>> 1.) The transmitting transducer as well as the whole device will vibrate
>>during the TX period. They will continue to vibrate slightly at 40KHz for a
>>few microseconds or so after the TX time. If you start listening for an
>>echo  soon enough to hear one which comes from only a few inches away, you
>>will most likely hear the vibrations instead of the echo.
>
>True.  This is a big problem with the Polaroid unit, from what I hear.  Since
> the same transducer is doing both transmit and receive functions, it is even
> more susceptible to false triggering if its "blanking interval" (the time
> period it doesn't look for an echo) is set too low.
>
>I was thinking that the transducers should be fairly snugly mounted in
some sort
> of damping medium.  Maybe something along the lines of cork or rubber, or
maybe
> some sort of styrofoam would work.  Also, placing them in tubes to increase
> their directivity should help.


You may want to send out a distinct pulse that varies.  Example: A 4 ping
pulse
then a 20 pin pulse....etc.  This might negate the receiver reading the
wrong pulse or outside influences or bounces.


John Adams ------- http://pobox.com/~electronics ---------
electronics@pobox.com
Check out the H.W.S Internet Guide to Electronics book as well. Links on my
site