Cheap Quick and Short term.
3C Alkaline cells, 7 X 100ohm resisters and 7 leds wired in parellel.
buy 18 white leds , 7 to fry accidentally, and a few spares.
get 10 X 100ohm resisters, just in case you lose a few.
put the 3 c cells in series, stacked , or get a Series holder at the electronics store.
solder each of 7 resisters to one of the legs of the led.
wire the leds and resisters all the same
hook the batteries in series making about 4.5v then parellel this with your led resister stack.
|--{~}---@---|
|--{~}---@---|
|--{~}---@---|
|--{~}---@---|
|--{~}---@---|
|--{~}---@---|
|--{~}---@---|
|+-{=]{=]{=]--|
(3 bats in series)
Runtime about 60 hours, till it dwindles down to very dim, and just keeps on going (or glowing) losses about 30%
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if you have a simple meter
|--@---|
|--@---|
|--@---|
|--@---|---{/~}---- to battery negative
|--@---| Variable resister
|--@---|
|--@---|
|+{==]{==]{==]-
wire the leds simply in parelel, then use one single variable resister of 20+ ohms, preferably a good wire wound, so it lasts, probably you will find a nice 100ohm easily.
roll the variable resister to its highest resistance setting
put the probes for the voltage meter on one of the leds, and read the voltage, as you slowly turn the variable resister up, stop when it reads about 3.1-3.2v or untill the leds turn blue and smoke starts pouring out
.
Or put a ampmeter between the last connection to the battery, and set the miliamps to about 105ma, Remembering that the Meters OWN resistance will exist, so when you pull the meter out and reconnect the battery you will slightly readjust to the same visual levels.
the Resister will be at about 15ohms with new alkalines
when it starts going dim, just visually recognise about how bright it is supposed to be and slowly turn it up, eventually when the cells are neer dead, you will be at the end of the resistance.
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after i look at trying to run it that long (168hours), an option of using rechargables really looks like a minimum (AC being a good idea again), or running them well below max spec, like at 5ma each or something. but then again my math is pretty bad.
http://led.linear1.org/led.wiz but you can check my math.
After you hook up the variable resister, which gives you cheap easy full control, you might find that as you bring it slowly up, you can stop with it looking more like a candle as opposed to a flashlight, and it will probably make it the whole 168 hours. i would just guess, feel out about where i want it, and let the nature of batteries do thier own thing.
notes: dont turn the resister "full Up" with full batteries, as then you will need the spare leds
but then again that is what they are for
the "math" is based on WHITE leds, if your using Yellow or Amber Leds and not White ones, you could probably use just 2 Cells, and you would have to recalculate the resistance, but that would be a good idea. with yellow Leds and 2 cells the math stuff doesnt change that much, but it should be checked at least.
Off the cuff, without doing precise math, approx. 2.5 days continuous on three C batts? Nonetheless, I have some additional thoughts on the matter overall, as I think some of the suggestions could be missing the mark for this project.
