Hi
I was looking at a method for measuring the speed of sound a few years ago
at school where you measured a distance to a wall, clapped your hands from
that distance and clapped on each echo.  This gave surprisingly good
results.  A method like this for sonar could work quite simply as follows:

40kHz ping is output

Analogue electronics receives this and starts a feedback loop where another
ping is instantly sent

The rangefinding PIC then counts the number of pings in a given time.

An inverse function is applied to this to give a large number for larger
distance using the speed of sound, time taken for measurement and number of
pings received.

This leaves the pic dealing with only counting and maths.



In terms of comms, keeping it simple is wise.  Suggestions of multi node
networks would complicate things and could be implemented in another pic
for that particular design.

I would stick to a serial output perhaps the first byte indicating with a
binary sequence the range (mm, cm, m etc) and the second the actual data.
A spare pin could be used to fix the range for applications where a wide
range is not required.  This would save problems with 256 bits and allow a
high accuracty.

An alternatice method would be to use three bytes.  The first two of the 8
bits indicate range (as data would only need to be up to 9mm, 99cm etc) and
the last 6 indicate data.  This would allow high precision on even the
metre ranfe where all three bytes could be used.


Some sort of calibration function would be useful where a distance of 1
metre is measured and the unit calibrated into its eeprom (on the f84) with
the speed of sound in those conditions.


If you need help on checking that components are available in the UK then I
have a pile of catalogues in my bedroom and I am more than happy to help.
I also have unlimited web space with my ISP so if you need it, it is there.



Tim Kerby


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