How to Wire a Stack Light (Andon Tower) for a Production Line

A stack light — sometimes called an andon tower or signal tower — is one of the simplest devices on a production floor and one of the most consistently miswired. They are cheap, the colors mean something obvious, and the wiring looks trivial. Then someone runs a single common wire too far, mixes 24 V DC tiers with a 120 V AC buzzer module, or hangs five lights off a PLC output that can only sink 0.5 A, and the tower dies in a way that takes an hour to figure out.

Here is how to wire one correctly the first time, whether it is driven by a PLC, relays, or just panel-mount switches.

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What a Stack Light Actually Is

A modern industrial stack light is a column of independent illuminated tiers, usually three to five high, often with an audible buzzer module on top. Each tier is its own little lamp with its own positive lead and a shared common (negative for DC, neutral for AC). They are NOT one bulb with multiple inputs.

The most common configurations:

• 24 V DC LED: standard in any modern PLC-driven panel
• 120 V AC LED or incandescent: legacy panels and some retrofit setups
• Pre-wired multi-conductor cable: each tier comes out as its own colored conductor, plus one common
• Modular tiers with terminal blocks: more flexible, more wiring

Patlite, Werma, Edwards, Banner, and AutomationDirect all make towers that follow the same basic rules. The brand does not matter much for wiring; the voltage class and whether you have continuous or flashing tiers does.

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Standard Color Meanings (ANSI/ISA-TR18.2 and IEC 60204)

Pick the colors based on convention, not preference. Mixing this up confuses operators and breaks audit trails.

• Red: machine fault, emergency stop active, critical failure requiring immediate attention
• Amber/Yellow: warning, attention needed but not stopped — low feed, near setpoint, operator action requested
• Green: normal operation, machine running and producing
• Blue: maintenance condition or specific operator-defined state (often a call for material handler)
• White: clear, ready, or an additional process state

A buzzer module is usually paired with red and amber and silenced by acknowledging the fault at the HMI.

If your facility already has a standard published in a work instruction, follow that instead. The worst thing is two adjacent machines using opposite color meanings.

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Wiring Configuration: Common Anode vs Common Cathode

This is the single most common source of confusion. Read the datasheet for your specific tower before pulling wire.

• Common positive (most 24 V DC towers, including most Patlite LE-A models): a single +24 V wire feeds the common terminal, and each tier is switched by pulling its individual lead to 0 V. This is how PLCs with sinking (NPN) outputs drive the tower.
• Common negative: 0 V is the common, and each tier is switched by applying +24 V to its lead. This is how sourcing (PNP) PLC outputs and most relay-driven schemes work.

If you wire a common-positive tower assuming common negative, the tower will not light up and you will spend an hour with a meter. Worse, if you accidentally feed 24 V to a tier on a tower that is wired for common positive, you may have applied 48 V across the LED if the common is already at +24 V — and you have just popped that tier.

Rule: identify the common terminal, identify which polarity it carries, then drive the other side of each tier with the opposite polarity through your switching element.

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Sizing Your PLC Output or Relay

A single 24 V LED tier typically draws 20 to 80 mA depending on size and brightness. A buzzer draws more — often 100 to 200 mA. A 5-tier tower with an active buzzer can pull close to 0.5 A total, but you usually only have one or two tiers on at a time.

That said, a few rules:

• Most PLC discrete outputs are rated for 0.5 A per point, 2 A per common group. A single tier per point is always fine. A buzzer on one output is also fine.
• If you are driving the entire tower from a single 24 V relay, size for the worst case (all tiers plus buzzer on simultaneously, even if logic says that will not happen). Use an interposing relay with at least 1 A contact rating.
• Never drive a tier through a long unshielded run with no flyback or load — even LED tiers benefit from a clamping diode across the load if the run is over a few meters and is switched by a mechanical relay.

If you are wiring inductive loads (older incandescent towers, mechanical buzzers, contactor coils stacked on the same output), put a flyback diode (1N4007 for 24 V) across the load. This protects the PLC output transistor.

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A Working Wiring Example (PLC, 24 V DC, Sourcing Output)

A typical setup for a 4-tier tower (red, amber, green, buzzer) driven by a PLC with PNP/sourcing outputs:

1. Run 18 AWG stranded wire from the panel to the tower
2. Land the 0 V (common) wire on the tower’s common terminal
3. Land each tier’s lead on its own output terminal block in the panel
4. Wire each terminal block to its assigned PLC output (e.g., Q0.0 = red, Q0.1 = amber, Q0.2 = green, Q0.3 = buzzer)
5. The PLC output, when energized, applies +24 V to the tier lead; current flows through the LED and back through the common
6. Add a clear ferrule on each tier lead labeled with the color so the next person who opens the panel does not guess

For a sinking (NPN) PLC, swap step 2 — land +24 V on the common and let the outputs pull each tier to 0 V.

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Mounting and Cable Management

The tower itself is usually mounted on top of the panel or on a pole bracket beside the equipment. Two things matter:

• Cable strain relief: use a proper compression gland or a strain-relief bushing where the cable enters the tower base. Tower bases are usually open at the bottom and the cable will pull out of the terminals over time without one.
• Conduit or flex: if the run goes more than a few feet, use liquidtight flex or rigid conduit to keep the cable off the floor and away from coolant.

Inside the panel, treat the tower’s tier wires as DC control wiring: keep them in a wire duct away from 480 V drive cables and shielded encoder runs.

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Common Mistakes That Kill Stack Lights

A short list of things that have all happened on real lines:

• Wrong voltage: feeding 120 V AC to a 24 V DC tier. The LED is gone instantly and usually takes the diode bridge with it. Always verify with a meter before applying power for the first time.
• Both polarities energized: someone wires +24 V to both the common and a tier lead. The tier never lights up because there is no voltage across it. Trace from the supply both directions.
• Buzzer wired to a flasher tier: some towers have a flasher module that drops voltage to about 12 V average. Wiring the buzzer through that tier makes it chirp arrhythmically and reduces its life.
• No common bonded to panel 0 V: the tower’s common terminal must be tied to the panel’s 24 V DC return. Floating commons cause intermittent lighting.
• Daisy-chained tier leads: never join two tiers’ leads together to “save a wire.” They are switched independently.

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Testing Before Handoff

Before declaring the install done:

1. Energize +24 V at the panel with all PLC outputs off — all tiers should be dark
2. Force each tier output one at a time and confirm only the correct tier illuminates
3. Force the buzzer and confirm it sounds without affecting any tier
4. Power-cycle the panel and verify all tiers default to off (or whatever the spec says)
5. Trigger a fault condition through the actual PLC program and verify the correct combination lights up
6. Walk to the floor and look at the tower from operator stations — if anyone has to lean to see the red tier, the mount is wrong

A correctly wired stack light is a quiet, reliable indicator that should outlast the rest of the machine. A miswired one is a recurring callout. Five minutes of datasheet reading and one careful meter check up front avoids both.