well, all this is pretty disapointing indeed...
ok so, my circuit would only be good for resistive loads...
but i absolutelly need to dynamically vary the speed of those fans from the arduino with some simple circuit, if not in a completelly continuous way, at least by some steps (4 o 5 steps would probably be enough)
i need to control 6 fans in total and i need a cheap and simple solution even if rudimentary, a too complicated circuit is totally out of my reach right now
it is for an art installation and my budget and time are quite limited, and as you see, i'm not a genius of electronics, i could just implement simple circuits.
a previous version of control was made by a friend using relays that where switched on and off at variable frequency, that did resulted in a rudimentary control of the fans which was enough for our purpose, but this was much too noisy.
i guess that was a brute way to "chop" the waveform ?
my first idea was to simply substitue the normal relays with silent solid state relays, that is what brought me to the triac/optocoupler idea.
KREMMEN, i will look for this moc3011 you recommend.
do you know of any link to some specific schematic example i could follow and adapt for the arduino?
thanks to all