I had done that at one time. Driving a piezo tranceiver, I was able to
produce a realtively loud tone. Therefore, I assumed that driving a
40 kHz transducer would also be relatively loud (though it was not
audible).  If you would like, I can mail you the source which gives
40 Hkz exactly using a 16c84 and a 1 MHz oscillator.

So the answer is YES it can be driven by the pin alone. I'm sure it would
be much louder if you drove the xcvr with a transistor though.

 I was having problems with the receiving end of my device.

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 Rolan Yang            http://hertz.njit.edu/~rxy5310   Electrical Engineer
 rxy5310@hertz.njit.edu                             kyurius@tsb.weschke.com
 VR,ROBOTICS,FENCING,HACKING,INDUSTRIAL MUSIC,ART,EXPLOSIVES,INLINE SKATING
                    THESE ARE A FEW OF MY FAVORITE THINGS.
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4 out of 10 people are annoyed by ^ this.