Photoelectric Sensor Symbol
Definition: The Photoelectric Sensor symbol represents an industrial proximity and detection device, typically designated PE or BG in schematics, that uses a modulated light beam (visible red, infrared, or laser) to detect the presence, absence, or position of objects without physical contact, per IEC 60947-5-2 and ANSI/IEC standards.
Also known as: photoelectric switch, optical sensor, photo eye, PE sensor, through-beam sensor, retro-reflective sensor, diffuse sensor, light barrier.
What the Photoelectric Sensor symbol means
The Photoelectric Sensor symbol denotes an optoelectronic detection device used in industrial automation, conveyor systems, and machine safety to sense objects passing through or interrupting a light beam. Unlike a photocell (which controls lighting), a photoelectric sensor outputs a discrete signal to a PLC input or control relay.
In control and automation schematics, the Photoelectric Sensor symbol appears in input circuits connected to PLC digital input modules or relay control panels. Its four pins — Sense, V+, Signal, and V− — represent the optical detection element, power supply, and logic output connections respectively.
How to identify the Photoelectric Sensor symbol
The Photoelectric Sensor symbol is drawn as a rectangle or circle representing the sensor body, with an arrow or beam symbol (chevron or dotted line) projecting from one face to indicate the emitted or received light. Four terminals are shown: V+ (positive supply), V− (negative supply/ground), Signal (the switched output), and Sense (the optical sensing face or beam reference). Some representations use the IEC 60617 general sensor symbol (a diamond or circle) with a light-wave annotation. The label 'PE' or the IEC designator 'BG' may appear on the symbol.
Function in a circuit
A Photoelectric Sensor contains an LED or laser emitter and a photodiode or phototransistor receiver. In through-beam mode, the emitter and receiver are separate units facing each other; an object interrupts the beam and the output changes state. In retro-reflective mode, emitter and receiver are in the same housing and a reflector returns the beam; an object blocks the path. In diffuse (proximity) mode, the sensor detects light reflected directly from the target. The Signal output is typically a PNP or NPN transistor switch providing a 0/1 logic level to a PLC or relay.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-5-2 (low-voltage switchgear — proximity switches) defines performance, testing, and marking requirements for photoelectric sensors. IEC 60617 represents sensors generically; specific photoelectric sensors may be drawn as a switch symbol with optoelectronic element annotation. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEC 60947-5-2 is adopted in North America. UL 508 covers industrial control equipment including photoelectric sensors used in machine control. NFPA 79 governs photoelectric sensor use in industrial machine electrical standards. |
| Key difference | IEC 60947-5-2 is the primary international performance standard and is adopted by both IEC and ANSI regions. Glyph representations vary between manufacturers and software tools; IEC drafting convention uses a generic sensor rectangle with beam annotation, while North American drawings often use a simplified circle-with-arrow or the manufacturer's custom symbol. |
Terminals / pins
| Pin | Name |
|---|---|
| sense | Sense |
| vplus | V+ |
| signal | Signal |
| vminus | V- |
Typical values
Supply voltage V+/V−: 10–30 V DC (typical NPN/PNP sensors) or 20–264 V AC (AC-powered sensors); current consumption: 15–50 mA; output current: 100–300 mA (transistor output); sensing range: 0.05–60 m depending on mode (diffuse 0.05–2 m, retro-reflective 0.1–15 m, through-beam up to 60 m); response time: 0.5–10 ms.
Where the Photoelectric Sensor symbol is used
- Conveyor systems: detecting the presence of boxes, bottles, or parts as they move along production lines, triggering counting or diverting operations
- Packaging machinery: verifying label presence, cap placement, or fill level on containers before sealing
- Machine safety (light curtains): arrays of through-beam sensors forming a safety barrier that stops machinery when a person enters the danger zone
- Part counting and inspection: counting parts on conveyor, detecting missing parts, or measuring dimensions using time-of-flight
- Door and gate automation: detecting vehicles or persons in the opening path of automatic doors, roller shutters, or parking barriers
- PLC input circuits: providing discrete 24 V DC input signals to PLC digital input modules indicating object presence or absence
Example
In a bottling line schematic, a Photoelectric Sensor (retro-reflective type) is mounted on the conveyor frame with its Sense face aimed at a reflector on the opposite rail. Its V+ and V− pins connect to the 24 V DC supply rail; its Signal (PNP output) connects to PLC input I0. When a bottle breaks the beam, the Signal output switches from 24 V to 0 V, triggering the PLC to activate a pneumatic diverter to remove the bottle from the line.
Key facts
- The Photoelectric Sensor symbol (IEC designator BG, commonly labelled PE) represents a non-contact light-beam object detection device with four terminals: Sense, V+, Signal, and V−.
- IEC 60947-5-2 is the primary performance and testing standard for industrial photoelectric proximity switches.
- The three main operating modes are through-beam (best range, separate emitter/receiver), retro-reflective (one housing with reflector), and diffuse/proximity (one housing, no reflector).
- Photoelectric sensor outputs are either PNP (sourcing — Signal goes high when object detected) or NPN (sinking — Signal goes low when object detected); the choice must match the PLC input module type.
- Supply voltage is typically 10–30 V DC for industrial sensors, or 20–264 V AC for older AC-powered models; signal output current is typically 100–300 mA.
- Sensing range varies by mode: diffuse sensors detect up to ~2 m, retro-reflective up to ~15 m, and through-beam types up to 60 m or more.
- Photoelectric sensors are not the same as photocells (dusk sensors); photocells control AC lighting circuits based on ambient light, while photoelectric sensors output digital signals for automation control.
- In safety applications (light curtains, safety barriers), photoelectric sensors must meet IEC 61496 (electro-sensitive protective equipment) for the appropriate safety integrity level.
Frequently asked questions
What does the photoelectric sensor symbol mean in a circuit diagram?
The Photoelectric Sensor symbol means that an optoelectronic object-detection device is present at that point in the circuit. It detects the presence or absence of objects using a light beam and outputs a discrete signal (typically 24 V DC logic) to a PLC input module or control relay. It does not control power circuits directly; it provides an input signal to the control system.
What does the photoelectric sensor symbol look like?
The Photoelectric Sensor symbol is drawn as a rectangle or circle (representing the sensor body) with a beam symbol (arrow or dotted line) indicating the emitted light direction. Four terminals are shown: V+ (positive supply), V− (negative supply/ground), Signal (logic output), and Sense (the detection face). The label 'PE', 'BG', or a photoeye graphic may accompany the symbol.
What are the pins on a photoelectric sensor?
A Photoelectric Sensor has four terminals: Sense (the optical detection reference), V+ (positive DC supply, typically +24 V), Signal (the switched logic output — PNP or NPN), and V− (negative supply/ground). Some sensors use a three-wire connection (V+, V−, Signal) with the Sense being the physical optical face of the sensor rather than a wiring terminal.
What is the difference between PNP and NPN output on a photoelectric sensor?
A PNP (sourcing) photoelectric sensor output goes HIGH (to V+ voltage) when an object is detected; an NPN (sinking) output goes LOW (to V− / ground) when an object is detected. PNP outputs are used with PLC input modules that have common ground (sink-type inputs), and NPN outputs with PLC inputs that have common positive supply (source-type inputs). The choice is determined by the connected PLC hardware.
What standard governs photoelectric sensors?
IEC 60947-5-2 (low-voltage switchgear and controlgear — proximity switches) is the primary international standard governing photoelectric sensor performance, testing, and marking. For safety photoelectric devices (light curtains), IEC 61496 applies. In North America, UL 508 covers industrial control equipment including photoelectric sensors.
What is the difference between a photoelectric sensor and a photocell?
A photoelectric sensor (PE sensor) is an industrial automation device that uses a focused light beam to detect object presence and outputs a 24 V DC digital signal to a PLC. A photocell (dusk sensor) is a residential/commercial device that measures ambient light level and directly switches AC power to a lighting circuit. They serve completely different purposes and operate at different voltage levels.
What are the three types of photoelectric sensor operation?
The three main types are: (1) through-beam — emitter and receiver in separate housings facing each other; object detection when the beam is interrupted; (2) retro-reflective — emitter and receiver in one housing, beam bounced off a reflector; object detected when beam is blocked; (3) diffuse (proximity) — emitter and receiver in one housing, detects light reflected off the target itself; no reflector needed but range is shorter.
Place the Photoelectric Sensor symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.