I started biking to work this summer and needed a good headlight and taillight. I didn't want to spend a lot, but I wanted extreme visibility. For about $150 I ended up with a headlight that puts out somewhere around 1200 lumens, and a really effective tail light. The power source is an 18 volt Ryobi power tool battery which is both easily replacable, and quickly charged.
Warning! This project is not a beginner's project. As such I'm not documenting it as such. This will be more of a guide and list of critical elements rather than a comprehensive How-To.
Step 1: Design Choices, thermal considerations
The first step is to determine what you want/need for your bike light. Total light output and beam shape will determine the type, number, and configuration of LEDs needed. Additionally mounting, portability, weather resistance, and access to machine tools will weigh in.
I started out with the intention to build the brightest lights around for the purpose of being noticed. This means for the headlight over 1000 lumens (although there are a few 1000+ lumen bike headlights commercially available, they cost close to that amount in dollars).
I settled on using 6 Cree XR-E LEDs, which from the appropriate bin will put out in the range of 180 - 230 lumens each at a 1 amp drive level. (UPDATE: The new R2 bin XR-E LEDs put out up to 275 lumens) This gives me a headlight which (ignoring losses from the lenses) will put out between 1080 and 1380 lumens. A number of considerations have to be made when using an array of LEDs of this magnitude.
- At 1 amp, these LEDs will requrie about 22 watts of DC power, carefully regulated to avoid over current and overvoltage conditions.
- With the LEDs running somewhere under 50% efficiency, the array will dissipate somewhere between 10 and 15 watts of heat. This must be disposed of properly to keep the LEDs within their rated junction temperature limits.
- Having 6 emitters will allow customization of the beam pattern. Each LED will have its own lens. The end result is a superposition of narrow spots, medium spots, and wide angle oval. This ensures side visibility, while providing good, even lighting towards the path ahead.
- A suitable power source needs to be provided. The array is a series connected string of 6 LEDs, each with a Vf in the 3.7v range, meaning that a power source in the range of 15 to 25 volts is required (This is to keep the regulator from working too hard to boost or buck the native supply voltage.)
- With the weight of the 6 LEDs, a suitable heat spreader, and heat sinks the headlight is going to have a pretty decent mass and needs a solid, adjustable mount that allows for quick dismount when parking the bike outdoors.
- Brightness control is handy so that when you are riding towards oncoming cars you don't piss off drivers by blinding them. This light is so bright that in darkness it can be blindingly bright. A measure of caution and restraint is needed when using a light this bright.