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How Photoelectric Sensors Work: Detection Methods & Signal Output

Detailed Working Principle of Photoelectric Sensors

I. Basic Principle of Photoelectric Sensors

A photoelectric sensor is a non-contact sensor that detects the presence or movement of objects by emitting and receiving light.
The basic working principle is: when a target enters the detection area, it changes the transmission of the light path. The sensor detects this change and determines whether an object is present or has moved.
A photoelectric sensor typically includes three key components:

  • Light source (emitter): Emits infrared light, visible light, or laser beams.

  • Optical system: Focuses or directs the beam.

  • Receiver (photodiode, phototransistor, etc.): Receives the reflected or interrupted light and converts it into an electrical signal.

II. Three Main Detection Methods of Photoelectric Sensors

Photoelectric sensors are classified by their emission and reception method into three main types:

  1. Through-beam (Opposed)

  • Principle: The emitter and receiver are placed opposite each other, forming a beam path. An object is detected when it fully blocks the beam.

  • Features: Strong anti-interference, long detection distance (up to tens of meters), but more complex installation.

  • Applications: Heavy logistics, access control, long-distance detection.

  1. Retro-reflective

  • Principle: The emitter and receiver are on the same side. A reflector bounces the beam back. An object is detected when it interrupts the reflected beam.

  • Features: Easy installation, medium range (dozens of centimeters to a few meters), suitable for shiny or transparent objects.

  • Applications: Packaging lines, transparent bottle detection, box sorting.

  1. Diffuse-reflective

  • Principle: The sensor detects light scattered back by the object itself—no reflector needed.

  • Features: Easy setup, sensitive to object color and reflectivity, ideal for short-range detection.

  • Applications: Small part detection, assembly line recognition, label sensing.

III. Advanced Detection Modes: Background Suppression & Laser Focusing

Beyond the basic types, advanced photoelectric sensors offer more precision:

  1. Background Suppression (BGS)

  • Based on triangulation, sensors only respond to objects within a set range, ignoring background noise.

  • Common in high-precision use cases like PCB detection or automatic labeling.

  1. Laser Photoelectric Sensors

  • Use focused laser beams for pinpoint accuracy, suitable for tiny parts, long distances, or fast production lines.

IV. Signal Output Principles of Photoelectric Sensors

When the object disrupts the light path, the receiver detects the change in light intensity. Internal circuits compare this to a threshold to output signals:

  • Discrete (NPN/PNP): Used for simple object detection.

  • Analog (0–10V or 4–20mA): For continuous displacement or distance measurement.

  • IO-Link Smart Communication: Allows remote configuration and digital communication for modern automation systems.

V. Conclusion

Photoelectric sensors use light reflection, interruption, or refraction to achieve accurate detection. Simple in principle yet powerful in function, they are indispensable in modern automation—from counting on conveyor belts and displacement detection to identifying small parts. Understanding each type's working principle helps users choose the best solution for their application.

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