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

[Joe Grisso]

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