Bike Lighting: Automotive Specs

Having recently taken a thorough drubbing on the ‘Bentrider forums for having a rear-facing white light on my bike, I should accelerate my plans for a red / amber taillight.

This Philips LumiLED app note gives some specs on automotive lighting. The one we bikies all tend to ignore is the surface area: greater than 37.5 square centimeters for rear combination stop-turn fixtures. Call it a scant 4 inches in diameter. You’ve never seen a bike light that large, have you?

LED combo tail stop light
LED combo tail stop light

Maybe the right thing to do is start with a street-legal truck light and build some electronics around it. This is a 4 inch diameter, 44 LED rear light with both taillight and brake light terminals. At 12 V, the taillight draws 10 mA and the brake light is 250 mA. Got it from Gemplers with a recent order, but they’re certainly not the optimum supplier if that’s all you’re buying.

Obviously, it’s unreasonable to run a 3 watt taillight on a bike, as the most recent crop of single-LED killer headlights are merely a watt or three. Battery life remains a problem.

At 10% duty cycle the brake LEDs would average 300 mW. That might be roughly comparable to the running lights on some cars these days.

With the taillight constantly energized and the brake flashing at 4 Hz, it’d be 120 + 0.5 * 300 = 270 mW.

That’s more reasonable. With a 50% efficient upconverter to 12 V, that’s half a watt. Start with 4 AA cells, triple the voltage, draw 100 mA, runtime is 1500 / 100 = 15 hours. Good enough.

And it ought to be attention-getting enough for anybody! The only trouble will be fitting the damn thing on the back of the bike; fortunately, ‘bents have plenty of room behind the seat, so maybe attaching it below the top seat rail will work.

Memo to Self: The rear reflector must be something like 3 inches in diameter, too. We ignore that spec, too.

4 thoughts on “Bike Lighting: Automotive Specs

  1. Hey wow you *are* my doppelganger.
    I’ve made some LED lights (since I’m an electrical engineer designing LED drivers.) Packaging them in, say, a trailer taillight enclosure is pretty good if you want a regulation-sized light. A lot of our chips (LM3402, 3409, in particular) do a really super job of driving 350mA LED’s at high efficiency and have a dim pin that you can toggle fast, to get dimming, or slow to get both dimming and a strobe effect.
    One thing we (most of my coworkers are pretty serious bicyclists, too) have been talking about is a more complicated lighting system: as bright as possible, maybe in the 1000 lumens range, just before and at dusk, probably strobing, so that people can see us, and then dimming back to maybe 500 lumens when it’s good and dark because 500lm is almost overwhelmingly bright and you might as well get the extra battery life. We’ve been playing with some small controllers for this, using photodiodes, so they automatically handle lighting conditions and auto-turn-off if you’re not in motion and haven’t been for over 3 minutes.
    One thing I’ve found with DIY LED lighting systems on vibration-prone platforms is that it’s a good idea to, when you finish and test the circuit board, dip the whole thing in epoxy. That keeps anything from moving and helps reduce corrosion. The first and second set of LED lights I made (one for a friend’s pinzgauer rear lights, the other for my trailer) had many of the LED’s fail when they jolted loose from the board, despite massive solderjoints. The epoxy-dipped ones have been running, in one case, for 2 years without failure.

  2. Link to the appnote is broken (you forget http://, so the navigator thinks it’s a relative path, and tries to download the file from your server)

    You can, of course, delete this comment once it’s fixed. I expect it won’t go through moderation ;)


    [Ed: Fixed! Thanks for the alert.

    You’re on the approved list, having already typed something meaningful, so you can say nearly anything and get away with it…]

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