On Mon, 23 Nov 2009 10:16:57 +0000, Donwill wrote:

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



Yeah - I saw that one too. I decided that
a) it was probably a bit of overkill (you don't really need 0-100%).
b) it might be difficult (if not impossible) to get working from 3v.

I also considered using a PIC chip as those will work down to 2v. It
would be possible to use software PWM to drive a LED output MOSFET and an
analogue input from a "brightness" control. The problem is efficiency -
it probably isn't very... ;-) One of the little 8-leg chips like 12F683
would be the animal. This approach has one advantage over the others -
you can make the brightness control linear (to all intents and purposes),
no matter how the LEDs behave. You can also do silly things. :-)

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.