Photocells are sensors that allow you to detect light and hence detect the motion. A Photocell is basically a resistor that changes its resistive value (in ohms) depending on how much light is shining. A photocell consists of a semi cylindrical photo sensitive metal plate (emitter) and a
wire loop (collector) supported in an evacuated glass. When light of suitable wavelength falls on the emitter, photoelectrons are emitted and are drawn to the collector and photocurrent is obtained. A photocell converts a change in intensity of illumination into a change in photocurrent. This current can be used to operate control systems and in light measuring devices. They are used in a control of a counting device which records every interruption of the light beam caused by a person. So photocells help count the persons entering an auditorium provided they enter the hall one by one. In burglar alarm, ultraviolet light is continuously made to fall on a photocell installed at the doorway. A person entering the door interrupts the beam falling on the photocell.
