Calvin Sambrook wrote:
"Donwill" wrote in message
...
I purchased a string of 10 Leds powered by 2 AA batteries connected
in series giving 3 volts or thereabouts presumably.
I was a bit sceptical about the life of the batteries so I left them
on 24 hrs to check, I was surprised that they lasted over a month
which makes them very useful as party lights in a damp conservatory,
let's say running them for 4hrs a day which should make them last for
6 months perhaps.

Hang on a minute, many LEDs won't turn on at 3v as that's quite a low
Vf. Not only that but the normal arrangement for multiple LEDs is to
put them in series chains. You could put them in parallel but there
are issues. I suspect then that the 3V from your batteries is steped
up to something rather higher.

LEDs require a current source rather than a voltage and there are some
nice little ICs available which do all of the step up and current
control in one package, you may well have one of these in the lights
you've bought. Have a look near the batteries, if they're cheapo it
may be a small black blob rather than a chip as such.
The leds are wired in parallell and I have looked, and checked again
after your email for a black blob or any indication of some kind of
electronic gizmo that might control current or whatever, I am pretty
sure there is nothing there. the leds have transparent plastic housings
and I can see nothing in the housings that might indicate that they have
anything significant in there either.


I got to thinking about dimming them, a series resistor would surely
do that, but it would reduce the battery life by warming up the
resistor.

If the lights use an IC to do step up and control then no, a resistor
won't work, or at least not in the way you think.
Even for simple resistor in series with LED circuits you wouldn't
reduce the battery life by increasing the resistor value. You would
waste power of course by heating the resistor but the overall current
draw would be less so the battery would last longer.

Now you can dim filament bulbs by using an astable multivibrator and
varying the mark to space ratio, relying on the chopping frequency
and the time constant of the filament to provide a non flickering and
dimmable light.

Again anything like this will screw up an IC controller, if that's
what you've got.
I don't think there is one, it's just a simple translucent battery
holder which includes a switch, no sign of a "black blob" there either.
The possible explanation that occured to me is that they have a
constant current diode built into the leds during the manufacturing
process and is visually quite undetectable.


Doing that to an led would cause flickering of the light but would
the relativly slow response of the eye smooth out the flicker? as in
film movies which I think have to be in excess of 25 fps in order to
fool the eye.

Movies are 24Hz frame rate *but* they are double or treble shuttered
to avoid flicker. 24Hz is objectionable and unwatchable, 48/50Hz is
right on the limit and anything you'll see in a UK cinema will reach
your eye at 72Hz.
OK, thanks, I hadn't realised that, I was thinking to set the multivib
to 1KHz at least.

In the interests of battery life, are there any dimmers of this type
available for leds?

I purchased them on ebay and also bought a string of 20 and the only
discernable difference between them is that the 20 string has 3 AA
batteries in series.
I eventually cut off the battery housings and connected the two strings
in parallell powered by a Nokia mains charger ACP-7X which were being
sold off cheaply, if I remember correctly for 99p, the charger got a
little warm after 10mins use so I inserted a 2ohm power resistor in
series which reduced the current sufficiently to keep the charger
temperature within bounds. The subsequent light output was reduced
obviously but still adequate for my purposes.
This was just an experiment as I did not want to introduce mains
voltages to the still drying out conservatory (damp).

Thanks
Don