This is the circuit diagram of automatic light controller which use 78xx voltage regulator IC series. The voltage regulator ICs deliver a constant output voltage, as against a extensively fluctuating input supply, when the common terminal is grounded. Any voltage about zero volt (ground) interconnected within the common terminal is added to the output voltage. That indicates the enhance in the common terminal voltage is reflected at the output. However, when the common terminal is disconnected from the ground, the maximum input voltage can be obtained at the output.
This characteristic is utilised in the present circuit. When the common terminal is interconnected towards the ground, the regulator output is equivalent to the rated voltage, and when the terminal is disconnected from the ground, the output increases up to the input voltage.
The common terminal is controlled by a transistor, which operates as a switch on the terminal. For automatic control of light, a light-dependent resistor (LDR1) is joined towards the base of the transistor. In this way, the voltage regulator is capable of operate a light bulb automatically as per the ambient light.
For detailed and complete explanation about how this automatic light controller circuit work, you can download the document in PDF version: