Proximity Sensor: Inductive Capacitive Ultrasonic and Photoelectric

Proximity sensor are used to detect nearby objects without coming in physical contact. Their applications include detection, position, inspection and counting in automated machines and consumer products such as mobile phones. In this article we will discuss various types of proximity sensors, their working principal and applications.

What is Proximity Sensor?

Proximity sensor are used to detect the presence of nearby objects without coming in physical contact. They works by emitting an electromagnetic field and change in field is detected by proximity sensor. 

Types of Proximity Sensor ?

Various types of proximity sensing technologies are available to detect different objects in different scenarios.

  • Inductive Sensor
  • Capacitive Sensor
  • Ultrasonic Sensor
  • Photoelectric Sensor
  • Magnetic Sensor
Inductive Proximity Sensor

Inductive Proximity sensors are used to detect metallic / ferrous objects only. Their range is limited by magnetic field generated by sensor. Therefore Inductive sensors have narrow sensing range.

Inductive sensors works by creating a symmetrical, oscillating magnetic field close to the sensing surface. Presence of metallic object near sensing area cause the change in magnetic field. This change in field is detected by sensor.

Capacitive Proximity Sensor

Capacitive sensors can detect both metal and non metallic objects. They detect the change in capacitance between the sensing object and the sensor. Objects that can be detected using capacitive sensors depends on their dielectric constant.

Working Principle

Capacitive sensor works as an open capacitor with two plates and air acts as an insulator between them. One plate capacitance is measured whereas other plate works as sensing surface. Any change in capacitance between two plates is detected. 

Similar to inductive proximity sensor, Capacitive proximity sensors have small range.

Magnetic Proximity Sensor

Magnetic proximity sensors are used to detect magnets only. They are actuated by the presence of magnetic field. Their operating range is high compared to inductive sensors in very small size. 

Magnet Proximity sensor can also detect magnet objects through non-ferrous metal, plastic or wood walls.

Photoelectric Proximity Sensor

Photoelectric proximity sensors utilize light sensitive element to detect objects. These sensors consist of an emitter and a light receiver. They have range from few mm to 60 meter. 

Photo electric sensors are classified in three types based on the way light beam is emitted and received by sensor.

Through Beam Photoelectric Proximity Sensor

In through beam photoelectric proximity sensors, light emitter and receiver sensors are housed in different housing. Any interrupt in between light emitter and receiver is detected by the sensor. 

These sensors can be used to detect objects at relatively larger distances. They have range upto 60 meter. 

Retro-Reflective (Reflection with Reflector) Photoelectric Sensor

In Retro reflective proximity sensors, both emitter and receiver are housed in same housing. Reflector is required to reflect light back to the sensor. Any object in between sensor and reflector is detected by the sensor. These sensors have range approx. 10 meter.

Compared to through beam proximity sensors, retro-reflective proximity sensors offer an advantage of convenience of wiring and saves cost.

Diffuse Light Type (Direct Reflection) Photoelectric Proximity Sensor

In diffuse light type proximity sensor both emitter and receiver are housed in the same housing. In this light emitted by sensor is reflected by object, back to the receiver.

Diffuse light sensors has applications to detect objects at relatively small distance.

Ultrasonic Proximity Sensor

Ultrasonic sensors utilize sound waves (higher than human audible limit ~ 25 to 50 KHz) to detect objects. Therefore object color and transparency do not affect their ability to detect an object. Ultrasonic Proximity sensors has application in manufacturing, robotics and automation for detecting long range objects.

Working Principle

Sensor sends an ultrasonic pulse and receive it back. Time difference between send and received signal time is used to calculate the distance between the objects.

Applications of Proximity Sensors

  •  Used on automated production lines for object detection, position, inspection and counting.
  • For detecting objects on industrial conveyor system.
  • For collision detection on robots.
  • Capacitive proximity sensors are used as touch switch on consumer electronics products.
  • Diffuse sensors are used in public washrooms, where they control water flow.


To sum up, Proximity sensor is a sensor that can detect the presence of nearby objects without coming in physical contact. They have applications in consumer electronics products, robotics, manufacturing and industrial automation.  

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