays as it provides intelligent street lighting mechanism. It provides light automatically during night without any human interference. These energy saving street lights make use of incandescent lamps instead of LEDs. The heart of this circuit is a LDR (Light Depended Resistor) which is connected as a potential divider with a resistor. The voltage drop across LDR is used for switching the transistor. The following circuit that described below which perform this job automatically. Working of Automatic Street Light Controller Circuit: The circuit diagram present here is that of a street light that automatically switches ON when the night falls and turns OFF when the sun rises. In fact we can use this circuit for implementing any type of automatic night lamp. The circuit uses a Light Dependent Resistor (LDR) to sense the light .When there is light the resistance of LDR will be low. So the voltage drop across R2 will be high. This keeps the transistor Q1 ON. The collector of Q1 (BC547) is coupled to base of Q2 (BC547). So Q2 will be OFF and the LED bulb will remain OFF. Similarly, when there is night the resistance of LDR will be high. So the voltage drop across R2 will be low. This keeps the transistor Q1 OFF. The collector of Q1 (BC547) is coupled to base of Q2 (BC547). So Q2 will be ON and the LED bulb will remain ON. So, because of this circuit, Street Lights turn on at night and turn off during the day automatically. Circuit Diagram: Components Used in this Circuit:  LDR  BC547 - 2  R1,3,4 (1K) – 3  R2 (100K) – 1  LED  9V Battery  Wooden Board  WPC Board Procedure:  Insert first transistor Q1-BC547 (NPN) on breadboard (or general PCB) as shown in the circuit diagram 1.  Connect another transistor Q2- BC547 (NPN) on breadboard as in step 1.  Connect wires across emitter pin of both transistors and –ve terminal of battery (lowest/ bottom row of breadboard.)  Connect a wire across Collector pin of transistor Q1 and Base pin of transistor Q2.  Connect a resistor 1K across positive terminal of battery (topmost row of breadboard) and Collector pin of transistor Q1.  Connect Light Dependent Resistor (LDR) across positive terminal of battery (topmost row of breadboard) and base terminal of transistor Q1.