With all components assembled in the proto board, I tested the system and, as such, there were some flaws in the arduino program and in the zero-cross detection circuit. How would I fire the Triacs in the correct time? Also, how the Arduino is receiving the signal from the zero-cross detector? I figured that the zero-cross circuit was faulty as when I have set the dimmer to a 50% of power the lamp was just blinking as it was a disco! Reviewing the circuitry connections revealed that I have connected the pin of the Arduino input at the non-connected pin if the TIL 111, where it should be connected to the pin 5 (the colector) of it. Correcting the connection and testing it afterwards revealed that there was still a problem with the Arduino logic, as the firing of the triacs was correct, but the scale on the display (where we control the luminosity using the individual scroll bars) was wrong. More calculation needed, as we have in Brazil 110V / 60 Hz, and the propagation of the firing in the Triac has to be taken into account. 

Adjustments made to the logic, program compiled, uploaded to the Arduino, and finaly it worked as it should!