The normally open (NO) proximity switch symbol represents a non-contact sensor that keeps its output circuit open (off) until a metallic or dielectric target enters its electromagnetic or electrostatic field. In the IEC 60617 standard, it is drawn as a standard NO limit switch contact paired with a diamond-shaped actuator or a dashed line indicating non-contact operation.
Proximity Switch Symbol & Pinout Reference Table
Below is the master reference for identifying normally open proximity sensors on schematics and wiring them to programmable logic controllers (PLCs) or relay boards. This table covers the most common industrial configurations using standard M8 and M12 connectors.
| Sensor Configuration | IEC 60617 Symbol Description | NEMA / ANSI Symbol Description | Standard Wire Colors (M12) | Practical Function & Load Wiring |
|---|---|---|---|---|
| 3-Wire PNP (NO) | NO contact symbol with a diamond actuator pointing to the contacts | NO limit switch symbol with a 'wobble' actuator line or 'PROX' box | Brown (+24V), Blue (0V), Black (Output) | Outputs +24V when target is detected. Used with sinking PLC inputs (e.g., Siemens S7-1200 standard inputs). |
| 3-Wire NPN (NO) | NO contact symbol with a diamond actuator, arrow points inward on transistor | NO limit switch symbol with target line, often marked 'NPN' inside the box | Brown (+24V), Blue (0V), Black (Output) | Outputs 0V (sinks to ground) when target is detected. Used with sourcing PLC inputs (e.g., Allen-Bradley Micro800). |
| 2-Wire AC/DC (NO) | Standard NO contact with a proximity actuator, no transistor symbol | Simple NO switch in a box labeled with voltage range (e.g., 24-240V) | Brown (Line/L+), Blue (Load/L-) or Black/White | Acts as a simple mechanical switch. Beware of off-state leakage current (typically 1-2mA) which can ghost-trigger high-impedance PLC inputs. |
| 4-Wire Push-Pull (NO/NC) | Two contacts shown: one NO and one NC, sharing a single diamond actuator | Box with two switch outputs, one NO and one NC, labeled 'Push-Pull' | Brown (+V), Blue (0V), Black (NO), White (NC) | Provides both NO and NC signals simultaneously. Push-pull output can source or sink current, making it universally compatible with most modern PLCs. |
Regional Standards & The 'Rows People Get Wrong' Notes
Electrical schematics are not universally standardized, and misinterpreting a proximity sensor symbol can lead to short circuits or fried PLC input cards. The two dominant standards you will encounter are IEC 60617 (used globally and heavily in Europe/Asia) and NEMA/ANSI/JIC (used primarily in North America).
Regional Symbol Variants
- IEC 60617 (Global): Uses a distinct diamond shape on the actuator line to differentiate proximity sensors from mechanical limit switches. The transistor output (PNP/NPN) is explicitly drawn with an arrow indicating current flow direction.
- NEMA / ANSI (North America): Often relies on a standard limit switch symbol but adds a dashed target line or encloses the switch in a box labeled 'PROX', 'CAP', or 'IND'. NEMA diagrams prioritize ladder logic readability over strict component physical representation.
- Old UK (BS 3939): Largely obsolete but still found in legacy British manufacturing plants. It used a different actuator notation (often a simple circle or triangle) and outdated wire colors (Red for +V, Black for 0V, Green/Yellow for Earth). If you see these colors, the panel is pre-2004 and requires extreme caution.
The Rows People Get Wrong
When wiring or troubleshooting based on the table above, bench technicians consistently make three specific errors:
- Assuming Black is Always NO: On a 4-wire sensor, Black is the NO output, but White is the NC output. If you wire White to a PLC input expecting an NO signal, your logic will be inverted, causing immediate safety hazards on automated machinery.
- Ignoring PNP vs. NPN Load Placement: A PNP sensor sources voltage; the load (PLC input) must be connected between the Black wire and 0V. An NPN sensor sinks current; the load must be connected between +24V and the Black wire. Swapping these will result in a dead short or a permanently floating input.
- Forgetting 2-Wire Leakage Current: A 2-wire NO proximity switch requires a minimum load current to operate correctly (usually >5mA) and leaks a small current when open (<1.5mA). If wired to a high-impedance solid-state relay or a modern PLC input with an optocoupler, the leakage current can falsely trigger the 'ON' state even when no target is present. Always check the IEC 60947-5-2 datasheet for 'no-load leakage current' specs.
Frequently Asked Questions
How do I identify a normally open proximity switch symbol without a datasheet?
Look at the switch contacts in the schematic. A normally open symbol will show a physical gap between the stationary contact and the moving contact arm. The actuator line (the line extending from the contacts) will feature a diamond shape (IEC) or a dashed line/target box (NEMA) to indicate it is a proximity sensor rather than a mechanical pushbutton. If the contacts are touching in the resting state, it is normally closed (NC).
What is the exact wiring difference between PNP and NPN normally open sensors?
The physical sensor might look identical, but the internal transistor topology dictates the wiring. A PNP (Sourcing) sensor switches the positive voltage (+24V) to the Black output wire when a target is detected; your PLC input must be configured to sink this current to 0V. An NPN (Sinking) sensor switches the 0V (ground) to the Black output wire; your PLC input must be configured to source +24V through the input circuit. Connecting a PNP sensor to a sourcing PLC input card will result in the input reading 'High' permanently, regardless of the target.
Why does my 3-wire NPN normally open proximity switch not trigger my PLC?
The most common cause is a mismatch between the sensor output type and the PLC input card configuration. If you are using an NPN sensor, the PLC input card must be a 'sourcing' type (providing +24V to the input terminal). If your PLC card is strictly 'sinking' (like many standard European PLC cards designed for PNP sensors), it expects the sensor to provide +24V, which an NPN sensor cannot do. To fix this without replacing the PLC card, install an intermediary signal inverting relay or swap the sensor for a PNP equivalent. Always verify the PLC input card's common terminal wiring: for NPN sensors, the common terminal must be wired to +24V, not 0V.
