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To provide visual and audible indicators of the status of a machine to equipment in industrial manufacturing and process control environments. This stack light is also known as signal tower lights, indicator lights, andon lights, warning lights, industrial signal lights, and tower lights. Stack lights are commonly used on equipment in industrial manufacturing and process control environments. It is a type of andon system, which is used in manufacturing to detect faults as they occur in the production process. 

Stack lights are employed in applications that are comparable to those of beacon lights and strobes. However, the information that they normally display spans a greater range of machine and process conditions. Stack lights are commonly illuminated by incandescent, LED, or xenon-type strobes as a source of illumination.

patlite revolving lights

Color-coded indicator segments are layered on top of one another in a “stacked” orientation in stack lights, which are generally columnar structures that can take on a number of geometries. A stack light will normally contain up to five segments that are each coloured differently to signify different situations on the machine or during the process.

Segments in any combination of usually red, yellow, green, blue, or clear white are actuated independently and can be turned off, solid-on (continuous), or flashing depending on their configuration.

To control stack lights, programmable logic controllers, distributed control systems, personal computer control systems, or machine controls such as timers, sensors, and latching relays must be hardwired to the stack light circuit.

Discrete signals activate lit segments at standard industrial control voltages (including 12Vdc, 24Vac/dc, 115Vac, and 230Vac), whereas continuous signals operate at lower voltages. Some devices are capable of fieldbus networked control using popular industrial networks such as Modbus, DeviceNet, Profibus, CAN-Open, or ASi, while others are not. The most advanced are the Patlite revolving lights

Flashing control may be given by the stack light’s internal circuitry, or it may be controlled externally by timers or logic controllers, depending on the configuration. Stack lights are offered for use in a variety of industrial situations, including washdown (IP65) and explosion proof applications.

There are specific colours for a specific purpose. 

Commonly used colour codes for machine state conditions include:

  • RED: Failure conditions such as an emergency stop or machine fault
  • YELLOW: Warnings such as over-temperature or over-pressure conditions
  • GREEN: Normal machine or process operation
  • BLUE: External help request, where an operator might be requesting raw materials, scheduling or maintenance personnel assistance
  • WHITE: User-defined conditions to a specific machine, often related to productivity monitoring optionally an audible alarm buzzer, typically in the range of 70–105dB, may be added to alert machine operators to high priority conditions.

The international standard IEC60073 handles machine status colour-coding and audible warning, which can be applied to a wide variety of devices such as panel pilots and stack lights. 

In most cases, machine operator action is required in the red and yellow machine states, as they are often mistakes or warnings, respectively. Intervention with a manual device may be required in the blue and white circumstances.