mick wrote:
On Sun, 22 Nov 2009 11:03:02 +0000, Donwill wrote:


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.

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




This gets to be fun if you are restricting your supply to 3v. Here's a
dimmer for 12v: http://www.reuk.co.uk/LED-Dimmer-Circuit.htm
You may be able to get it to work using a CMOS 555 (which works down to
about 1.5v) and, maybe, a MOSFET such as 2N7000 instead of the transistor
to give better efficiency at the low voltage. You could decrease C1 to
increase the frequency if you get any flickering. R2 can be omitted if a
MOSFET is used.


Found this also:
http://www.opensourcepartners.nl/~costar/leddimmer/
which might be interesting to play with.
Cheers
Don