On 22 Nov, 23:48, "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.

Was thinking that but presume OP has red, green and amber chains,
red ,amber and green LEDs will run off sub 3V, blue and white need 3V+
to light and typically have 3 AA batteries.

B&M Homestores have a good range of both mains and battery operated
LED sets this year.

Cheers
Adam


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.

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.

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.

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