> I found a very interesting pic ultrasonic range circuit

Tal has sent me this address for it

http://www.eb.uah.edu/~wells/cpe437_sp_00/hrf/index.htm

I haven't had a chance to look at the code properly yet, but
some comments anyway

If you were to use a 20MHz PIC (F628 or F84A) the performance
could be improved. The author seems happy with a 4MHz. If 4MHz
is acceptable, then a 628 running with internal oscillator would
have the 4 pins available to get the LCD into 8-bit mode. Migrating
to a larger PIC would be another option. Two pins are being used
to generate the reciprocal drive for the transmitter, but it could be
done with one and an external inverter, but those are just nit-picky
comments as the circuit works (allegedly) OK as it is

The LCD is probably a standard 16x2 - the picture of it shows
two chips. It could be run faster, as a fixed pause is used instead
of checking the Busy flag, but that's not really important in this
application

I'm sure that issue with the output transistor could be dealt
with in another way. Rather than find a particular transistor
it would seem to make more sense to deal with the noise.
Not having built one I can't say what the noise is, but his
pictures of the prototype show it to be breadboarded with
no evidence of any attempt at filtering. A few filter caps on
a proper PCB with shorter wiring would go a long way to
cleaning it up. The author seems aware of that, but I don't
know what he means by "a generic copper-clad PCB", unless
that's his term for a breadboard

Overall it's probably not a bad starting place for a rangefinder
although I'm sure it could be improved upon with a little work

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