You may have noticed that the world of light emitting diodes has started to evolve rapidly. After decades of use in low output applications, high powered LEDs are now popping up in flashlights, traffic signals, and car tail-lights. The newest LEDs are starting to rival incandescent bulbs in their output, so I've been spending some time trying to figure out how we can take advantage of these little gems in our airplanes.
Why bother? Well, there is the coolness factor. But there are real benefits to LEDs too: they can be cheaper, they'll last longer than your airframe will, and they only use a tiny fraction of the power and current that an equivalent bulb would use. Sounds like a natural fit for airplanes, right?
Don't think that I'm the first person with this idea. Whelen has started to sell nav lights based on this technology ($600 a pair), and some vendors are also selling kits for the experimental market (using dozens of LEDs per nav light) for several hundred dollars. Fear not -- using the newest and brightest LEDs out there, you should be able to forge a pair of nav lights for about $60.
In most forms, Luxeons are a small circuit board about the size of a penny, with a clear dome in the middle that emits the light. The packaging shown below is called the "star" layout. It includes a number of scallops which can be used for mounting with #4 screws. You can also see that there are multiple input solder pads (2 positive, 2 negative), which allows easy and compact wiring in many different configurations.
The 1-Watt Star units sell for about $6 each, direct from Lumileds. This model takes a maximum current of 350mA and drops 3-4 volts (depending on color). Various radiation patterns are available, including some with integrated optics for a tightly focused beam. For our purposes the "lambertian" radiation pattern seems most useful; this model puts most of the light roughly straight ahead in an 80-degree cone, then tapering off linearly as you increase the viewing angle up to about 100 degrees off-axis.
How bright are they? Well, it's a little painful to look at the 1-watt model indoors. You could certainly use it as a smallish flashlight if the beam were focused tighter. I brought a three-LED demo to the meeting last month and I think everyone agreed that it was "bright enough", for nav lights anyway. There are also 3 and 5 watt models available, but not in the colors or life expectancy that we want (yet!) These higher-powered devices require careful heatsinking, and are dangerous to the eyes -- so I'm avoiding them for now.
The other big thing to think about is current regulation. While most other devices in our airplanes require a fixed voltage, LEDs require a limited current instead. The LED, being a diode, offers little resistance to any current you provide -- it trusts you not to fry it. Also like other diodes, it creates a well-defined voltage drop (which varies by color). By arranging LEDs and resistors in series we can match the voltage drops to the ship's voltage, providing a means to control the current. For example, 4 green Luxeons drop 3.4V each for a total of 13.6V, so you want your resistor to drop the remaining .8V coming from your 14.4V bus. At this known voltage, the resistor can then be sized to give us the maximum allowed current of 350mA.
So, Ohm's Law will tell us the appropriate resistance value at the maximum allowed current, and Watt's Law will give us the power rating we need on the resistor. If that was Greek to you, no problem, search on the web for "LED resistor calculator" (or check the links below) which will do all the math for you. Note that the power dissipated by the resistor may not be small -- values exceeding one watt are common. So be sure to buy an appropriately-rated resistor.
You may wish to add additional sophistication to compensate for variation in bus voltage. On a 12V system where the voltage may move from 12V or less (battery power) to 14.5V (alternator), you will experience significantly less current and less illumination in your LED circuit when at the lower voltage. You may not even have enough voltage to turn on the LEDs at all. Exactly how this plays out depends on your circuit and can be calculated on the web as above. You can then decide if you care about the diminished lighting in an alternator-out situation. If you do, I have seen people use the LM317T voltage regulator for this application. This part can be had for less than a dollar, mounts easily with a screw, and the datasheet includes a sample circuit which should work.