Incandescent Indicator / Pilot Light Trivia

Indicator lamps, also called pilot lights, are used on just about every kind of electrical or electronic equipment imaginable.  Most new applications use Light-Emitting Diodes (LEDs), with the exception of AC mains indicators or pilot lights where neon lamps are still popular.  LED prices have dropped dramatically in recent years;  LEDs have a typical average life of 100,000 hours (more than 11 years of continuous use); LEDs are extremely resistant to shock and vibration, LED's run cool, and there are many other advantages.  It is no surprise LEDs have become so popular.  Incandescent indicators are seldom seen on new equipment, but were very common in the past.  The balance of this discussion focuses on various tricks to avoid frequent relamping of incandescent indicators.  (Note the terms bulb and lamp are used interchangeably herein.)

Longer Life Incandescent Replacements

Operating voltage has a profound effect on incandescent bulb life.  See the Approximate Effect of Incandescent Lamp Voltage on Light Output and Bulb Life table below for a rough idea of the relationship.  The dynamic range of the human eye is such that some reduction in light intensity can be tolerated or even barely noticeable in exchange for dramatically longer bulb life by selecting a higher voltage rated lamp.  In fact, the longer lived 12 volt bulbs in common use are actually 14 volt rated lamps.  A knowledge of the operating voltage / light output / bulb life relationship allows the selection of a lamp which will yield sufficient illumination for the application, yet possibly last much longer than the "standard" bulb that came with the equipment.

A Few Tried and True Incandescent Substitutions

There are some notable and easily obtainable bulb substitutes that bear mentioning.  For 12 volt applications, the very common 1815 miniature bayonet based lamp with a rated life of 3,000 hours can usually be replaced with the 1893 lamp rated at 7,500 hours.  For 24 volt applications, the 1819 miniature bayonet based lamp with a rated life of 2,500 hours can usually be replaced with the 757 lamp rated at a nice 15,000 hours.  The above two substitutions are unusual in that the longer lived bulbs (1893 and 757) actually produce more light than the lamps being replaced at the expense of consuming a bit more power.

For 6 volt applications, the common 47 miniature bayonet based lamp with a rated life of 3,000 hours can usually be replaced with the 755 lamp rated at an amazing 20,000 hours.  For 24 volt applications, the 327 midget flange based lamp with a rated life of 4,000 hours can usually be replaced with the 376 lamp rated at an extraordinary 25,000 hours, or if a light output reduction is tolerable, the 385 with a mind boggling rated life of 50,000 hours can be used.

For automotive indicator and marker applications, the 168 and 194 wedge based lamps with rated lives of 1,500 and 2,500 hours can usually be replaced with the 193 lamp rated at 5,000 hours.  The above listed specific substitutions are just a sampling of a few that have proven to be useful; more detailed information is contained in the table below.

NEON Replacements

If the voltage is high enough and the available colors can be used, neon lamps are a good choice to replace incandescent bulbs.  The 120MB  120PSB 130 volt miniature bayonet based lamps are notoriously short lived despite their published life rating of 10,000 hours perhaps due to sensitivity to vibration; we have sometimes had luck replacing them with the NE-51H (B2A) neon lamp rated at 25,000 hours (a 47 kilohm resistor must be added).  In any neon bulb retrofit, make sure the proper series resistor is installed.

LED Replacements

The advantages of LEDs are discussed briefly above.  For new projects, we almost always use LED indicators except for the occasional neon indicator as a basic power status pilot light.  Several manufacturers now offer replacement LED assemblies for various voltages and with an assortment of lamp bases.  These products typically internally contain the required current limiting resistor for the particular rated voltage, and are thus direct drop-in replacements with no lamp holder modifications required.  Some use a cluster of LEDs as in the automotive tail light replacement lamp assemblies now available.  These LED replacements tend to be relatively expensive, but given the potential for a 100,000 hour run between relampings, they can be quite attractive.

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Last 10-20-10