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).
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.