It is a simple and powerful concept which uses a transistor as a switch to switch ON and OFF the street lights automatically.
It automatically switches ON the lights when it is dawn.
It automatically switches OFF the lights when sunlight falls on LDR.
Case 1: When light falls on the LDR, resistance will be less, Current will flow from negative terminal of the battery and enters the base of the transistor. Due to a high resistance of 1k, The positive potential entering the base of the transistor will be very less. Hence transistor will be in off state and LED will not glow.
Case2: When no light falls on the LDR, resistance will be more. Current from negative terminal will not be able to cross LDR. The positive potential will enter the transistor through 1K resistor. Hence transistor will be in on state and LED will glow.