DC light driving circuit using Darlington pair

Description

To DC light driving circuit a large number of LEDs for lighting purposes, we need transistors of high power. Darlington pair of transistors 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 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 resistances R1 and R2 to form a voltage divider network.

In case of darkness, the DC light driving circuit resistance of the LDR remains very high, hence the voltage at the base of Q2 remains high and the transistor gets turned on. Due to the high current value of the emitter current of Q2, the base current of Q1 also becomes high, and hence both the transistors get turned on and sufficient current flows through them to turn on LEDs(D1 and D2). During daytime, more light falls on LDR hence its resistance becomes very low, and hence base voltage of Q2 also goes to a lower value, and Q1 and Q2 get turned off. So the LEDs will be turned off.