Hi Team,

Last fall I discovered that the incandescent landing light installed in the nose of my experimental Velocity aircraft had failed. I had two choices - I could either spend six bucks and buy a new one, or I could roll my own... well, winters are long here in Minnesota, and I needed something to do, so I went at it.

I used three Cree CXA-1304 9.6V LEDs mounted on a heat sink and then mounted into the original incandescent bulb reflector on a heat sink. I'm driving these three LEDs with about 1.9 amps total (around 640ma/LED), which, according to the datasheet, should provide me around 2000 lumens total. The driver I designed is using a LM338 linear regulator for current limiting, and an IRF1501 mosfet controlled by a PIC controller to allow either a flashing or a steady-on mode.

I've got two problems... I sure appreciate some help...

First... I think I've chosen the wrong LED, because I can't get the full 1.9 amp drive current on the 14V supply I've got available on the airplane. The problem appears to be that I don't have enough voltage margin to completely turn the mosfet on. Here's the simple math... 14V supply minus 9.6V turn on voltage for the LEDs leaves 4.4V that gets dropped across the LM338 regulator and the mosfet. The LM338 drops about 1.4V of that 4.4V... so the mosfet only sees about 3V... which isn't enough to completely turn it on...

So... I think I need a different LED - one with high efficency, high light output, and much lower voltage... Any suggestions?

Second problem - even when I fully drive the LEDs with 1.9A, the resulting brightness just isn't that great. I suspect the problem here is that the reflector I'm using (the old shell from the incandescent bulb) isn't working... or, perhaps, I need a lens? I can do the electronics, but I could sure use some help on the optics!

Here's a picture of the bulb I created:


Here's the plane it goes in: