How to build a simple Luxeon LED bike headlight
This is a simple bike light which can be contructed with a minimum of tools. It is bright enough to be useful as a headlight, is fairly robust, and cheap to construct. It's perfect for commuting and does a surprisingly good job for mountain biking too.
The whole thing, including rechargeable batteries, weighs about 200g and will run for over two hours.
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The light and battery installed on my road bike
It's based around a Luxeon III Star. This is a very high-power LED that supposedly produces about the same amount of light as a 5W halogen bulb while using far less power. To focus the output, I used a Fraen narrow lens. It's designed to mount directly over the LED and produces a nice tight beam without wasting a lot of light to the sides.
Everything else is just there to support the LED and its lens. I used four rechargeable AA cells to power it (why?), which should last for about two hours. One of the nice features of the LED is that when the batteries go flat, the LED will continue to glow dimly for a while, so you're not invisible in traffic. The AA cells are contained in a battery holder and attached to the stem of the bike using velcro. A 1.5 ohm resistor prevents the LED from overheating. Everything is attached to a sheet of folded aluminium, which acts as a heatsink for the LED and prevents the light from dazzling the rider.
Performance
I built this light mostly for commuting, and it does nicely for this task. It's bright enough to see where you're going on unlit streets. It does get drowned out in street lights, but in that situation you really only need something so that drivers can see you anyway.
I've used the light for the mountain bike legs of a few adventure races, and it's held up quite well. It's a bit marginal in technical terrain, but for 95% of what's out there it's good. When you're out in the middle of nowhere with no street lighting for miles, it's remarkable how far 3W of LED light will go.
The light itself 
The battery pack, completely assembled
Luxeon bike headlight: construction
Parts
You will need:
The light itself
- Luxeon III Star
- Fraen lens. I used the narrow type.
- 1.5 ohm, 5W resistor. A 2W resistor will do, but 5W gives you some headroom and is easier to mount.
- A piece of aluminium, about 10cm long and 5cm wide
- An old reflector mount with a hole that you can put a bolt through
- A nut and bolt to put through the reflector mount
- A battery holder that will hold four AA cells
- Four rechargeable AA cells. I used 2400mAh NiMH cells
- A DC plug and matching socket
- Some self-adhesive velcro - both the hook (scratchy) and loop (furry) varieties
- Epoxy or hot glue
- Some bits of wire to link it all up
- Thermal grease, if you have some handy.
The head unit
Might as well get the hard part out of the way first.
Drill the aluminium sheet to fit the lens. You need four holes that you can poke the legs of the lens through. When everything is assembled, the LED will be firmly held between the aluminium and the lens. Also drill the hole(s) to fit your reflector. In this diagram, the sheet will be folded along the dashed green lines.
Drilling diagram for the aluminium base
Bend the aluminium sheet to make a U shape. While you're at it, do something about the sharp edges and points. I just dabbed some hot glue on mine, but I recommend that you fold them over again so that the outside is curved. If you skip this, you'll get nicely scratched up when you knee the light by accident.
Now is a good time to solder some short wires to the LED, since it'll be difficult once the LED is installed. You won't be able to get a strong mechanical connection, so I used some hot glue to hold everything in place once it was installed. Epoxy would probably be more secure. Using nothing will probably result in the joint shaking itself loose. Don't get glue or epoxy on the transparent surface of the LED!
You can tell which side is the positive by the + marking on the LED base. I soldered a red wire to this side so that I knew which was which later on. Don't get them confused!
Closeup of a Luxeon Star
Install the LED and lens on the base. Ideally, the LED should have some thermal grease between it and the aluminium, but it's not critical. I used epoxy to retain the lens - hot glue is probably not strong enough. Try to keep the lens and base held tighly together until the adhesive sets.
Mount the resistor somewhere on the base. This is a bit of a tricky one, as hot glue isn't strong enough and epoxy-on-metal is too brittle for long-term use. My epoxy shook loose after six months or so. The easiest long-term solution would be to drill more holes for cable ties and wrap one around the resistor.
Wiring diagram
Finish off the soldering. You need a wire from one side of the resistor to the positive side of the LED, a wire from the negative of the LED to the negative of your socket and a wire from the positive of the plug to the open side of the resistor. Make sure no bare wiring is touching your base! Copious amounts of hot glue will help retain everything securely in place.
Traditionally, the negative wire connects to the outside (barrel) of the plug, and the positive connects to the inside of the plug. This is to reduce the chance of sparking and badness if your live plug comes loose and touches things.
A common mistake is to solder the wires onto the DC plugs and sockets without sliding the cover over the wires first. If you do this, you won't be able to get the cover on, and will have to desolder everything. Cover slides over the wires first!
Finally, you can bolt the reflector mount to the base. After some testing, I added hot glue to make sure the bolts didn't shake themselves loose.
The battery pack
The assembled battery pack
Cut back the velcro to fit the two edges of the battery pack and stick it on. The other type of velcro goes on the stem, so you can attach the batteries like so:
The battery pack hangs under the stem, and the velcro holds it securely in place. I put the scratchy (hook) type on the battery pack to prevent abrasion on the stem.
Solder on the DC plug, and you're done! Install the AA cells, connect the plug and socket, and the LED should glow nicely. If it doesn't, check your wiring. If it glows brightly and gets dimmer, or smokes, or glows and turns off with a cracking noise, you've probably connected the LED directly to the battery, and will need a new LED.
The battery pack, installed on the bike
Luxeon bike headlight: improvements
Casing
I'll be the first to admit - the casing is butt-ugly. It also wouldn't be much fun to fall on. It's not particularly aero, if that matters to you.
It is very lightweight, however, and this is hard to maintain with a nicer casing. Suzy Jackson's light has a very nice casing, but it looks heavy, and I also lack access to a lathe with which to build such a thing. PVC tubing is another option, though heat may become an issue (unless you leave the front and back of the case open, and have a heatsink in the middle to catch the air rushing through).
I'm pleased with the reflector mount - it seems more than strong enough. The only thing that I'd change is to make it easier to mount and unmount the light without a screwdriver. This may be possible with a commercial light bracket.
Batteries
The batteries have served me extremely well. I love the modern NiMH AA cells; they store a lot of energy, are very cheap, and are not even particularly heavy. Commercial fast chargers are cheap and reliable.
The main improvement here would be convenience. It's a pain to remove the holder from the bike, remove the cells from the holder, plug them into the charger, and reverse the process later. If I had commercial battery sticks that clip to the frame like a pump, that would be far more convenient. I've never purchased commercial lights, so I didn't have any motivation to keep compatibility there.
Future revs of the battery system will probably use stick-compatible casing and a DC socket so they can be easily removed and charged. This adds a lot of hassle during construction, since I also have to build a charger. One big advantage of the frame-mounted batteries is that I don't have to install high-current cable all over the bike.
A bottle battery is another option, but it uses a whole bottle cage, which is an issue for long-distance riding. My road bike has nowhere good to put a third bottle cage (and if it did, I'd probably want it for another water bottle). I did take the bottle battery option for my mountain bike lights (coming soon!)
The li-ion battery packs for my laptop are quite cheap to replace (AUD$70) and extremely good capacity (66Whr in 400 grams), so it'd be nice to be able to use them. They're somewhat unwieldy to mount to the bike. Alternately, I could get the 18650 cells off eBay and build a charger. With the li-ion cells, I'd have an extremely good weight to power/battery life ratio.
LEDs
I'd like more light, but experiments with some alternate brands of LEDs and lenses have been disappointing. I'll probably stick with the Lumileds emitters and Fraen lenses, but go for the new Luxeon K2 variety for better efficiency. I might also install a second emitter/lens pair - they add very little additional weight.
The use of a resistor as regulator is somewhat dumb. It burns off about 20% of the power used by the whole circuit. So far, battery life hasn't been an issue for me - but the engineer part of my brain just won't let it go. A proper regulator like a Buckpuck or a Boostpuck would reduce this wastage to about 5% and make it possible to use any old batteries that happen to have a compatible plug on the end.
Miscellaneous
A power switch would be nice. From my mountain bike lights, I've also found a 'dim' setting to be useful, especially when moving slowly or sitting at traffic lights.
Heavier cabling and better reinforcing on the battery pack would make me less nervous, but it's held up fine so far.
The future
So my next road bike lighting system will probably look something like:
- Frame-mounted battery containing AA cells (so they can be swapped over later), with the option of using higher-capacity (heavier) cells - or a second battery pack wired in parallel. Maybe a li-ion battery pack. There are some tiny li-ion cells with great capacity - the iPod battery packs store about 5 watt-hours and weigh almost nothing.
- Buckpuck or boostpuck regulator to allow flexible battery options and better power efficiency.
- Higher-grade LEDs; possibly two of them. Or maybe three, so I can use the same system for road and mountain biking.
- More solid casing.
- Hopefully, not much heavier. Lighter would be great if I can arrange some li-ion batteries.
Some of these have already been addressed in my mountain bike lights - check back for more information!