[Fwd: Re: [sdiy] super-nice LED interface ..]

[Sumanth Peddamatham]

Here's a quick experiment you can do.  Grab and led, hook it up to a
DMM, shine a light on the led, and watch the voltage fluctuate.  I
don't remember what the animation on Jeff's site demonstrates, but
with a microprocessor, you could have each led in the matrix on for a
tiny fraction of a second, and measure the voltage on the adjacent
leds.  It would take a bit of tweaking, but it shouldn't be too hard.

As for the Jason's question, no, it's not *currently* possible to turn
on the led and have it measure it's own reflection.  I don't have time
right now to hammer out the math, but at the distances that we're
working with, the roundtrip time for the light waves is way too short.


Sumanth Peddamatham

On 10/13/05, Joe Grisso <jgrisso at det3.net> wrote:
> It seems the light is reflected to adjacent cells, and then an average
> intensity is taken. I also found a quick online resource summarizing the
> photodiode effect with LEDs:
>
> http://archive.chipcenter.com/eexpert/akruger/akruger042.html
>
> One thing I think most people missed was that the video was taken in the
> dark. If we were to do it in normal or bright ambient light conditions,
> would the representative wavelengths in the ambient light overload the
> photodiode effect and make what little information you get off of the
> adjacent LEDs indistinguishable from the noise in the surrounding light?
>
> Of course this can be corrected by using a polarizer or a light filter of
> some kind, but why? The more cost effective way is to use either a
> resistive or capacitive material, a-la touchpads.
>
> Best Regards,
>
> Joe Grisso
> Detachment 3 Media, Ltd.
>
> > ----- Original Message -----
> >> but .. all LED's are on in that demo, so does this mean that the array
> has LED's that are 'off', but still being used as a detector, or does
> it mean that LED's can be used as detectors even when lit?
> >>
> > No, as you can see from the diagram on the page, the leds alternate in
> such a way that ones adjacent to the point being examined are on, and
> the light diffuses through the finger (or possibly reflects from it) to
> the middle one, which is at the same time in receive mode. I'm not
> personally impressed by this, I think  capacitive sensors are more
> practical. You are going to need a certain amount of processing to try
> to work out what is going on.
> >
> > paul perry melbourne australia
>
>
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