A pilot lamp is a light that illuminates under specific conditions, most commonly when an electrical circuit is energized. It may also be known as an indicator lamp or pilot light, although this should not be confused with a small flame kept burning in a gas appliance to provide a source for ignition when the appliance is turned on. Pilot lamps are used in a wide variety of settings and in some cases are required by law for safety reasons.
One example of a pilot lamp is a light used in railway signaling. Railway signaling is complex and problems with the system could pose a significant risk. Many systems have lights that activate when shorts or other electrical errors are present, alerting train engineers to the fact that a track may be unsafe because the signaling system is down. Pilot lamps can also be used to provide signals to track controllers, activating when a train is passing and completing an electrical circuit so operators know when trains are on the track.
The pilot lamp can also be used on a variety of other types of circuits, including very small electrical circuits. Many electronics use a pilot lamp to indicate when the system is energized, or when there is a problem. The illumination behind the “power” button on many electronics is a simple example; when there's a light, people know the circuit is energized. Similarly, many systems use orange or red backlighting for the power button when the system is powering up or down, or when there is a problem.
Setting up a pilot lamp is usually very easy, especially when it only needs to provide basic functionality like lighting up when an electrical circuit is energized. The lamp can be connected to the circuit so it will light up as power moves through the circuit. When power is cut, it will go out, as it will have no source of energy. Lights set up to operate in the presence of faults and other problems are slightly more complicated, especially if they are also designed to activate with a trip to turn the circuit off until the problem is resolved.
A variety of bulbs are used for pilot lamps. Usually, highly durable bulbs with a long life are used, and increasingly people rely on light emitting diodes (LEDs) for their brightness and longevity. The goal is to prevent a situation where the lamp fails and an unsafe condition is present with no indicator.