Mike Keitz wrote:

> On Wed, 22 Oct 1997 16:53:30 -0400 Andy Kunz <montana@FAST.NET>
> writes:
> >At 11:09 AM 10/22/97 -0200, you wrote:
> >>Hi to All,
> >>
> >>I'm trying to do a ultrasound distance measurement with PIC, I don't
>
> >want
> >>to use the Polaroid module. Does anyone has some experience to
> share?
> >>I know how to generate the transmition pulse but I need help in the
> >>receiver part.
> >>The range will be from 20 to 200 inches.
>
> The Polaroid module is quite well-done, an excellent example of USA
> engineering in the early 1970's.  If you're needing the full range the
>
> stepped increasing gain of the receiver will be necessary.  As someone
>
> else said, there is nothing in that module that isn't necessary.   I
> doubt you can buy the receiver chip seperately anymore.
>
> >
> >Why not optically?
>
> Since light travels much faster than sound, resolving a distance would
>
> require wider signal processing bandwidth.  The target must also
> reflect
> light to some degree (at long range, must reflect quite well).
>
> This does seem to be the preferred solution for modern autofocus
> cameras
> though.  Some use a CCD to look for optimum fine detail in the
> picture.
> Others have a couple of infrared windows in the front.  Do those use
> the
> main lens too, or is it an independent distance- measuring system?
> How
> does it work?

   I can answer the wuestion of what the little extra window is, scince
I too am trying to measure distances and have explored many paths. I
took apart a camporder with autofocus to examin hoe it used it's little
window. behind it were 2 led's. one.
I assume that one was an infrared reciever, one a detctor. they both had
focusing lenses. One permanant, the other could change it's angle.
Apparently, it shined a beam of light, then changed the angle of the
other lens until the recieved signal was the greatest, it used
triangulation i guess.