Light-dependent resistors, also known as "photoresistors," are passive electronic components that resist electric current. Unlike a normal resistor, though, the resistance of a photocell varies according to the amount of light energy that is illuminating the device. Consequently, photoresistors are typically used in light-sensing applications.
Photoresistors are often made of cadmium sulphide, which provides a light response that is similar to the response of the human eye. This important quality allows photoresistors to detect light in a way that is similar to how a person would detect it. The resistance of a cadmium sulphide photodetector varies inversely with light, so the photoresistor circuit must be designed to interpret lower resistance values as higher light intensities.
In general, photoresistors are used in sensor circuits. A sensor is a device that detects a condition in its environment and converts that condition into a measurable signal. Because its resistance varies according to light intensity, a photoresistor can be used to produce a voltage or current --- the measurable signal --- that varies according to the lighting conditions of its environment. A photoresistor cannot produce a measurable signal by itself --- it must be part of a properly designed circuit.
Day and Night
There are many situations in which a device needs to perform certain actions according to the time of day. A common example is a lamp that automatically turns on at night. By detecting the intensity of ambient light, photoresistors can distinguish between day and night.
Measuring Light Levels
Photoresistors can also be used to measure light intensity for applications that require greater precision. Cameras can use photoresistors to determine the proper exposure time; laptops may use photoresistors in a circuit that varies screen brightness according to ambient lighting conditions.