WiremanWireman · Photo-eye or proximity sensor not detecting: troubleshooting for electricians · 2023 NEC ·
Photo-eye or proximity sensor not detecting
Power, the sensor’s own LED, the output, then alignment and target.
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How it works
Most industrial sensors are 3-wire DC devices: brown is +V (often 10–30 V DC, check the label), blue is 0 V, and black is the switched output. A PNP (sourcing) output switches +V onto the black wire; an NPN (sinking) output switches the black wire to 0 V. Inductive proximity sensors see only metal, and their range drops for non-ferrous metals like aluminum or brass. Photo-eyes need a clean lens, alignment and the right mode (light-operate or dark-operate).
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Test: Check power at the sensorExpect: Within the sensor’s supply range on its label, commonly 24 V DC.
- No voltage or lowNo power to the sensor
- Power OKDoes the sensor’s output LED change when the target is there?
- LED changes, PLC or relay doesn’t see itTest: Measure the output wireExpect: About the supply voltage when the output is on, near 0 V when it’s off.
- Switches correctlySensor works; check the input side
- Doesn’t switch, LED doesFailed sensor output
- LED never changesWhat kind of sensor is it?
- Inductive or capacitive proxTarget out of range or the wrong material
- Photo-eye (thru-beam, reflector or diffuse)Alignment, dirt or the wrong mode
- LED stays on all the timeSensor always sees something
- LED changes, PLC or relay doesn’t see itTest: Measure the output wireExpect: About the supply voltage when the output is on, near 0 V when it’s off.