I took a look at AN521 just now to refresh my memory. I'm a little
puzzled about the comments of "connecting the AC line directly to the
PIC". The input to the PIC is current limited through a 5 Meg resistor.
I have seen this type of circuit (combined with the clipping diodes
which are internal on a PIC) used in commercial applications many times.
Even before PICs were common. Another variation is to use a voltage
divider ahead of the current limit resistor. I think that even on a PIC
running from a transformer isolated supply, that the AC line coupled via
a (say 5 meg resistor) to a PIC input would still give enough signal to
work. The AC "return" would be a capacitive/inductive  coupling through
the transformer back to AC neutral. This is somewhat along the same
lines as how some touch switches work.


Frank Richterkessing
Experimental Methods Engineer
GE Appliances

FRANK.RICHTERKESSING@APPL.GE.COM