Capacitive Proximity Sensor

A capacitive proximity sensor detects targets by measuring a change in capacitance at its sensing face. The sensor contains a high-frequency oscillator connected to internal electrodes. Many metals, plastics, liquids, wood products, and granular media create a detectable change, but reliable range depends on target size, dielectric properties, distance, mounting, and sensitivity adjustment.

**Capacitive vs inductive proximity sensors** — this is the most important distinction: - An inductive sensor detects only conductive metal targets. It is unaffected by non-metallic objects. - A capacitive sensor can detect many conductive and non-conductive materials when their dielectric contrast and target geometry produce enough signal. Verify the application with the selected sensor rather than assuming every target is detectable. - Use inductive for metal-only detection in oily environments; use capacitive when the target is non-metallic or when you need to sense through a thin non-metallic wall.

**Sensing through containers** is a key application. A capacitive sensor can be mounted flush against the outside of a plastic tank or conveyor guard and detect the presence of liquid or product inside — the material changes the effective capacitance through the tank wall. The sensitivity potentiometer on the sensor is adjusted so the sensor triggers on liquid but not on the tank wall alone.

**NPN and PNP output wiring** follows the same three-wire convention as inductive sensors: - **NPN (current sinking):** the signal switches toward 0 V when active, so the input circuit must source compatible current through the load. - **PNP (current sourcing):** the signal switches toward the positive supply when active, so the input circuit must provide a compatible path to 0 V.

Brown for positive supply, blue for 0 V, and black for the primary output are common IEC-style conventions on three-wire DC sensors, but connector pinouts, cable colors, extra outputs, and indicator behavior vary. Verify the exact sensor and PLC input manuals before wiring; never choose the input common from a naming shortcut alone.

**Sensitivity adjustment** is unique to capacitive sensors — a potentiometer on the body lets you tune the sensing threshold. This is necessary because different materials have very different dielectric constants: water (εr ≈ 80) triggers much more strongly than dry cardboard (εr ≈ 2–4). The adjustment prevents false triggering on condensation, dust coating on the face, or thin container walls.