Electronic component: Light Dependent Resistor (LDR)

Part of the project is to find out what a LDR is and does. As the name suggests it has something to do with resistance and with light. Depending on the type the resistance will be lower when more (or less) light falls on the LDR. But you can  build the circuit either way.

Today's shopping list contains:
1 x LDR
1 x 10 kOhm Potentiometer
2 x 330 Ohm resistor
1 transistor BC547B
1 x green LED
1 x 9V battery

The following circuit will light a green LED when there is little light. By adjusting the potentiometer you can control the amount of light required.

This works basically because when the LDR recieves a lot of light, the resistance will drop. The current will not go through the transistor. As the amount of light drops, the resistance of the LDR will decrease. After a certain point, the current will prefer the transistor instead of the LDR, enabling the green light.