Michel Polder wrote:

> The time between the AC zero crossing and the triggering determines
> the light intensity. This is just like a standard triac based light
> dimmer.

  ... however the problem with the "standard" Triac dimmers is their
sensitivity to transients and Zellweger (ripple-control) tones.

  My suggestion it to run a counter using the PIC timer with a period
corresponding to the power line frequency (different in your case to
mine), and use a compare function to determine the trigger point for
the Triac.  No different so far.

  The trick is not to reset the counter at the zero crossing, but to
accumulate the error between the timer and the zero crossing, making
incremental adjustments until they coincide.  This is called a "Software
Phase Locked Loop" (soft PLL) or in TV terminology, a "flywheel", and
should allow much more stable control and freedom from flicker.  The
counter pre-load can be interrupt-driven, but the ZC detection and
trigger comparison can probably be done quite well in the main loop.

  Don't forget the "phase un-lock" inhibit and soft-start algorithms
of course!

  Cheers,
        Paul B.