Stack lights andon light (aka: signal tower lights, indicator lights, andon lights, warning lights, industrial signal lights, or tower lights) are commonly used on equipment in industrial manufacturing and process control environments to provide visual and audible indicators of a machine's status to machine operators, technicians, production managers and factory personnel. It is a form of andon - systems in manufacturing which identify errors as they happen.

andon light are used in a variety of machines and process environments; specific colour-coding is assigned by the system designer.

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.

Common applications include, but are not limited to:

In conjunction with SCADA supervisory control systems and user interface/HMIs: SCADA/HMIs provide more specific machine/process status data; stack lights complement them by providing visual/audible feedback away from the machine operator console.

Assembly station workcells

Maintenance call stations

CNC machining equipment and process monitoring and feedback

Broadcast studios (commonly used in broadcast radio studios) to display status of things such as a studio on air, live microphones, phone calls and even as a doorbell in an environment where silent indication is critical.