Hi, this a simple project mostly used in high school. Even simple it has got a lot of significance to save energy.

It is a simple and powerful concept , which uses transistor ( BC 547 NPN) as a switch to switch ON and OFF the system automatically .

It automatically switches ON lights when the light goes below the visible region of our eyes. ( e.g in evening after Sunset ).

It automatically switches OFF lights when light fall on it ( e.g in morning ) , by using a sensor called LDR (Light Dependent Resistor) which senses the light just like our eyes.

Needs no manual operation for switching ON and OFF...

Step 1: Getting the Materials...

Materials you'll need are :

* BC547 transistor

* 220 k resistor

* 330 ohms resistor

* LED (any colour )

* 9v battery

* battery clip

* LDR ( light dependent resistor )

* Breadboard

Step 2: Know What You Are Doing...

Ldr circuits are simple which are controlled by light, they are also called photovoltaic resistors. In this circuit the transistor will receive positive charge from the 220K resistor. To regulate the input at the base , let will send negative charge to the base. Positive charge and negative charge will nullify each other so that no charge is left behind at the base. But when the lights go off led will resist the current flow to provide the base a positive charge. When the base receives charge the switch will turn on and LED will glow completing the circuit.

Thus the circuit works!!

For reference Google out how the electric components work...