Stack lights and Andon systems both signal production status. You'll find both on factory floors around the world.
But they work very differently. Stack lights are passive—they display status for anyone who happens to look. Andon systems are active—they push alerts to specific responders regardless of where they are.
Understanding the difference helps you choose the right tool for your operation, or decide whether you need both.
What Stack Lights Do Well
Stack lights have been factory fixtures for decades, and for good reason.
Simple and Reliable
A stack light is straightforward: when a condition occurs, a light comes on. Red typically means stop or fault. Yellow means caution or attention needed. Green means running normally.
There's no software to configure, no network to connect, no training required. People instinctively understand what a red light means.
Visible in the Immediate Area
A bright red light on top of a machine is hard to miss if you're nearby. For local visibility—showing people in the immediate area that something needs attention—stack lights work well.
Low Cost and No Maintenance
Stack lights are inexpensive to purchase and require almost no maintenance. Hardwired lights don't need batteries. LED versions last for years.
Standard Color Coding
Red, yellow, and green have near-universal meaning in industrial settings. No interpretation required.
The Passive Alert Problem
The fundamental limitation of stack lights is that they're passive. They display information but don't push it to anyone.
"Right now it's like a light or a radio—not everybody carries the radio."
That comment from an HVAC manufacturer captures the issue. A light only works if someone sees it. What if:
No One Is Looking
The maintenance tech is on the other side of the facility. The supervisor is in a meeting. The quality inspector is at lunch. The light is on, but no one who could respond is there to see it.
The Facility Is Large
If you can't see the light from where you're standing, you don't know it's on. In large facilities, stack lights have a limited radius of usefulness.
The Call Gets Lost in Visual Noise
Walk through a busy production floor, and you'll see lights everywhere—on machines, on vehicles, on stations. One more red light may not register, especially if some are always on for various reasons.
There's No Escalation
What happens when a stack light has been on for ten minutes with no response? Nothing automatic. It just keeps shining until someone notices or the condition clears on its own.
There's No Record
Stack lights don't log anything. You can't answer questions like:
- How long was the light on?
- Did anyone respond?
- How often does this station have issues?
What Andon Systems Add
Andon systems address the limitations of passive visual alerts with active notification and data capture.
Active Notification
Instead of displaying a status and hoping someone sees it, Andon systems push alerts to responders.
One manufacturer asked: "Is this an add-on system where we're going to need to get lights out there and stuff?" The answer: "No, you don't need to get lights."
With Andon, the alert comes to you—on a pager, watch, smartphone, or computer screen—regardless of where you are in the facility. You don't need to be looking at the machine to know it needs attention.
Digital Visibility
Andon systems can replicate the visual status display of stack lights through digital dashboards.
"You'll see everything in green. Once anything gets triggered, it'll go to red and you'll know that area needs attention."
These dashboards can be displayed on any screen: dedicated monitors on the floor, TVs in break rooms, laptops in offices. Unlike a physical light that only works in one location, a digital display can show facility-wide status from anywhere on the network.
Data and Tracking
Every Andon call creates a record:
- When it started
- Who responded
- How long it took
- When it was resolved
Stack lights provide no such data because they have no way to capture it.
Escalation
Andon systems can automatically escalate unanswered calls. If no one responds within a defined time, the system notifies a backup or supervisor.
This ensures that calls don't sit indefinitely—something stack lights can't prevent.
When Stack Lights Are Enough
Stack lights aren't always the wrong choice. They work well when:
The Area Is Small
If everyone who needs to see the light can see it from their normal position, active notification may not be necessary. Small production cells, single-room operations, or stations with dedicated supervision can work with passive visual alerts.
Supervision Is Always Present
If a supervisor is stationed where they can see all relevant lights—and they're always there—they can coordinate response manually. This makes stack lights viable in closely supervised operations.
Records Don't Matter
If no one will ever ask about response times, call volumes, or patterns, the lack of data isn't a problem. Some operations simply don't need this information.
Call Volume Is Low
If problems are rare—a few per day—the limitations of stack lights matter less. The occasional missed response isn't catastrophic when issues are infrequent.
When to Upgrade to Andon
Consider upgrading from stack lights when:
Your Facility Is Large
"Currently only basic maintenance alerts with stack lights."
If responders can't see the lights from where they work, passive visual alerts aren't sufficient. Active notification becomes necessary.
You Have Multiple Response Teams
When different types of calls go to different people—maintenance, quality, materials—routing and coordination matter. Stack lights don't differentiate; they just turn on.
You Need Accountability
If you need to know who responded to what, and how quickly, stack lights can't help. They have no mechanism to capture this information.
You Need Data for Improvement
Continuous improvement requires data. If you want to identify which stations have the most issues, which shifts have the slowest response, or whether new procedures are working, you need systems that track these metrics.
Escalation Is Required
If unanswered calls must escalate automatically, stack lights aren't sufficient. They have no concept of "unanswered"—they're either on or off.
Hybrid Approaches
Many facilities use both stack lights and Andon systems. This isn't redundancy; each serves a different purpose.
Local Visual + Remote Notification
Stack lights provide immediate local visibility—the machine operator can see at a glance that the light is on. Andon provides remote notification—responders across the facility know help is needed.
Lights Trigger Andon Calls
Some systems allow stack lights or machines to trigger Andon calls automatically. When a machine faults and lights up red, it also sends an Andon alert. This bridges the two systems.
Best of Both Worlds
The combination gives you:
- Immediate visual feedback at the station
- Active notification to remote responders
- Data capture and escalation
Cost Considerations
The cost comparison isn't straightforward.
Stack Lights: Low Upfront, No Ongoing
Stack lights are inexpensive and widely available. Installation requires wiring but no network infrastructure. There's nothing to maintain beyond occasional bulb replacement.
Andon: Higher Upfront, Low Ongoing
Andon systems have higher initial costs: buttons, pagers, transmitters, software. But most don't have recurring fees for the base system.
Hidden Costs of Stack Lights
The real cost of stack lights is the cost of missed responses—and you'll never know what that cost is because there's no data. How many times did a light stay on for twenty minutes with no response? Without tracking, you can't say.
ROI Comes from Response
Andon ROI comes from faster response times and the ability to improve based on data. If response time matters to your operation, the investment in active notification typically pays back quickly.
Frequently Asked Questions
Can Andon replace stack lights entirely?
Yes, for notification purposes. Digital dashboards can show the same red/yellow/green status. However, some operations keep stack lights for immediate local visibility at the machine.
Can stack lights trigger Andon alerts?
Yes, with integration. When a light condition triggers, it can also send an Andon call. This requires connecting the light signal to the Andon system.
Which is easier to install?
Stack lights are simpler—just wire them to the machine or condition they monitor. Andon requires additional components (transmitter, pagers) but modern wireless systems minimize installation complexity.
Do I need both?
It depends. If your facility is small and closely supervised, stack lights alone may work. If it's large, if you need data, or if responders aren't always in view of the lights, you need Andon—with or without stack lights.
Making the Decision
Stack lights and Andon systems solve different problems.
Stack lights answer: "What's the status of this machine right now?" for anyone standing nearby.
Andon systems answer: "Who needs help, where, and did they get it?" for the entire operation.
If passive visual display is sufficient, stick with stack lights. If you need active notification, data, and escalation, evaluate Andon systems.
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