Head unit
The head unit is constructed around a small diecast aluminium case. The power socket and brightness switch are mounted on the sides. The LEDs are bolted to the back surface and use the case as a heatsink. The wiring is contained within the case and can be weather-sealed, although I didn't bother. With the exception of the lenses (see below), the whole assembly is extremely robust.
The case that I chose measures 51x51x32mm - just big enough to fit the LEDs on the back. Larger cases are excessively heavy - this one was about 60 grams.
Assembly was remarkably LEGO-like. I guess this is what happens when I have plans before I begin construction! I drilled holes in the case to accomodate the mounting bracket, power socket, switch, LEDs and power wires.
The Cree XLamps: I also bought some PCBs which make the Cree LEDs mechanically similar to Luxeon Stars
The Cree LEDs soldered to their PCBs
The Cree LEDs don't come with a mounting PCB by default. Soldering them to their mounting PCBs turned out to be difficult - the PCBs soak up most of the heat from the soldering iron (as you'd hope). So they came out pretty messy. Spread lots of heatsink goop under the mounting PCB before bolting them down. The case isn't perfectly flat, and the bolts do a good job of squeezing out the excess. I also used some insulating washers underneath the bolts since they run close to the PCB pads.
The back of Luxeon Star LEDs is not electrically neutral. If you use them instead of the Cree XLamps, you need to insulate the back of the LED from the aluminium case somehow.
My drilling wasn't perfectly accurate, so I was concerned about the centre bolt being able to hold in all three LEDs. To compensate, I put in a large washer that would have plenty of overlap. To avoid shorting the LED contacts, I used the insulating washers off an old computer motherboard.
The LEDs mounted on the case
I completed the external wiring. This should be pretty solid - moreso once it gets covered in hot glue. At this point, the LEDs can be tested. A current-limited power supply is extremely useful for this, especially since the LEDs are rather painful to look at at full power.
LED wiring
Testing the LEDs
At this point, I tested to see how hot the case would get while running. After half an hour the case was hot and the power through the LEDs had increased measurably, but everything was still within acceptable limits. The LEDs are rated up to 85 degrees Celcius. In a normal operating environment where is plenty of airflow and the operating temperature is lower, I expected no trouble at all. Actual use showed that the case only got warm, which was quite pleasant!
Lenses hot-glued to the case
Mounting the lenses turned out to be tricky. They're made to fit precisely over the LEDs, which meant that they didn't fit my LEDs once they'd been hand-soldered to their carrier boards. They also clipped the mounting screws. I wound up cutting a lot of plastic out of the back of the lenses so that they'd actually fit over the LEDs.
I got the lenses mounted in a near-ideal position, but in use there was a lot of light being lost out of the side of the lens case. The focusing was also pretty lousy - I'd ordered six degree lenses, and I was observing more like a 30 degree beam. I'm not convinced that the lens design matches the LED design. The Fraen lenses from my original light were definitely more efficient, even when poorly mounted. The beam was also far tighter.
I wound up hot-gluing the lenses to everything else. This was my biggest concern with the whole design - that the lenses could come loose after a collision or crash. The lenses don't have any obvious mounting mechanism. I think they're designed to friction-fit inside a purpose-built case. As it turns out, they're surprisingly solid. The lenses got a fair beating in shipping and during the race (two crashes; I went through a lot of painkillers). They remain firmly attached to this day.
I also took the opportunity to cover the external wiring in hot glue; this adds a bit of stability and weatherproofing.
All that is left is to complete the internal wiring and bolt the case together.