>I recently found a source selling very cheap plastic 3 white LED
> flashlights with great reflectors. I've dissected other lights for 
> use ...

Many of the cheap LED torches have the LEDs wired directly across 3 x 
AAA batteries in series. They aren't designed in any real sense and 
the makers don't care. They work well enough to sell and that's all 
that counts. (If they don't work at all and still sell then that's OK 
too. For example the examples of the "shaker" torches in which the 
genuine ones generate power when you shake them, but the "cheap" ones 
have instead a few small batteries in them and a fake magnet and a 
coil that goes nowhere. Pure fraud and they don't care).

If you are using the LED torches that you describe as your component 
sources you have no guarantees at all about lifetime, performance from 
batch to batch or any other spec that counts. What you can be almost 
certain of is that the LEDs were either bought from reject stock that 
didn't meet QA for a more reputable product or perhaps were found in a 
jumbo bin (dumpster to you probably). If you are making small volumes 
of products and don't care about the results then you may get away 
with this. But, as you express a desire for eg 90,000 hours of service 
but definitely not hundreds of hours then you probably do care.

You probably want to try and source your LEDs as discrete components 
somewhere if you care that much.

As far as driving goes, LEDs are specified at rated current and 
voltage will vary. You can "bin" them - ie sort for similar voltage 
drop at a given current - but this is a poor compromise. The "proper" 
method is to control current. How tightly you need to control current 
depends on your need for consistent brightness across LEDs and how 
well matched the LEDs are for brightness. I know how good the latter 
is likely to be if you source them from the torches that you describe 
:-).

Even a volt or so of voltage drop in a series resistor will help 
greatly and even less than that is better than nothing at all.
If you have the available voltage then 3 LEDs in series is more 
economical on resistors and energy consumption.

While you can get LEDs rated at all sorts of current ratings (eg 
Luxeon 1 Watt at 300 mA ish) the sort of LED you have are quite 
possibly specified at around 20 mA operating. Getting them from a 
supplier rather than a torch will help greatly in establishing this. 
You could look at the current voltage curves of a number of white LEDs 
and their ratings and get some idea of how your torch LEDs are rated. 
Measuring If versus Vf in say 5 mA steps would give you some idea of 
the V/I curve and some idea of how vertical you are getting at say 90 
mA. BUT I'd be surprised if cheap junk LEDs were rated at anything 
like 90 mA continuous.

If using 7805 then a series resistor will be better than a series 
diode or two. If the 7805 is supplied from a much higher voltage then 
a series resistor from that supply will dissipate no more power than 
the 7805, will cost less and will be a much better approximation of a 
constant current source.

eg 7805 and 20 mA at 3V Vf (so R = 100r) would give 10 mA at Vf = 4V 
= -50% change.

BUT 20 mA at 12V into a resistor and 3 V Vf (so R = 450r) would give 
17.8 mA at 4V Vf = -11% change.
Pres < 0.2 Watt.



         Russell


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