Proximity Sensor (Inductive) Symbol
Definition: The Proximity Sensor (Inductive) symbol represents a non-contact electronic sensor that detects the presence of ferrous and non-ferrous metal targets within its sensing range by measuring eddy-current losses induced in the target by an oscillating electromagnetic field, depicted in schematic diagrams as a rectangular sensor block with Sense, V+, Signal, and V- pins per IEC 60947-5-2 and IEC 60617 conventions.
Also known as: inductive proximity sensor, inductive prox, metal detector switch, inductive switch, eddy-current sensor.
What the Proximity Sensor (Inductive) symbol means
The Proximity Sensor (Inductive) symbol in a control-system schematic indicates a sensor that detects metal objects without physical contact by generating an oscillating magnetic field at its sensing face and detecting the energy absorbed by metallic targets placed within that field. Inductive proximity sensors reliably detect all metals — steel, aluminium, brass, copper — with response times typically under 1 ms.
In wiring diagrams, the inductive proximity sensor symbol communicates that the Signal output switches when a metal target enters the sensing field. The symbol distinguishes this sensor from capacitive types (which detect non-metals as well) and photoelectric types (which require optical line-of-sight), making it the standard choice for metal position detection, machine guarding, and high-speed counting applications in industrial automation.
How to identify the Proximity Sensor (Inductive) symbol
The inductive proximity sensor symbol is drawn as a rectangle with rounded corners representing the sensor body, with a short vertical Sense line entering the top (the active detection face) and three vertical lines exiting the bottom representing V+ (supply positive), Signal (switched output), and V- (supply negative / ground). Two arc-shaped lines inside the rectangle — resembling an inductor or coil cross-section — distinguish the inductive sensor from the capacitive type, which uses parallel vertical lines (capacitor-style) inside the same block outline.
Function in a circuit
An inductive proximity sensor contains an LC oscillator circuit whose coil is embedded in the sensing face. The oscillating coil generates an alternating magnetic field extending a few millimetres in front of the sensor. When a conductive metal target enters this field, eddy currents are induced in the target, absorbing energy from the oscillator and reducing its amplitude. An internal evaluation circuit monitors this amplitude: when it drops below a threshold (target present), the output transistor switches, producing an NPN (current sink) or PNP (current source) digital output. When the target moves away, the oscillator amplitude recovers and the output resets.
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 full performance, test, and marking requirements for inductive proximity switches. IEC 60617 general sensor block symbols and IEC 81346 designation rules apply to schematic representation. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 provides general sensor/transducer block symbols. No dedicated ANSI symbol exists for inductive proximity sensors; the IEC block-symbol convention is standard in North American industrial schematics as well. Electrical ratings comply with NEMA and UL standards. |
| Key difference | IEC 60947-5-2 fully governs inductive proximity sensor performance for international markets. North American practice follows the same IEC-derived block-symbol convention with NEMA electrical ratings. The schematic symbol is identical across both traditions. |
Terminals / pins
| Pin | Name |
|---|---|
| sense | Sense |
| vplus | V+ |
| signal | Signal |
| vminus | V- |
Typical values
Supply voltage: 10–30 V DC (3-wire DC types), 20–250 V AC (2-wire AC types). Output current: typically 100–400 mA. Sensing range (Sn): 1–40 mm depending on body size (M8: 1–2 mm; M12: 2–4 mm; M18: 5–8 mm; M30: 10–15 mm). Switching frequency: up to 3000 Hz. Operating temperature: -25 °C to +70 °C typical.
Where the Proximity Sensor (Inductive) symbol is used
- End-of-stroke detection on pneumatic and hydraulic cylinders with steel or aluminium piston rods
- Gear-tooth counting and rotational speed measurement on metal gear wheels and camshafts
- Part presence detection on metal stamping, machining, and welding production lines
- Conveyor pallet position confirmation in automated material handling systems
- Safety guarding interlocks detecting metal safety doors and guards without mechanical contact
- Tool-length measurement and broken-tool detection in CNC machining centres
Example
In an automated machining cell, the inductive proximity sensor symbol is positioned with its Sense pin adjacent to the tool holder carousel; the Signal pin connects to a PLC input DI2. When a steel tool is present in the pocket, the sensor detects the metal and switches Signal high, confirming tool presence to the PLC tool-change sequence.
Key facts
- The Proximity Sensor (Inductive) symbol is a rectangular block with a Sense (detection face) pin and three wiring pins: V+ (supply positive), Signal (switched output), and V- (ground), with arc-shaped inductor lines inside the rectangle per IEC 60947-5-2 and IEC 60617.
- Inductive proximity sensors detect only conductive metal targets by sensing eddy-current losses; they cannot detect plastics, wood, glass, or liquids, unlike capacitive proximity sensors.
- Standard output types are NPN (current sink) and PNP (current source), each available in normally open (NO) or normally closed (NC) configurations, with 3-wire DC or 2-wire AC variants.
- Nominal sensing range (Sn) is specified for a standard mild-steel target (Fe 360); for non-ferrous metals such as aluminium and copper, the effective range is shorter — typically 40–60% of Sn.
- IEC 60947-5-2 defines the inductive proximity switch standard; short-range cylindrical types are available in M8, M12, M18, and M30 body diameters.
- Switching frequency of inductive proximity sensors can reach 3000 Hz or higher, enabling pulse counting and speed measurement of rapidly rotating metal components.
- The standard schematic designator for an inductive proximity sensor is B (transducer) or SQ (proximity switch) per IEC 81346, or PROX/PX in North American practice, followed by a reference number.
Diagrams that use this symbol
- proximity sensor diagram
- 3 wire proximity sensor wiring diagram
- proximity sensor wiring diagram
- proximity sensor circuit diagram
Frequently asked questions
What does the inductive proximity sensor symbol look like?
The inductive proximity sensor symbol is a rectangle with a Sense line at the top and three lines at the bottom for V+, Signal, and V-. Inside the rectangle, two arc-shaped lines (representing the inductive coil) distinguish it from the capacitive sensor, which uses two parallel straight lines inside the same block shape.
What does the proximity sensor (inductive) symbol mean in a wiring diagram?
The inductive proximity sensor symbol indicates a non-contact metal detector at that point in the circuit. When a metal target enters the sensor's magnetic field, the Signal output switches, allowing a PLC or control circuit to detect the target's presence without physical contact.
What metals can an inductive proximity sensor detect?
Inductive proximity sensors detect all electrically conductive metals, including ferrous metals (steel, cast iron) and non-ferrous metals (aluminium, copper, brass, stainless steel). The effective sensing range is longest for mild steel and shorter for non-ferrous metals — typically 40–60% of the rated range for aluminium.
What is the difference between inductive and capacitive proximity sensors?
An inductive proximity sensor detects only conductive metals using eddy-current effects. A capacitive proximity sensor detects any material with a dielectric constant above air — including plastics, liquids, powders, and wood — using electric field changes. Inductive sensors are preferred for metal detection in harsh environments; capacitive sensors are used for non-metallic targets or through-wall level sensing.
What standard defines the inductive proximity sensor?
IEC 60947-5-2 (Low-voltage switchgear — Proximity switches) defines the performance requirements, test methods, and markings for inductive proximity switches. Schematic block symbol conventions follow IEC 60617 and IEC 81346.
What are the pins on an inductive proximity sensor schematic symbol?
The inductive proximity sensor symbol shows four pins: Sense (detection face at top), V+ (positive supply, brown wire, +24 V DC typically), Signal (switched output, black wire — NPN or PNP), and V- (negative supply / ground, blue wire). This matches the standard 3-wire IEC sensor cable colour code.
What is the designator letter for an inductive proximity sensor?
Per IEC 81346, inductive proximity sensors are designated B (transducer/sensor) or SQ (proximity switch) with a sequential number, such as SQ1 or B3. In North American industrial schematics, the designator PROX or PX followed by a number is also common.
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