Capacitive Proximity Sensor

A capacitive proximity sensor detects targets by measuring the change in capacitance at its sensing face. The sensor contains a high-frequency oscillator connected to two internal electrodes that form a capacitor with the air in front of the face. When any material — metal, plastic, liquid, wood, or granular media — enters the sensing field, it changes the dielectric constant between the electrodes. This increases the capacitance, which the sensor's electronics detect and convert to a switched output.

**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 detects any material with a dielectric constant greater than air (εr > 1). This includes metals, but also plastics, liquids, glass, wood, and powders. - 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):** signal wire pulls to 0V when active. Connect to a PLC input wired to a positive common (PNP input type). Common in Asia-manufactured equipment. - **PNP (current sourcing):** signal wire pulls to supply voltage (+24 V DC) when active. Connect to a PLC input wired to a 0V common (NPN input type). Most common in European and American industrial practice.

The cable is always: brown = +24 V supply, blue = 0V/common, black = signal output. A yellow LED at the rear of the sensor lights when the output is active.

**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.