Are you working with the indoor or the ruggedized Polaroid transducer? The
indoor (the round black mesh-looking one) has a lot better gain, but doesn't
like being splashed alot. The outdoor is elipsoid-shaped and has a more
focused pattern.

In any case, you have to be careful of:

The ringing of the input signal after sending the sonar pulse (You must blank
for x amount of time.)
targets that will bounce back an even stronger signal. (sometimes)
signals from other transducers firing in the same frequency.
broadband sonar noise -- Try shooting white noise ala air brakes or an air
hose nearby and watch a real strong echo.
Both frequency and voltage on the output transducer are critical with rather
narrow bands.

I've got a patent in that field and had so solve a lot of stupid little
problems to get the devices to work reliably. (PIC C57 which also powered the
RS-485 network.) You can get more accurate than a foot -- Ours is running .6
inches reliably. I'd like to get more detailed with code, etc. but I'll bet
our customer would get reasonably upset.

Tom

----------
From:   pic microcontroller discussion list on behalf of Frank B. Scalzo
Sent:   Friday, July 12, 1996 11:32 AM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        ultrasonic ranging

I am working on a project that needs to find the distance to objects.
Currently I am using using a PIC16C84 as my controller. I'm using 40kHz
transmit and recieve transducers from polaroid. I am driving the
transmit transducers by attatching one of it's leads to RA1 and the
other to RA2. In my code RA1 is always the opposite of RA2 and I switch
them at 40kz 20 times, to make an outgoing train of pulses. My reciever
transducer has one pin to ground and one to a 2 transister amp
(2n2222's). I am having problems with range, I fear recier amp is not
strong enough, but if I increase it's gain, I detect ghost (false)
signal returns. Any suggestions to improve my design? I want a range of
2-15 feet accurate within only a foot. Also,does anyone know of a way to
narrow by field of detection. My current design detect objects off to
the side.