Proximity Sensor (Capacitive) Symbol
Definition: The Proximity Sensor (Capacitive) symbol represents a non-contact electronic sensor that detects the presence of any material — metallic or non-metallic — within its sensing range by measuring changes in capacitance caused by the target's dielectric properties, depicted in schematic diagrams as a rectangular sensor block with Sense, V+, Signal, and V- pins per IEC 60617 and industrial sensor wiring conventions.
Also known as: capacitive proximity sensor, capacitive prox, capacitive switch, non-metal detector, CAP sensor.
What the Proximity Sensor (Capacitive) symbol means
The Proximity Sensor (Capacitive) symbol in a control-system schematic indicates a sensor that detects targets without physical contact by sensing changes in the electric field capacitance in front of its active face. Capacitive proximity sensors can detect metals, plastics, wood, liquids, powders, and glass — any material with a relative permittivity (dielectric constant) greater than air.
In wiring diagrams, the capacitive proximity sensor symbol communicates that the Signal output switches from low to high (or high to low for NC types) when a target enters the sensing field. The symbol distinguishes this sensor from inductive types (which detect metals only) and photoelectric types (which require optical transmission), making it the appropriate choice for level detection, material presence sensing, and non-metallic target detection.
How to identify the Proximity Sensor (Capacitive) symbol
The capacitive proximity sensor symbol is drawn as a rectangle with rounded corners representing the sensor body, with a short vertical line entering the top edge (the Sense detection face), and three vertical lines exiting the bottom edge representing the V+ (supply positive), Signal (output), and V- (supply negative / ground) wires. Two vertical parallel lines inside the rectangle symbol indicate the capacitive element — similar to a capacitor symbol — distinguishing it from the arc-line representation used for inductive sensors.
Function in a circuit
A capacitive proximity sensor generates a high-frequency oscillating electric field at its active sensing face using internal electrodes that form a capacitor open on one side. When a target material with a dielectric constant greater than air (typically ε_r > 1.5) enters this field, it increases the effective capacitance between the electrodes, increasing the oscillation amplitude. An internal threshold circuit detects this amplitude increase and switches the output transistor, producing an NPN (current sink) or PNP (current source) output signal. Sensitivity is adjustable via a potentiometer to tune the detection range for specific target materials.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-5-2 (Low-voltage switchgear and controlgear — Control circuit devices and switching elements — Proximity switches) defines the performance and test requirements for capacitive proximity switches. IEC 60617 general sensor block symbols apply to schematic representation. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 (IEEE 315-1975) covers general sensor and transducer block symbols. NEMA standards apply to electrical ratings. No ANSI-specific dedicated symbol exists for capacitive proximity sensors; the IEC block-symbol convention is universally used in industrial schematics. |
| Key difference | IEC 60947-5-2 fully defines capacitive proximity sensor performance. ANSI/IEEE 315 provides the general block-symbol framework. Symbol representation is essentially identical across both traditions, using a rectangular block with labeled pins. |
Terminals / pins
| Pin | Name |
|---|---|
| sense | Sense |
| vplus | V+ |
| signal | Signal |
| vminus | V- |
Typical values
Supply voltage: typically 10–30 V DC (3-wire) or 20–250 V AC (2-wire). Output current: typically 100–200 mA. Sensing range: 2–20 mm for standard cylindrical types; up to 40 mm for large-face types. Switching frequency: 10–100 Hz. Operating temperature: -25 °C to +70 °C typical.
Where the Proximity Sensor (Capacitive) symbol is used
- Liquid level detection through plastic or glass tank walls without penetrating the vessel
- Bulk material level sensing for grain, powder, or granule hoppers and silos
- Plastic bottle or cap detection on high-speed packaging conveyor lines
- Detecting non-metallic targets (wood, cardboard, foam) on automated assembly lines
- Position sensing for pneumatic cylinders with non-metallic bodies (e.g. composite or aluminium with plastic fittings)
- Presence detection of PCBs or electronic assemblies in SMT pick-and-place machines
Example
In a liquid filling machine, the capacitive proximity sensor symbol is shown with its Sense pin pointing toward a plastic bottle on a conveyor; the Signal pin connects to a PLC DI (digital input) channel. When the bottle is present and filled to the correct level, the sensor's electric field detects the liquid dielectric, switching Signal high and enabling the PLC to advance the conveyor.
Key facts
- The Proximity Sensor (Capacitive) symbol is a rectangular block with a Sense (detection face) pin and three wiring pins: V+ (supply positive), Signal (switched output), and V- (ground / supply negative), per IEC 60947-5-2 and IEC 60617 block conventions.
- Capacitive proximity sensors detect any material with a dielectric constant greater than air (ε_r > 1), including metals, plastics, wood, water, and powders — unlike inductive sensors that detect metals only.
- Standard output configurations are NPN (current sink, common in Asian-manufactured sensors) and PNP (current source, common in European sensors), with NO (normally open) or NC (normally closed) contact logic.
- Sensing range for capacitive proximity sensors is typically 2–20 mm for cylindrical M18 or M30 body types and depends strongly on the target material's dielectric constant — higher dielectric materials produce longer effective sensing ranges.
- IEC 60947-5-2 defines the electrical and mechanical performance requirements for proximity switches including capacitive types; supply voltage is typically 10–30 V DC.
- Capacitive sensors can sense targets through thin non-metallic barriers (tank walls, conveyor belts up to ~2 mm thick) making them ideal for hygienic liquid level detection without process contact.
- The schematic designator for a proximity sensor is typically B (transducer/sensor) or SQ (proximity switch) followed by a reference number, e.g. B1 or SQ1 per IEC 81346 designation conventions.
Frequently asked questions
What does the capacitive proximity sensor symbol look like?
The capacitive proximity sensor symbol is a rectangle representing the sensor body with a Sense line at the top (detection face) and three lines at the bottom for V+ (supply), Signal (output), and V- (ground). Two parallel vertical lines inside the rectangle indicate the capacitive sensing element.
What does the proximity sensor (capacitive) symbol mean in a wiring diagram?
The capacitive proximity sensor symbol indicates a non-contact detector that switches its Signal output when a target material enters its electric sensing field. It signals to the reader that the connected circuit responds to material presence — including non-metals — without physical contact.
What is the difference between a capacitive and an inductive proximity sensor?
An inductive proximity sensor detects only conductive metal targets by sensing eddy currents induced in the metal. A capacitive proximity sensor detects any material with a dielectric constant greater than air — including plastics, liquids, wood, and powders — by detecting changes in capacitance at its sensing face.
What standard defines the capacitive proximity sensor?
IEC 60947-5-2 (Low-voltage switchgear and controlgear — Proximity switches) defines the requirements for capacitive proximity switches including test methods, ratings, and marking. The schematic block symbol follows IEC 60617 and IEC 81346 conventions.
What are the pins on a capacitive proximity sensor symbol?
The capacitive proximity sensor symbol shows four pins: Sense (the active detection face, at the top), V+ (positive supply, typically +24 V DC), Signal (the switched output — NPN or PNP), and V- (negative supply / ground). These correspond to the brown, blue, and black wires in standard IEC colour-coded 3-wire sensor cabling.
What is the designator letter for a proximity sensor?
Per IEC 81346, proximity sensors are designated B (transducer) or SQ (proximity switch), followed by a sequential number such as B1 or SQ3. In North American practice the designator PX or PROX is also used, followed by a number.
Can a capacitive proximity sensor detect through walls or containers?
Yes, a capacitive proximity sensor can detect targets through thin non-metallic walls such as plastic tank walls (up to approximately 2–3 mm thick), enabling liquid level sensing without penetrating the container. The sensor's sensitivity must be adjusted to ignore the container wall and respond only to the liquid or material inside.
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