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Update on bike light design

February 1, 2007

It’s always better to use a canned solution, assuming you can find it.

It turns out that there is a company LEDdynamics designing and selling power supplies customized for driving LEDs. The BuckPuck, BoostPuck, and BuckToot all look useful. Their products are generally aimed at the Philips Lumileds “Luxeon” LEDs. These are high-current LEDs, starting at 350mA and going up. Definitely, this is the way to go for the power supply, and probably for the taillight (they sell a very efficient orange-red light with 110-degree spread — perfect for a taillight). One minor difficulty is deciding whether to go boost or buck, and also to figure out the best way to convert generator AC to DC. The BuckPuck claims to run on AC, but it appears that it uses conventional (0.7v forward drop) diodes to rectify, and this is not a good match for low bicycle generator voltages.

The choice is not quite so clear for the headlight; other people propose to use a cyan-green color (good for the low-light rods and middle-color cones in our eyeballs) but it seems to me that this would destroy night vision (it IS night vision, but if you got a pop in the eye from this thing, good luck seeing anything else in low light for a few minutes), plus the rods don’t give a sharp focus. There’s also the issue of GREEN=GO reactions from car drivers; these guys were talking about mountain bikes, not commuter bikes. I’m going to have a look at how the whites I already bought do (they’ve got efficiency comparable to the Luxeon whites, and a slightly better color mix) but it seems that an amber light might be a much better choice. Amber (590nm) might win because: (1) lower input power, (2) better transmission through the eye itself, (3) YELLOW=CAUTION, (4) it seems to have little overlap with the rod wavelengths, but still overlaps the the “red” cone wavelengths. I can always recycle the whites for indoor lighting; it’s pretty easy to wire a boatload of them in series to get to a line voltage, and the result is both efficient and dimmable.

Another useful thing about newer LEDs, especially these high-powered ones, is that that for as much as a 50% current reduction, they still produce light, and it is reduced only linearly — that is, given a 60mA drive, 4 white LEDs and 3 white LEDs produce just about the same amount of light. This means that it should be possible to fine-tune the voltage of a parallel array, if necessary, and also to provide some headroom for power supply glitches. Under-driving a parallel array of lights also provides protection against runaway failure, which can occur if one LED fails and the current is concentrated onto the remaining LEDs. The amber/orange/red Luxeon III LEDs can be driven through a large range of currents; anywhere from .14A to 1.4A (continuous) produces light, and pulses of 2.2A can be tolerated.

Someone else has looked at this issue, though I’m not sure where they went with it.

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