Steve Hardy wrote (among other things):
 >For proper control, you
>will need to know at least the zero crossing points of the mains
>voltage.  Preferably, you should also have a current sense so that
>the thyristor turn-off point can be known.

What method of current sense is available commercially?  If you knew the
phase difference for each cycle, then you could program the pic to turn
off and on at explicit points of the phase using zero cross detection as
the timing reference.  Before that I suppose you would need to
empirically view the phase difference on a scope, eh?

-Pete