DC light driving circuit using Darlington pair

To drive large number of LEDs for lighting purpose we need transistors of high power. Darlington pair of transistor is commonly used to improve the power performance of general purpose transistors(like BC547). We can see from the circuit diagram , two transistors are connected in such a way to represent a single transistor. The LDR(light dependent resistor ) is also there . These are combined to make an automatic street light. The resistance of LDR decreases as the light falls on it. In the given circuit there are two resistance R1 and R2 to form a voltage divider network. In case of darkness, the resistance of the LDR remains very high, hence the votlage at the base of Q2 remains high and the transistor gets turn on. Due to the high current value of emitter current of Q2, base current of Q1 also becomes high and hence both the transistors gets turned on and sufficient current flows through then to turn on LEDs(D1 and D2). During day times, more light falls on LDR hence its resistance becomes very low and hence base voltage of Q2 also goes to lower value and Q1 and Q2 get turned off. So the LEDs get turned off.