Stack lights are found everywhere in industry. And for a good reason. They are a quick and easy way to visualize the status of a machine, which otherwise would need some effort to see. They are—sort of—a mini-andon. While sometimes underappreciated, they do serve a valuable purpose in the factory, and they usually do a good job. Let’s have a look at stack lights!
Stack lights are simple stacks of lights that sit on top of a machine, line, or generally a process. They are also known as signal tower lights, indicator lights, andon lights, warning lights, industrial signal lights, or tower lights. They are commonly used in industrial settings to visually communicate important information about the status of machines, equipment, or processes. Technically they are quite simple, merely a light in a colored transparent pipe. Older machines still may have light bulbs, but nowadays low-maintenance LED lights are common. Feeding it the right data from the machine requires a bit of thought.
Stack lights can have different colors. Obviously there should be at least one color, but often there are more.These colors have different meanings, which often also differ from factory to factory. In some factories only one light is turned on at any given time, in other places multiple lights can be active simultaneously. Some factories also include blinking lights, either to increase its visibility, or to squeeze another bit of information in to distinguish it from a non-blinking light. But here I advise to keep it simple. Too many lights may be more confusing than helpful.
- Red often indicates a breakdown or stop, or generally a problem. This is probably the most common color included in a stack light. As this is often critical and needs urgent attention, the red light often also blinks for increased visibility. Sometimes a solid red light for a lack of material is used in addition to the blinking light.
- Green often indicates normal operations. Green is usually good.
- Amber or yellow may indicate a warning, minor problem, or irregularity, possibly without an actual machine stop. If you don’t want to have too many lights, this could be included in the red status light, and not every stack light includes yellow.
- Blue is often used to request help from maintenance or the supervisor. This could be used together with red. For enhanced visibility, this may also blink. A solid light is sometimes used for service needs in the near future, with a blinking blue light indicating urgent maintenance attention. It is also sometimes used to request material, or to indicate quality problems. There are indeed quite different definitions for blue lights in different factories. This is not a common color on a stack light.
- White is sometimes but not often used if you need another color for another type of information. This could be the completion of a production run, a shift change, or anything else that may be helpful. If you can’t think of anything, then don’t include or at least don’t activate the white light to avoid information overflow.
- All off: Machine is most likely powered down.
There even exists a European norm IEC 60204-1, chapter 10.3 lights and displays, where red is for emergencies, yellow for abnormalities, blue for mandatory actions, green for normal conditions, and white for any other neutral condition. Flashing indicates higher urgency, a discrepancy in status, or merely a transition between process statuses.
Be aware that there are many different opinions on what which color should mean. I even came across a definition where red lights indicate normal operation, and green lights for a machine that has stopped after the completion of a cycle. The logic is that after a stop it is safe to open the machine door, whereas you should not open the door when the machine is running (red).
Stack lights are pretty straightforward. Yet, it is possible to mess them up too. In my view, the most common problem is trying to squeeze too much information into a single stack of lights, leading to confusion. Keep it simple! Also, keep it consistent. A red blinking light should have the same meaning across the shop floor, or even the entire company. Unfortunately, the definition of the lights is often merely whatever the machine maker thought best, which may vary across machine makers and hence within your shop floor.
This in turn leads to neglect. People will ignore lights, and instead of conveying information they will blend into the background of all the bells and whistles of a modern shop floor. While stack lights are not really high tech (for exceptions, see below), they do help to get attention to where it is needed, and to provide understanding of the status of a shop floor quickly.
A smaller issue is having the stack lights well visible. If you can’t see them, they are useless. Luckily, in almost all cases they are on top of the machine and visible from faraway. With the common use of LEDs, burned-out light bulbs and other stack-light-related maintenance problems are also less common.
Historic Stack Lights
Stack lights have been around for quite some time, even before major electrification. The picture here shows a red signal on the Toyota Model G loom from 1924. The white signal is down. While I don’t know the exact definitions, I would assume that red is a problem, white is a call for attention, and both being down meaning everything is fine. Such machine indicators probably appeared around 1900. The modular stack light probably dates to around 1950, which got computerized around 1970.
Advanced Stack Lights
Stack lights are evolving. While the basic stack light is still good and useful, I have seen some variations and evolutions of stack lights. The oval brick in this picture is a KeyProd, a small computer that measures performance using a vibration sensor and indicates the status using a LED ring. This computer from JPB Systeme is an easy way to bring older machines into the digital world. I wrote a whole blog post about it on What’s Buzzing?—Vibration Sensors in Industry.
Another newer stack light I have seen is from ABB Stotz-Kontakt near Heidelberg. They had a triangular LED running text that gives more details on the error through an error code and an error text. This way it is visible from faraway what the problem at the machine is. Read more on ABB in my blog post Industry 4.0 Tour in Germany – A Van Full of Nerds – ABB Stotz-Kontakt.
Transition to Andon Lights
Andon lights generally show the status of multiple processes in a system, although it is also quite valid to see a stack light as a simple andon light. The term andon comes from the Japanese word for paper lantern (行灯, あんどん) . The picture here shows an older andon from the Toyota Museum for Industry and Technology in Nagoya.
This animation represents a more modern andon. You will find colors similar to a stack light, but multiple processes are displayed on one board. In this example a line has eight processes, of which process six is stopped and process two is expecting problems. These type of andons also often include current and target outputs, quality information, and much more. Nowadays these are often LCD monitors, and at Toyota they even switch the entire monitor display to a message if urgent attention is needed.
Overall, stack lights are easy and useful tools for visual management, and are commonly found in industry. Now, go out, light up your Christmas tree (but green only please), and organize your industry!