Assuming the pump runs at a relatively reasonable speed (maybe 1800RPM = 30
pulses per second?) you might be able to do everything inside the PIC with
software based on the existing A/D value (if the PIC isn't too busy
already):

> >In analog electronics, this could be done by low-pass filtering the
> >signal

Low pass filtering in software is very easy. The simplest technique is
probably an IIR filter, for example:

NewFilteredValue = OldFilteredValue * (1-k) + NewSample*k

Values of K like 1/2, 1/4, 1/8, 1/16 make for a particularly efficient
implementation.

>> AC coupling it

This can be done by maintaining a longer term average (ie: a much slower
digital filter) to track the average value of the signal.

> and applying a zero-crossing detector with

Just compare the outputs of the two filters.

> >some hysteresis.

Before doing the compare of the two filters, add a small value to the one
which was higher on the last pass. This will help it continue to be higher,
hence adding hysteresis.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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