Just a word of thanks here to Tom McGahee for his outstanding help on this
subject.

He sent me a PDF with a schematic and explanation of how it works (does that
ever help!), along with suggestions on what to change (and why or why not) if it
didn't fit exactly with my needs.  Not being very strong on analog, this is
_exactly_ the kind of help I needed.

I urged him to post the PDF on his site, since it is _very_ helpful.  I'm sure
others with similar problems will find it a useful resource.
The next one is to Chris Eddy for his input on the _hows_ of connecting it up.
Chris made some good suggestions on what to do, and they will be helpful as well
when it comes time to the actual implementation.

Thanks again, both of you!

Andy