Stack Light Symbol

The Stack Light schematic symbol. Standard-specific details are listed below where available.

Definition: The Stack Light symbol represents a multi-segment machine status indicator (signal tower) whose stacked coloured lenses signal machine state at a distance, drawn as a vertical column of segments on a pole base with a Common terminal plus one terminal per colour (Red, Amber, Green), with colour meanings assigned by IEC 60204-1.

Also known as: signal tower, tower light, andon light, indicator tower, machine status light, signal beacon column, light tower.

What the Stack Light symbol means

The Stack Light symbol denotes a modular column of independently switched indicator segments mounted on top of a machine or control cabinet so operators and supervisors can read machine status across a production floor. Each segment is electrically a separate lamp sharing one common conductor, which is why the symbol carries a Common pin and one pin per colour: energizing the Red pin lights only the red segment, and multiple segments can be lit simultaneously.

Colour semantics are standardised by IEC 60204-1 (Safety of machinery — Electrical equipment of machines): RED = emergency, immediate action required; YELLOW/AMBER = abnormal condition, impending critical state; GREEN = normal operation; BLUE = a condition requiring mandatory operator action; WHITE = neutral/monitoring (e.g. cycle running). In lean-manufacturing (andon) systems the stack light is the visual core of the escalation process — an amber light typically summons a line supervisor before a stoppage occurs.

How to identify the Stack Light symbol

The symbol is a vertical stack of two to five rectangular or lens-shaped segments on a short pole and base, with each segment distinguished by hatching, shading, or a colour label (R/A/G), and wire stubs for Common plus one per segment. In IEC-style schematics each segment may alternatively be drawn as a standard pilot-light circle (IEC 60617 lamp symbol: circle with X or with the colour letter) grouped and mechanically linked by a dashed line to show they share one housing.

ANSI/NEMA ladder diagrams typically draw each segment as a separate pilot-light circle with the NEMA colour letter inside (R, A, G) on its own rung, with a note tying them to one tower; the physical-outline stack symbol is more common on layout and interconnection drawings than on ladder logic. The pole base and multi-terminal wiring distinguish it from a single pilot light.

Live editor identity: Live-library identity: `stack-light`, 40 × 70 canvas units, 4 logical terminals. Match this exact SVG and terminal list when identifying the placeable editor symbol; do not identify a physical package from the schematic outline alone.

Function and usage boundary

Each segment is driven by a separate control output — typically a PLC digital output, relay contact, or machine controller output — while all segments share the Common return (common-negative wiring on 24 V DC PNP-output PLCs is the most frequent arrangement). The controller logic maps machine states to colours: green steady for auto-cycle running, amber for low material or pending fault, red (often with a buzzer segment) for fault or emergency stop, flashing modes for acknowledged-but-active conditions.

Modern towers are modular: LED segments snap onto a base that provides the common bus, and an audible sounder segment (buzzer, 85–105 dB) can occupy the top position. Smart variants replace discrete wiring with IO-Link or a single multi-colour LED tube addressed serially, but the schematic contract — one control signal per indicated state — stays the same.

Usage boundary: Use the Stack Light abstraction only when the intended circuit can be expressed through these logical ports: `com` (Common), `red` (Red), `amber` (Amber), `green` (Green). Select a versioned physical profile and exact manufacturer datasheet before assigning package pins, ratings, or fabrication dimensions.

Standards: IEC vs ANSI

IEC 60617	IEC 60204-1 clause 10.3 assigns indicator colour meanings for machinery: RED (emergency), YELLOW/AMBER (abnormal), GREEN (normal), BLUE (mandatory action), WHITE (neutral). IEC 60617 provides the underlying signal-lamp symbol; IEC 60073 covers general coding principles for indicators and actuators.
ANSI/IEEE 315	NEMA ICS and ANSI Y32.2 / IEEE 315 draw each indicator as a pilot-light circle with a colour letter; NFPA 79 (Electrical Standard for Industrial Machinery) adopts the same colour semantics as IEC 60204-1 for the North American market.
Key difference	IEC schematics tend to draw the tower as grouped lamp symbols with a mechanical-link dashed line, while ANSI ladder diagrams place each colour on its own rung as an output pilot light. Colour meanings are harmonised between IEC 60204-1 and NFPA 79, so a red segment means the same thing under both regimes.

The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.

Variant availability

Editor implementation (available): This is the exact SVG and terminal geometry placed by the CircuitDiagramMaker editor.
IEC 60617 (reference only): IEC 60617 is the official international source. This page does not present the editor SVG as a licensed or certified IEC reproduction.
ANSI/IEEE 315 (reference only): IEEE 315 is the North American reference listed by IEEE as inactive-reserved; use the edition required by the project.

Logical terminals

The live editor exposes `com` (Common), `red` (Red), `amber` (Amber), `green` (Green). These are stable logical terminal IDs for diagram connectivity, not a promise that they equal physical package pin numbers.

Logical ID	Editor terminal name
com	Common
red	Red
amber	Amber
green	Green

Polarity and direction

This editor symbol does not encode a universal positive/negative orientation. Do not infer physical polarity or package pin numbering from left/right placement alone.

Reference designator

Project-defined: No single reference letter is asserted for this broad symbol. Apply the project standard and equipment-classification rules consistently.

Where the Stack Light symbol is used

CNC machine tools and machining centres signalling cycle running, program end, and alarm states
Assembly-line andon systems where amber escalates to a team leader before a line stop
Packaging and bottling machinery indicating low material, jam, and guard-open conditions
Automated warehouse and conveyor systems marking zone faults visible above racking
Injection moulding machines showing auto/semi-auto/fault status to roaming technicians
Test stands and end-of-line testers indicating pass (green), testing (amber), and fail (red)

Key facts

IEC 60204-1 defines the colour meanings: red = emergency, amber/yellow = abnormal condition, green = normal, blue = mandatory operator action, white = neutral/monitoring.
Electrically a stack light is several independent lamps sharing one common conductor — the symbol has a Common pin plus one pin per colour segment.
24 V DC is the dominant supply in modern panels; LED segments draw only 20–50 mA each, so they can be driven directly from PLC transistor outputs.
Segments are modular and stack in any order, but convention places red at the top; a sounder (buzzer) segment usually caps the tower.
Flashing is a distinct signal state from steady — many towers offer built-in flash modes, otherwise the PLC pulses the output.
NFPA 79 aligns North American machinery indicator colours with IEC 60204-1, so colour semantics are effectively global.
The stack light is the hardware backbone of the lean-manufacturing 'andon' escalation system.
Smart towers using IO-Link replace one-wire-per-colour with a single communication line, but discrete-wired towers remain the schematic default.

Common interpretation mistakes

Treating the Stack Light drawing (40 × 70 canvas units) as a physical footprint. The schematic outline expresses electrical intent; package and panel dimensions come from the selected physical profile.
Copying `com`, `red`, `amber`, `green` directly to a wire marker or manufacturer package pin number. These are logical Stack Light port IDs and require an exact ordered-part terminal map.
Assuming every unlabelled or generic Stack Light terminal has the same role across manufacturers and variants.

Frequently asked questions

What do the colours on a stack light mean?

Per IEC 60204-1 (mirrored by NFPA 79): red means emergency or a condition requiring immediate action; yellow/amber means an abnormal condition or impending critical state; green means normal operation; blue means a condition requiring mandatory operator action; and white is neutral, often used for cycle-running or call-for-material signals.

How is a stack light wired to a PLC?

Each colour segment gets its own PLC digital output, and all segments share one common conductor. With PNP (sourcing) 24 V DC outputs the tower common goes to 0 V and each output feeds a colour pin; with NPN outputs the common goes to +24 V. LED segments draw 20–50 mA, well within a typical 0.5 A transistor output rating, so no interposing relays are needed.

What is the difference between a stack light and a pilot light?

A pilot light is a single panel-mounted indicator visible mainly to someone standing at the panel; a stack light is a column of multiple high-visibility segments mounted on top of the machine to broadcast status across the floor. Schematically a stack light is drawn as grouped lamp symbols (or a segmented tower) with a shared common, whereas a pilot light is one lamp symbol.

Should the red segment be at the top or bottom of the tower?

There is no mandatory order in IEC 60204-1, but industry convention places red at the top (traffic-light order: red, amber, green descending) so the most critical signal is most visible. What matters for compliance is the colour-to-meaning assignment, not position. Sounder segments, when fitted, sit above the lenses.

What voltage are stack lights?

24 V DC is the modern standard, matching PLC output modules; 24 V AC/DC, 120 V AC, and 230 V AC versions exist for retrofits and relay-driven systems. Always match segment voltage to the driving circuit — an LED segment specified 24 V DC will not survive on 230 V AC. Power draw is roughly 0.5–1.2 W per LED segment.

Sources and verification

IEC 60617:2026 DB — Graphical symbols for diagrams (International Electrotechnical Commission) — The current official IEC graphical-symbol database and the electrotechnical areas it covers. Exact symbol identity data requires IEC access.; claim scope: standards
IEEE/ANSI 315-1975 — Graphic Symbols for Electrical and Electronics Diagrams (IEEE Standards Association) — The official catalogue record for North American diagram symbols and reference-designation letters; IEEE lists the standard as inactive-reserved.; claim scope: standards, reference_designator
IEC 61666:2010+AMD1:2021 — Identification of terminals within a system (International Electrotechnical Commission) — General principles for identifying terminals within systems; it does not replace the exact ordered-device datasheet.
IEC 81346-2:2019 — Object classes and reference-designation codes (International Electrotechnical Commission) — The official classification scheme used to form reference designations across technical disciplines.; claim scope: reference_designator
IEC 61082-1:2014 — Preparation of documents used in electrotechnology (International Electrotechnical Commission) — General presentation rules for electrotechnical diagrams, drawings, and tables.; claim scope: standards, usage

Geometry fingerprint: d530d4b0e47e5c6b. A changed SVG, canvas size, or terminal definition invalidates the recorded review.

Related symbols

Physical breadboard, PCB, and panel mapping

No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.

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