Off the top of my head I'd say look into a 'joule thief' circuit.

You should find plenty of search engine hits on that if you include the
quotes.

Denny Esterline wrote:
> I found some extremely cheap white LED flashlights stuffing my stocking. Turns out they drive the three LEDs in parallel without any form of current control. Not surprisingly, one LED is much brighter than the others.
>
> Looks like I'll have to rewire it so the LEDs are in series and setup a little boost converter to drive it off the three AAA cells.
>
> Anybody have a favorite circuit you'd recommend?
>
> -Denny
>
>
>   
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