Thank you for all your answers, I will try to respond to some of them:
Many of you want more informations, I will ask the seller for the data sheet (but I doubt he is going to provide it), I can also open the plastic box which contains the dimmer and take up some measurements if it is useful.
TomGeorge:
Hi,
This is what the dimmer says s how to connect it.I would not even trust the unit at all, when the seller knows nothing about AC or DC.
The dimmer pot, could be at any potential, even at mains potential.
Can you tell us your electronics, programming, arduino, hardware experience?
Thanks.. Tom..
I'm good at programming and now I'm trying to interact with the real world, I have basic electronics knowledge.
Ralph_S_Bacon:
If this dimmer switch is on the wall in a house in the usual way then I go along with all the replies above.However, if you were willing to think laterally and move it (or have a bigger pattress) you could rotate the existing pot with a servo or stepper motor using the Arduino. Always a bit risky and given that you seem a bit new at this game it might not be something you should undertake (now there's a word not to be used in this reply) as you are dealing with mains electricity.
But I mention it only as a bit of lateral thinking and not because it answers your original query.
Just suggestin'
Yes, I thought about it as my backup plan but it's a bit tedious compared to a simple potentiometer.