A light-dependent resistor (LDR) is a photo-conductive cell. It is made from a semi-conducting material called cadmium sulphide (CdS). Its resistance (in ohms) changes in proportion to the amount of light falling upon it.
An LDR component that is often used by GCSE students is the ORP12. As a general rule-of-thumb, in bright light its resistance is around 5.4 kΩ. However, in the dark its resistance increases to around 1 MΩ. The precise amount can vary depending upon the manufacturer and the component tolerance. Therefore, please consult the ORP12 datasheet.
CdS is a material which in the dark becomes an insulator. In the dark, the material does not have enough electrons to conduct. When in light, the light energy called photons, falls upon it and releases electrons. These electrons then allow it conduct. The more electrons it has, the more it will conduct, thus lowering its resistance.
In this LED circuit, there is a 22 kΩ preset resistor and an LDR resistor forming a potential divider network. As you can see, they are connected across the 9 V power supply.
When the light level increases, the resistance of the LDR will fall and the voltage across it falls as well.
When the light level decreases, the LDR resistance increases, and the voltage across it also increases. When this happens, the voltage across the base and emitter junctions of the transistor also increases, because the LDR is essentially connected across them...
When the voltage across the base rises to a sufficient amount, the transistor will begin to conduct and therefore the LED will light. In this circuit, we are using a transistor as a switch, and the ORP12 controls it.
The preset is used to change the characteristics of the potential divider network. This in turn determines when the transistor will conduct. In other words, it lets the user set the ambient light level necessary to trigger the circuit.
BFY51 Transistor Pinout
For the transistor I am using the BFY51 which is an NPN silicon transistor used for general purpose amplifier applications. It has an Ic rating of 1 A and is capable of driving relay loads as well. It comes in a hermetic TO-39 package. If you were to look at it from underneath then this is how the pins are organised.
An equivalent transistor is the 2N3053 with a slightly lower Ic rating of 0.7 A. It has the same pinout and used for the same purpose.
This is a very simple circuit and you can use many different types of general purpose NPN transistors. Even the BC range of transistors would work in this circuit.
This is a very simple circuit that could be constructed on a modern Solderless breadboard, or tag board, or even screw-down terminal blocks. A permanent circuit could be built on PCB Prototype Matrix Board which you can get very cheaply from most electronics stores.
When you are constructing this, make sure you understand the pinout of the transistor. Also make sure you understand how to use an LED. The LED has a flat side which is the cathode. Please refer to the LED Resistor Calculator article for more information about using this component.