A photocell is a relatively simple device that measures the presence of light. Photocells fall into two main categories: highly evolved versions that work with complicated equipment to help scientists find detailed readings of the amount and types of light produced by natural (such as stars) or synthetic (such as light bulbs) sources, and the consumer-oriented, photocells that generally just detect light and are unreliable when it comes to measuring it. Neither of these should be confused with photovoltaic cells, which absorb incoming light and turn it into electricity.
The average photocell works like a resistor. It contains a simple circuit that lets electricity flow through the cell at a certain rate. When the sensor part of the cell, usually on top, receives the light, it lessens the resistance of the device. This received light causes the electricity to flow through easier, at a faster rate. Another simple sensor measures the change in resistance and usually sends a signal to the primary part of the device, turning it on or off. Resistors can be calibrated, as the electric force is measured based on the effect the light sources have on the cell. This leads to a change in how or when the cell sends signals to another part of the device.
Manufacturers make photocells in many different sizes. The cheapest are made with simple plastic coverings and a ceramic structure. Two electrodes are set on either side of the cell, connected to wire terminals through which the electric charge passes. The light sensor is placed between these two electrodes, made from a photoconductive material that turns the light into electric energy. This is covered by a clear protective shell.
These type of cells are also called CdS (Cadmium-Sulfide) cells, light-dependent resistors (LDRs) and photoresistors. They are usually sold separately in categories based on resistance range, or what range they can measure, from light to darkness. Most cells measure light between violet and orange rays on the spectrum. They usually aren't larger than 1/2 inch in diameter and are usually only about 1/5 inch.
Photocells are also used in a variety of applications. Simple, uncomplicated versions are used in toys that may be activated by extra light or sound. More complicated versions may be used in motion-activated lights or alarms. The photocell senses an incoming amount of light and sends a signal when that light source is changed or disrupted. These are different versions from the scientific photocells used to measure more precise properties of light.