I think ive been looking at this the wrong way round. Ive had the multimeter on one of the boxes Matt made (8 triacs are riveted to the side so i cant see under the PCB). From what i can see the interrupt pin isnt connected at all. I think he has a main loop which just waits for a certain pin to go low (for the zero-crossing) then executes the bit to control how bright everything is. As for the serial input i think that is where the interrupt comes in. Either that or there is no interrupt at all and he checks that in the main loop too. Now its starting to sound simple

For now im gonna use the same chips Matt used and see if i can make his circuit work on a breadboard. Then i can copy the hex code to the new chips. That gives me 8 outputs so ill need 2 chips per circuit but they are cheaper than 1 big one.

1 thing i find weird about his circuit is that MCLR doesnt appear to be connected to anything :S