>----------
>>Well, since the ultrasonic sensor I am trying to build it
>>interfaced to a PIC, its legal to talk about it here, right?
>>
>>OK, I tried a couple of different circuits.  To generate the
>>40KHz signal, I brute forced it with a 40Kc xtal and a 4049.
>>That works fine, good square wave driving (and I can hear it).
>>Rx circuit, not so good.  I tried a couple of ways, but in one
>>case my amp turned into a osc (of course), and the other works
>>better (using a 324 quad amp), but the final stages seem dead.
>>Tried the obvious (new amp, etc), but it doesn't appear sensitive
>>enough.  I going to get another good analog guy to sit down and
>>see what he can tell me, but just curious what (if any) circuits
>>are proven to work.  Even an inexpensive commercial solution is
>>an option now (need to have this thing done by mid-april).  I
>>need to have a sense distance of 4 to 6 feet. Comments anyone ?
>>
>>-Harrison Cooper
>> hcooper@es.com
>>

An internally-compensated general-purpose opamp like the 324 is generally
not suitable for 40 KHz signals, except maybe near unity gain (even then it
may not have the slew rate).  Try using a high-frequency op-amp like a 318
(or one of the many newer ones), a bipolar transistor amplifier, IF
amplifier chip (MC1330 ?) or maybe even a string of CMOS inverter sections
biased for linear operation.
Also, the Kapton film transdusers as used by Polaroid work similar to
condenser microphones:  they require a bias of a couple hundred volts DC.
In an echo-ranging system, the transducer is charged up with bias by
rectifying the transmit pulse so it is biased during the receive phase that
immediately follows.  Some provision for this would have to be made in a
split transducer system which I think is what you are trying.  Most likely
the piezoelectric types of transducer don't require bias.

-Mike