Hi,

You might think about using an opto on an input to the 8051 to sense the mains AC zero-crossing point. This way, you can : 1. count AC cycles, 2. know when to trigger the triac. For a fan, IMHO you don't need to work with phase angles; just turn on the triac for 1 full (half-)cycle, then off for x more (slow speed), 1 on & 1 off for 1/2 speed etc.

Regards,

Richard