Capacitive proximity sensor

A capacitive proximity sensor detects almost any material: plastic, wood, glass, paper, grain, water and of course metal. It is the go-to sensor for level detection and for non-metal parts.

How it works

The sensing face is one plate of a capacitor; the surroundings form the other. When an object enters the field, it changes the capacitance, because every material has a dielectric constant higher than air. The sensor's oscillator reacts to that change and switches the output.

The higher the dielectric constant, the easier the material is to detect:

Typical uses

• Level detection: mounted on the outside of a plastic or glass tank, it sees the liquid or granules through the wall.
• Detecting plastic bottles, cardboard boxes or wooden parts on a conveyor.
• Checking that a bag or hopper contains powder.

Sensitivity adjustment

Most capacitive sensors have a potentiometer (or teach button). Turn it up until the sensor reliably detects the target, then back slightly so it does not trigger on the tank wall, dust or humidity. When sensing through a wall, set it with the tank empty so the wall alone does not trigger it.

Things to watch

• Moisture, condensation and build-up on the face can cause false triggers.
• Range is shorter than an inductive sensor's on metal, and depends heavily on the material.
• Flush and non-flush versions exist, as with inductive sensors.

Capacitive vs inductive

A neat trick from the sorting line on our home page: use one inductive and one capacitive sensor at the same spot. Both on = metal part; only the capacitive one on = non-metal part.

Wiring is the same as any 3-wire DC sensor: see PNP vs NPN.