Q.Write any five applications of photo electric cells.

Because a photocell produces an electrical signal proportional to (or triggered by) incident light, it finds wide use wherever light needs to be converted into a control or measurement signal — from automatic switching and alarms to sound-on-film reproduction, counting, and exposure metering.

How a photocell is used, in general

A photoelectric cell (photocell) emits electrons, or changes its electrical resistance/current, when light falls on it, via the photoelectric effect. This makes it useful as a light-controlled switch or sensor in a wide variety of automated and measuring devices.

Five applications

1. Automatic switching (street lights, doors): A photocell can be arranged so that a relay circuit switches street lights on at dusk (when the light intensity drops below a threshold and the photocurrent falls) and off at dawn; the same principle is used for automatic door-opening systems, where a person interrupting a light beam falling on the cell triggers the mechanism.

2. Burglar/fire alarm systems: A beam of (often infrared) light is kept continuously falling on a photocell; if an intruder crosses and interrupts the beam, the sudden change in photocurrent triggers an alarm circuit.

3. Reproduction of sound in cinema films: The soundtrack on a cinema film is recorded as a strip of varying optical density/width alongside the picture frames. As the film moves past a steady light source, the light passing through the soundtrack (whose transmission varies with the recorded sound signal) falls on a photocell, generating a fluctuating photocurrent that, after amplification, reproduces the sound.

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