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

[Magnus Danielson]

From: "John L Marshall" <j.l.marshall at comcast.net>
Subject: Re: [Fwd: Re: [sdiy] super-nice LED interface ..]
Date: Thu, 13 Oct 2005 21:42:38 -0700
Message-ID: <008901c5d079$b50e9860$6601a8c0 at cheetah>

> A nanosecond is about a "Lightfoot".

It greatly depends on the medium of the lightfoot. In optical fibre for
instance a nanosecond is much shorter. It is rather about two lightdecimeters
and the delay varies with temperature. 

Cheers,
Magnus

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