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Current-limited variable voltage light circuit

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kananga

New Member
Here's the story:
- Multiple high-output led's, 3 or 4. Each led requires 2.9ish volts to drive at 250ma, and about 3.05ish volts to drive at 500ma.
- Generator hub for a bicycle. The generator saturates at 500ma, and will deliver at least a dozen, probably more, volts to deliver that current. Which is great for these led's, that's well within safe operating current! Just line them up in series.
- But all of that voltage isn't available right away, the potential increases with speed. Lots of series led's will eventually make a ton of light, but you'll be climbing hills in the dark. One led will light up at walking speed, but won't give enough light for more than 10mph!

So here's the plan: I'm working on efficient ways to add led's (one at a time) to the series circuit as voltage increases for a wider available range of light vs speed.

Divert current to ground following the first led (then the second, etc.) until the voltage rises to between a 2.9-3.0 volt drop across that led; the hub will generate 3 volts at walking speed. But I'll need 5.8 volts to get as much light from 2 series led's at 250 ma as I'd get from one led at 3.05v and 500ma! That's a big transition gap to make up with something like a simple comparator triggered switch...

So what do you think? 1) Would it be possible to rig up a reasonably simple circuit to ramp off the connection to ground following the first led as the voltage potential becomes available for the second? B) Or should I give in (like others have suggested) and sample the generator's frequency (initial output is ac), to reference a frequency-based switch by?
Of course, all powered by the generator, at a range of 2 to 12+ volts! Mwa, ha, ha.
 

ronsimpson

Well-Known Member
Most Helpful Member
I would a micro-processor.
Another way is to use a LM3914, LM3915 LED meter. It could be set to turn on 1 LED at 2 volts, 2 LEDs at 3 volts....10LEDs at 10 volts. I have not thought much on the subject but it might work.
 
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