the answer is a PWM circuit that manages the current to the LED - they
didn't say 24 hours a max brightness - the ones for spelunking, for example,
operate at reduced brightness most of the time. Just use a 555, or the CMOS
equivalent with some current feedback
"AL A." wrote in message
...
Hi all,
I know this is not strictly on topic, but this crew seems to know
something
about everythig, so here goes:

I am looking to build a "task" lamp (think dentist-type light) that uses
high output white LEDs for a friend who often does
work for extended periods of time in very remote places. I was looking at
the NTE electronics "lightwave" flashlights
http://www.nteinc.com/lightwave/ and they claim that they have circuitry
that makes it possible to run one of the 10
led lights for 24 days, continuously, on 3 D cells.

Any idea how they accomplish that? Most of the high output LEDs I have
seen
seem to want to run between 20 and 40 mA
current draw. Given the amp-hour ratings I find for D cells, I don't see
how
that would work out. Do they modulate the power to the LEDs
in some way that helps extend the battery life so dramatically?

I looked at just modifing a lightwave flashlight for the task at hand,
but
the beam is a bit too wide for what he needs. So I am hoping to
build something with maybe 3 LEDs and appropriate optics to give a fairly
narrow, even beam, mounted on a gooseneck type thing,
with a remote battery pack on a cord.

Any thoughts on the electronics involved? I can handle that part of it,
if
I know what I am trying to accomplish.Of course the body
of the thing will be METAL, nicely turned, naturally. (obligatory
metalworking content...)

Thanks for any ideas you can offer.
-AL