At 04:44 PM 11/11/97 -0800, you wrote:
>Sean,
>I was actually thinking of simply counting on the transducer's own
resonance to
> filter things down to just the 40 KHz pulse, and using a couple of
op-amps to
> boost the 40 KHz echo signal to the point that it clips and is detectable by
> the PIC.  It sounds like exactly what you are doing.
>

Yes, I agree that for the most part, you can rely on the transducer's
natural narrow bandwidth to exclude stray, non-ultrasonic signals. I was
simply using the PLL to tell the PIC when there was a signal/ when there
was not a signal. It was more laziness than need! I didn't want to bother
writing code for the PIC to detect pulses when the PLL would deliver a
simple yes/no answer via its "carrier detect" pin. But, as you suggest, it
looks like the direct pic detection scheme will work better.
        My receive amplifier is ONE transistor, operating in maximum gain
configuration. I have yet to design a clipper circuit to add to this. I
tried an op amp for the amplifier, but I have bad luck and little patience
with op amps as linear/semi-linear amps for high frequencies, so I tried
the discreet approach, and it turns out that the single transistor delivers
quite a sizable signal. I plan on adding an op amp in open-loop mode as a
comparator to clip the signal, same as you suggest.

>Are you using the Murata transducers from All Electronics?  Those seem to be
> cheap and readily available, so I was planning on using those.
>

They are not from All Electronics, they are from Mouser. I am not sure of
the manufacturer, it looks like it may be Kobitone, it's on the same page
as other Kobitone products. They are abt 1/2 inch diameter cylinders of
about 1/2 inch height also.
They are for 40KHz.

>As far as transmit goes, I was thinking of using a simple FET or BJT
amplifier
> stage as follows:
>                     Vcc
>                      |
>                      >
>                      < Rc
>                      >      C1
>                      |____||____
>     |             | /     ||    |
>PIC  |      Rb     |/            |
>Port |----/\/\/----|   Q1        [<   Transducer
>Pin  |             |\            |
>     |             | X           |
>                      |          |
>                      |          |
>                      V Gnd      V Gnd
>
>What sort of drive circuitry had you planned on using?

I am currently using a single bipolar transistor, base to pic pin, emitter
to GND, collector thru primary of transformer to positive. The transformer
is a 10:1 or so audio matching type. I'm using it to step up the voltage to
the xducer. Xducer is connected across the secondary, of course. I tried
using the transducer directly off two PIC pins, and pulsing them 180 deg
out of phase, to get effectively 10 V across Xducer but that did not work
as well as this method with transformer. This appears to be getting out a
good signal. I hooked up the receive transducer directly to a meter and at
a distance of about two feet from the transmit transducer, got an output
voltage on the receiver of about 400 to 600 mV. With the single transistor
amp, I get about 900 mV when bouncing off an object about 7 feet away.

>
> - Rick
>---
> You can help design a Serial Sonar Unit for model robots!
>  Vist http://www.notesguy.com/notesguy for details, or
>   send mail to Rick Dickinson at rtd@notesguy.com
>


Sean Breheny,KA3YXM
Electrical Engineering Student