Stack Light Symbol
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.
Function in a circuit
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.
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. |
Terminals / pins
| Pin | Name |
|---|---|
| com | Common |
| red | Red |
| amber | Amber |
| green | Green |
Typical values
The dominant control voltage is 24 V DC (also 24 V AC/DC, 120 V AC, and 230 V AC variants); LED segments draw roughly 20–50 mA each at 24 V DC, versus 3–7 W for legacy incandescent segments. Standard tower diameters are 30, 40, 50, 60, and 70 mm with two to five stackable segments. Sounder segments produce 85–105 dB at 1 m. Ingress protection is typically IP54–IP65. Wiring is commonly 0.5–0.75 mm² (20–18 AWG) via screw terminals, M12 connector (4-, 5-, or 8-pin), or pre-wired cable.
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)
Example
In a 24 V DC machine-control diagram, the Stack Light symbol's Common pin is wired to 0 V, while the Red, Amber, and Green pins are driven by three PLC transistor outputs (Q0.0, Q0.1, Q0.2). The PLC logic energizes Green during auto-cycle, Amber when the parts hopper runs low, and flashes Red together with a horn output when a guard-open fault trips the machine — exactly the colour semantics required by IEC 60204-1.
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.
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.
Related symbols
- Buzzer / Audible Alarm symbol
- Emergency Stop symbol
- Horn / Siren symbol
- Pilot Light / Indicator symbol
- PLC Output Module symbol
- Selector Switch 3-Position symbol
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