Photocells, or light dependent resistors, (LDR) are made from a piece of exposed semiconductor material such as cadmium sulphide that changes its electrical resistance from several thousand ohms in the dark to only a few hundred ohms when light falls upon it by creating hole-electron pairs in the material.
In general, when light strikes a photocell, the conductivity increases allowing more current to pass. The analogy would be in the use of a potentiometer that passes more current as the resistance goes down. Materials used as the semiconductor substrate include: lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb), which detects light in the infra-red range, and the most commonly used of all photo resistive light sensors being Cadmium Sulfide (CdS).
Cadmium sulphide is used in the manufacture of photoconductive cells because its spectral response curve closely matches that of the human eye and can be controlled using many types of light sources. Typically, it has a peak sensitivity wavelength of about 560nm to 600nm in the visible spectral range.
And, although the typical photocell is cheap and readily available, there are some downsides. The response times are relatively slow, and the variability between the same types of cell can be as high as 50 %. Despite the short comings, CdS photocells still have interesting applications. The video that follows demonstrates the light and dark sensor circuits and an unusual use of this mature technology.