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

[John L Marshall]

A nanosecond is about a "Lightfoot".

Thank you Gordon.

Take care,
John

www.sound-photo.com
47-30-39.02 N, 122-09-51.75 W, elev 440'
----- Original Message ----- 
From: "Sumanth Peddamatham" <peddamat at gmail.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, October 13, 2005 12:41 PM
Subject: Re: [Fwd: Re: [sdiy] super-nice LED interface ..]


> 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
>>
>>
>>
>>
>>
>