Photoelectric sensors (photo-eyes)
A photoelectric sensor, or photo-eye, detects an object when it blocks or reflects a beam of light. Photo-eyes detect almost any material at long range, which makes them the standard sensor on conveyors, packaging machines and doors.
The three main types
1. Through-beam
A separate emitter and receiver face each other. An object is detected when it breaks the beam. Through-beam gives the longest range (tens of metres) and the most reliable detection, even of dark or shiny objects, but you have to wire and align two devices.
2. Retro-reflective
Emitter and receiver are in one housing, aimed at a reflector on the other side. The object is detected when it breaks the reflected beam. Only one device to wire. Shiny objects can reflect the light back and fool it, so use a polarized retro-reflective sensor for bottles, foil and metal.
3. Diffuse (proximity)
The sensor sees the light reflected directly by the object. No reflector is needed, but the range is short and depends on colour: white objects are seen further than black ones. Background suppression versions measure the angle of the returning light and ignore anything behind a set distance, so a dark box is still detected in front of a shiny machine frame.
| Type | Range | Devices | Best for |
|---|---|---|---|
| Through-beam | longest | 2 | long distances, dirty environments, any object |
| Retro-reflective | medium | 1 + reflector | conveyors, doors |
| Diffuse / BGS | short | 1 | when there is no place for a reflector |
Light-on vs dark-on
Light-on: the output is on when the receiver sees light. Dark-on: the output is on when the light is blocked. Many sensors have a switch or a second wire to choose. For a through-beam "part present" signal you usually want dark-on.
Light sources
- Red LED: visible spot, easy to align.
- Infrared: longer range, invisible.
- Laser: tiny spot for small parts and precise positions.
Wiring
Photo-eyes are 3- or 4-wire DC devices: brown +24 V, blue 0 V, black output (PNP or NPN, see PNP vs NPN). A through-beam emitter only needs brown and blue. Some types have a white wire to select light-on/dark-on or for a second output.
Setup tips
- Align using the stability LED, not just the output LED.
- Keep lenses and reflectors clean; dust slowly reduces the margin.
- Mount it so parts cross the beam squarely and gaps between parts are larger than the spot.
- For counting, feed the signal into a counter with a rising-edge contact.
For personnel safety (not part detection) use a certified safety light curtain, never a normal photo-eye.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →