Photoelectric Sensor (Photoeye)

A photoelectric sensor — often called a photoeye — detects objects without physical contact. It emits a beam of light (usually infrared) and monitors whether that beam reaches a receiver. When an object interrupts or reflects the beam, the sensor's output switches state, giving the PLC a discrete ON or OFF signal on an input address like %I0.0.

There are three common operating modes. In through-beam (or opposed) mode the emitter and receiver are separate units facing each other; an object crossing the gap breaks the beam. Retro-reflective mode uses a single housing that both emits and detects light bounced off a reflective target — the object breaks the return path. Diffuse mode has the emitter and receiver in the same housing and relies on light scattered back from the object's surface itself.

NPN sensors sink current (the output switches toward 0 V), while PNP sensors source current (the output switches toward the positive supply). A PLC input channel is designed for a particular current direction or has a configurable common; many modules do not accept both on the same wiring. Check the input-module manual and match its common polarity, voltage range, and input current before connecting the sensor.

Photoeyes are widely used for counting parts, detecting labels, confirming stack heights, and triggering reject gates. Their non-contact sensing avoids mechanical contact wear. Some models respond in less than 1 ms, while others trade speed for range, filtering, or measurement capability; use the selected model's response time and the PLC input/filter timing to verify a high-speed application.