Photoelectric Sensor Working Principle

Photoelectric sensor

A photoelectric sensor is a device used to detect the distance, absence, or presence of an object by using a light transmitter, often infrared or LED, and a photoelectric receiver. Photoelectric sensors respond to the presence of all types of objects, be it larger, small, transparent or opaque, shiny or dull, static or motion. They can sense objects from distance of a few mm up to 100m.

Photoelectric sensors use an emitter unit to produce a beam of light that is detected by a receiver. When the beam is broken by any external object, a presence is Mechatronics, Sensors and Transducers detected. The emitter light source is light – emitting diodes (LED) that emit light when current is applied. The photo detector receiver contains a phototransistor that or produces a current when light falls upon it.

There are two modes of detection for photoelectric sensors.

  1. Through-beam
  2. Retroreflective

A through beam arrangement consists of separate emitter and receiver elements located opposite each other as shown in image. Therefore the light emitted by  the emitter falls directly on the receiver. In this mode, an object is detected when the light beam is blocked from getting to the receiver from the transmitter.


A retroreflective arrangement places the transmitter and receiver at the same location and uses a reflector to bounce the light beam back from the transmitter to the receiver as shown in image. An object is sensed when the beam is interrupted and fails to reach the receiver.

The detecting range of a photoelectric sensor is its “field of view”, or the maximum distance the sensor can retrieve information from, minus the minimum distance. A minimum detectable object is the smallest object the sensor can detect. More accurate sensors can often have minimum detectable objects of minuscule size.

Proximity Sensors

  1. Optical encoders
  2. Hall effect sensors
  3. Capacitive sensors
  4. Eddy current proximity sensors
  5. Inductive proximity sensors
  6. Pneumatic proximity sensor
  7. Proximity switches

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Santhakumar Raja

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