Proximity sensor
Detects the presence of a target without contact — inductive, capacitive, or photoelectric.
What it is
A proximity sensor detects whether something is present, without touching it, and gives an electrical signal a controller can read. It replaces a mechanical limit switch in many positioning jobs.
How it works
Inductive types sense metal by the effect it has on a high-frequency field — so they only detect metallic targets. Capacitive types sense most materials. Photoelectric types use a light beam: through-beam, retro-reflective, or diffuse.
Sensors are sourcing (PNP) or sinking (NPN); the type must match the controller input. Detection depends on the sensing gap, alignment, and target — set wrong, the sensor 'works' but triggers at the wrong point.
Where it's used
Position sensing, counting, presence detection, and end-of-travel on machines. Common faults: not detecting (gap/alignment/wrong type), stuck on (fixed metal in range), and intermittent (cable/vibration/noise).
What it looks like when it fails
Dead sensor (no LED at all — supply or cable), stuck on (fixed metal in range, swarf on the face, a bracket bent until the gap closed), and intermittent with vibration or cable flex — the classic conveyor fault that only shows at speed.
'Wrong point' faults are alignment, not electronics: the sensor works but triggers early or late because the gap or the target changed. On photoelectrics, a dirty lens or tired reflector does the same thing.
How to test it
Use the built-in LED first: present the target by hand and watch it switch. Then meter the output — a PNP sensor's output measured to 0 V should read supply volts when on; NPN is the mirror (output to +V). And check supply at the sensor before condemning anything.
Compare the actual gap against the rated sensing distance — inductives are rated on mild steel, and non-ferrous targets cut the range noticeably. Wiggle-test the cable at the gland while watching the LED to catch intermittents.
Gotchas
PNP vs NPN mixups: the replacement 'doesn't work' because it sinks where the input card expects sourcing. Match the output type, not just the thread size and colour.
Two-wire sensors leak a small current when 'off' — enough to hold a sensitive PLC input on. If an input never drops, suspect the sensor type before the card. And flush vs non-flush mounting matters: a non-flush sensor buried in a steel bracket is permanently made.
Safety first
Triggering a sensor by hand can operate the machine it feeds — conveyors start, cylinders stroke. Make sure any movement is expected and safe before you present a target.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Limit switch or proximity sensor not being detected
A limit switch or proximity/photo sensor isn't registering — the machine doesn't stop at position, the input never makes, or the sensor LED looks wrong for the target's position.
Proximity sensor permanently on (always made)
A proximity sensor reads 'detected' all the time — its output stays on even with no target present, so the controller always sees the input made.
Photoelectric sensor not detecting
A photo-eye (through-beam, retro-reflective, or diffuse) isn't detecting reliably — it misses the target, or the output doesn't change when something blocks/enters the beam.
Related definitions
PLC (Programmable Logic Controller)
An industrial computer that reads inputs, runs a program, and drives outputs to control machinery.
Limit switch
A mechanical switch operated by a moving part reaching a position — often for end-of-travel and safety.
Encoder
Gives precise position or speed feedback by producing pulses (or a coded value) as a shaft turns.