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driving white LEDs

2006-10-02 20:38:50.288213+00 by Dan Lyke 5 comments

From our Burning Man[Wiki] exploits, I've got a whole bunch of those solar powered walk lights in a bag downstairs. They each have two white LEDs in them.

Charlene and I have been riding the tandem in the early morning, and I'd like to go do a few double centuries, and I'm finding in both cases that as cool as the single LED off of 4 AA NiMH batteries headlight we've got is, it's much better suited to lower speeds in more lit urban areas.

So I'm thinking that there has to be a way to combine 20 decent white LEDs and a water bottle of NiMH AA batteries into something that'll give me more peripheral light, and let me focus my current spotlight a little further up the road.

In looking at the circuits in the walk lights I'm sure that the LEDs aren't being driven to anywhere near their full capabilities. I've also, over the years, run into articles which suggest that you can get more light out of an LED by running it with pulses at much higher currents than you'd normally draw, but every time I go searching for those articles, I can't find 'em now.

Anyone got resources on efficiently driving white LEDs (of unknown provenance) for as much light as I can get out of 'em?

[ related topics: Burning Man Hardware Hackery Dan's Life Bicycling Bicycling - Tandem ]

comments in ascending chronological order (reverse):

#Comment Re: made: 2006-10-02 20:45:33.439123+00 by: Dan Lyke

Of course then this article pops up, which references this one, and the notes further down the page might even help me make sense of the driver circuits I've already got.

#Comment Re: made: 2006-10-02 22:23:02.315571+00 by: Mars Saxman

What a coincidence - I recently received two giant black trash bags full of those lights, left over from someone's Burning Man camp a few years ago, and have been trying to think up a good use for them.

#Comment Re: made: 2006-10-03 12:42:59.314964+00 by: DaveP

I looked at this a few years back (fall 2001 into spring 2002), with the same purpose as you, Dan.

As you've probably seen, the basic idea on driving these is to use some sort of pulse-generator to bump up the voltage enough to drive the LED and then keep the duty cycle low enough that you don't burn out the LED.

Here are the articles that looked useful at the time:


http://www.cs.indiana.edu/~willie/lvr.html says:

LED's can be run on a constant voltage DC source but really they need constant current. There are many DC to DC current mode convertors that can be used for LED's. The problem with white LED's: are far less efficient (when driven hard IE > 50mA) at producing light than a good halogen incandescent light bulb at the lumen levels that most climbers, cavers, and cyclist need. If you only need 1 to 5 lumens LED's are nice, but if you want 40 to 100 lumens they are very expensive way to produce less light and light of poor color temperature with more power consumption than a halogen light bulb. However, for low light needs they can work great and if you build large array and keep the per LED current low (< 20mA) you can achieve higher efficiency than light bulb.

which matches what I found at the time. I ended up buying a two-halogen beam 12-V system for my bike headlight.

Everything I read suggested to me that PWM was the way to go.

The biggest problem I found with LEDs were that they were hard to focus. Even driving my 20-LED unit (from C.Crane's 4.5V flashlight) at 12V with the PWM set to about a 50% duty cycle, the light was plenty bright, but not focused enough to pick up a chunk of black tire on an asphalt bike trail. It was just too diffuse, at which point I said screw it and bought the halogen set.

I'm sure the technology has advanced, but I still use halogen on my trike, and in my flashlights where I need strong light (I really like the 5 D-Cell MagLite).

Anyway, hopefully there's something in here that'll give you a hint for directions to research today. If you're really curious, I can email you all my notes, but they're almost five years old at this point...

#Comment Re: made: 2006-10-03 17:19:45.456527+00 by: Dan Lyke

Thanks, Dave, that looks like a great start. I've got a good focused beam, what I'm most looking for from these is the more diffuse side lights that'll tell me things like where the shoulder is, and let me put that beam further ahead so that once I identify hazards I can track them as they approach.

And I've got all of these white LEDs anyway, so I may as well try 'em out.

Further updates as I build more.

#Comment Re: made: 2006-12-15 18:31:25.667845+00 by: ebradway

Here's another possibility: