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A light dependent resistor is a semiconductor electrical device that has a very high resistance to the flow of electrical current when not in the presence of light. If light strikes the device, it lowers its resistance, allowing electrical current to flow through it and on to other devices or electrical circuits. A light dependent resistor, because of this action, is often used in any type of device that needs to be turned on or off, depending on the presence of light. Typical uses for light dependent resistors are streetlights and light meters used for photography.
Light dependent resistors, also called photo-resistors or photo-conductors, are often constructed of a material that has been doped, which is to give the material an impurity that contains an electrical charge. These types are said to be extrinsic. Light dependent resistors can also be constructed of a single undoped material that possesses a weak electrical charge of its own, such as silicon. These are called intrinsic.
Whether extrinsic or intrinsic, all light dependent resistors work on the same essential principle. The electrical charge in them is not made of free electrons, but of electrons bound into the material matrix of the light dependent resistor. This gives the device a naturally high resistance to conducting electricity. When light strikes the device, however, the electrons bound in the material matrix become excited. If the light is strong enough, the electrons can become so excited that they can jump into the conduction band of a device, allowing it begin the conduction of electricity.
Most inexpensive light dependent resistors are constructed with cadmium sulfide and serve a number of practical uses. In streetlights, these devices save power by turning the lights on at dusk and off at dawn. These types of devices are also used in light meters for photography; the meters measure the resistance of the light dependent resistor, which decreases as light levels increase.
There are, however, a number of other materials used in light dependent resistors to alter their properties and make them useful in applications that do not involve visible light. Lead sulfide and indium antimonide are used to create devices sensitive to some frequencies of infrared, making them good infrared detectors. Germanium copper is another material that can make a light dependent resistor so sensitive to far infrared light that some astronomers use it in making their observations.