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What is Photoelectric Sensor?

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1. What is Photoelectric Sensor?

A photoelectric sensor is a device that emits a frequency beam of light onto a target or mirror. When an object passes by it will affect the frequency of the light receiver. This change is converted into an electrical signal due to the phenomenon of electron emission at the cathode pole when a quantity of light is shining on.

2. How does photoelectric sensor work?

The structure of the optical sensor is quite simple, including 3 main components:

+ Light Transmitter Unit
Optical sensors often use LED semiconductors. Light is emitted in pulses. A special pulse rhythm helps the sensor distinguish between sensor light and light from other sources (such as sunlight or room light). The most common types of LED are red LED, infrared LED or LED laser. Some special sensor lines use white or green LEDs. There is also a yellow LED.

+ Light Receiver Unit
Usually the light collector is a phototransistor. This unit senses light and converts it into proportional electrical signals. Many types of optical sensors now use ASIC (Application Specific Integrated Circuit). This circuit integrates all optical components, amplifiers, processing circuits and functions into one IC. The receiver unit may receive light directly from the transmitter (as in the case of a Through-beam Sensor), or light reflected from a detected object (Diffuse Reflection Sensor).

+ Output signal processing circuit
The output circuit converts an analogue signal from an optical transistor into an amplified ON / OFF signal. When the amount of light received exceeds the specified threshold level, the sensor output is activated. Although some of the previous generation sensors integrated the source circuit and used an output signal as a relay contact (relay), it was still quite common, today most types of sensors use semiconductor output signals (PNP / NPN). Some optical sensors also have output scale signals for measurement applications.

3. How many types of photoelectric sensors?

Photoelectric sensors currently have about 5 different types depending on the application. However, the working principle is still the same. But the details of each one are a little different, and you discover each type together.

3.1 Through-beam Sensors

Through-beam Sensor is a non-reflective light sensor, in order to work, a light emitting device and a light receiver are facing each other.


  • Long-detecting distance
  • Stable detecting position
  • Opaque objects detectable regardless of shape, colour or material
  • Powerful beam

The principle of operation is as follows:

  • Unobstructed state: Light emitting sensor and light collecting sensor. Emitting and receiving light continuously
  • Status with obstacles: The transmitter sensor still emits light. But light sensor does not capture light (obstructed by obstacles)

Principle of operation:
The introduction of an independent optical transceiver sensor to meet the environment with high reflectance properties, or light absorbing surface, … cannot use common transceiver sensor.

3.2 Retro Reflective Sensors

Retro Reflective Sensor is a sensor that has a light emitter and receiver on the same device. Reflector mirror is a special prism (comes with photoelectric sensor)


  • Reflector allows installation in a limited space
  • Simple wiring
  • Longer detecting distance than the diffuse-reflective sensor type
  • Easily-adjustable optical axis
  • Opaque objects detectable regardless of shape, colour, or material

Principle of operation:
When the sensor operates the light emitter will emit light to the mirror. When there are no obstructions, the mirror will reflect the light collector. When there is an obstacle, it will change the frequency of reflected light or loss of receiver light => The sensor will output electrical signals PNP, NPN

3.3 Diffuse Replective Sensors

Diffuse Replective Sensor is a type of sensor with a common receiver and transmitter. Often used to detect objects on automated systems. Monitor whether the devices are in the correct position.


  • Space-saving (requires installation of sensor unit only)
  • Adjustment of optical axis not required
  • Reflective transparent objects detectable
  • Colour differentiation possible

Principle of operation:

  • Unobstructed state: When there is no obstruction, light does not reflect back to the obtained position or the surface of the object does not reflect light to the receiving position.
  • Obstacle detection status: The sensor continuously emits light from the transmitter to the obstructing surface. The reflected light goes back to the light receiving position

3.4 Limited Reflective Sensors


  • Only detect objects in the limited detection area.
  • Not affected by the background color behind the sensor area.
  • Ideal for many applications that need background texting

3.5 Color Sensors

Rely on the spectrum of different colors to distinguish them

Principle of operation: Color detection optical sensor is also the type of transceiver sensor. It works similarly to diffuse reflective photo sensors. But the sensor only receives the learned color to output the signal

4. Advantages and disadvantages of photoelectric sensors


  • Detect an object but do not need to be near the object or directly touch it (Remote detection)
  • Detected from a distance (up to more than 100m)
  • Less wear, longevity and accuracy, high stability
  • Detect many different objects
  • Fast response time, sensitivity can be adjusted according to application.
  • Reasonable price.


  • The nature of the optical sensor will be changed when a light source is directly on the surface of the sensor or object
  • If multiple identical optical sensors are placed close to each other, noise may interfere with each other
  • Will operate poorly when dirty, so clean the sensor carefully

5. Application of photoelectric sensor

The main application of optical sensors is to detect many different types of objects, detect distance measurement or detect the speed of the object, … for example, detect a plastic bottle on a conveyor belt or check. See if the robotic hand has assembled the car components for installation?
Optical sensors play a very important role in the automation industry. Without an optical sensor, it’s hard to get automation.

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