I'm in the midst of redesigning a controller, and have asked
this in the past, but still trying to decide on what to do.
Of course, with any product cost and relibilty are the most
pressing design issues (no flames on that one), and I am currently
using 110 VAC loops to monitor the status of limit switches and
door interlocks on field mounted equipment (distance is variable maybe
even up to 100' away).  I'm using optocouplers, but the cost is about
$5 each.  So two choices...either interface the 110 VAC direct to a PIC
which I have done before as a reference input, but not sure how this
might work to sense if the AC signal is present. Sure, I could
rectify and filter, etc, but this adds cost.  The other method is to use
24VDC, a common control voltage found in alot of industrial control.
How is it for picking up noise and such? Of course, I could also implement
a software filter for noise on it (basically a debounce circuit), use a
zener to drop the 24VDC to 5VDC, and apply to the input (or perhaps a
voltage divider and cap for noise reduction).  What say all...private
email may be more acceptable.

hcooper@es.com