Introduction
A photocell is a technology application of the photoelectric effect. It is a device whose electrical properties are affected by light. It is also sometimes called an electric eye. A photocell consists of a semi- cylindrical photo- sensitive metal plate (emitter) and a wire loop. A (collector) supported in an evacuated glass or quarts bulb 💡. It is connected to the external circuit having a high- tension battery and micrommerter as shown in a picture.
Sometimes, instead of the plate C, a thin layer of photosensitive material is pasted on the inside of the bulb 💡. A part of the bulb is left clean for the light to enter it.
When light of suitable wavelength falls on the emitter C, photoelectrons are emitted. These photoelectrons are drawn of the collector. A photocurrent of the order of s few microampere can be normally obtained from a photo cell.
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. A photocell of lead as sulphide sensitive to infrared radiation is used in electronic ignition circuits.
In scientific work photo cells are used whenever it is necessary to measure the intersity of light. Light meters in photography cameras make use of photo cell to measure the intensity of incident light. The photocells, inserted in the door light electric circuit, are used as automatic door light electric circuit, are used as automatic door opener. A person approaching a doorway may interrupt a light beam which is incident on a photocell.
The abrupt change in photocurrent may be used to start a motor which opens the door or rings sn alarm. They are used in the control of a counting device which records every interruption of the beam caused by s person or object passing across the beam. So photocells help count the persons entering an auditorium, provided they enter the hall one by one.
They are used for detection of traffic law defaulters: an alarm may be sounded whenever a beam of (invisible) radiation is intercepted.
In burglar alarm (invisible) ultraviolet light is continuously made to fall on s photocell installed at the doorway. A person entering the door interrupts the beem falling on the photocell. The abrupt change in photocurrent is used to start an electric bell ringing. In fire alarm, a number of photocells are installed at suitable places in a building. In the event of breaking out of fire, light radiation fall upon the photocell. This completes the electric circuit through on electronic bell or a siren which starts operating as a warning signal.
Photocells are used in the reproduction of sound in motion pictures and in the telivision camera for scanning and telecasting scenes. They are used in industries for detecting minor flaws or holes in metal sheets.
