Reza,

I hope to add to what has been said.

Just for fun, I made a 20-level temperature controller for my electric shower (4kW) using AT89C2051 and a zero-crossing optocoupler (MOC3061) to trigger a TRIAC.

By trial and error, I found that twenty cycles of the mains voltage as control period were adequate for my control purposes __ control range from 0 to 100%, with steps of 5% (one mains cycle), resulted in a comfortable temperature controller, i.e., with temperature kept constant for any given setpoint.

When I made the number of cycles longer, say 30, then the water temperature undesirably went slightly up and down between every control period. Twenty, then, seemed to tune the controller pretty well to the shower reaction time.

The only problem with my design is the faint flickering of lamps in the house, which does not seem to be an issue with your 500W project.

Good luck.

J.R.