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 (output pulls to 0 V), while PNP sensors source current (output pulls to supply voltage). Most modern PLC input cards accept either, but it is critical to match the sensor wiring to the input card's common polarity.

Photoeyes are used everywhere in manufacturing: counting parts on conveyors, detecting label presence on containers, confirming stack heights, and triggering reject gates. Their non-contact nature means no mechanical wear, and response times under 1 ms make them suitable for high-speed lines running hundreds of parts per minute.