Your questions are quite broad, but I’ll try to answer you
- How can I wire the Arduino to the room light?
You need to use either mosfet components or relays. The most easy one to use is the relay. Usually, you drive the relay with a simple BC337 NPN transistor. What does all that mean? It’s all explained on that page.
But be careful, playing with electricity can be really dangerous.
If you want to make a light gradator, then things will become a bit more complicated, involving digitally controlled triacs, or digitally controlled potentiometer (for low current).
- Should I need an Arduino microcontroller for any electric input I’d like to control (and WIFI shield too? it won’t make the price too high?)
It depends, there’s no absolute answer for that. You can wire your arduinos (or even use ATTiny, which are way cheaper) to each of your switches, or you can have one microcontroller per set of switch etc.. That’s up to you to look at how your switches are organized in your place, and how you can design your system to better fit your needs.
The best option, imho, is taking control of “state switches” (you know the switches where when you push once, it’s on, and twice it’s off). On those you can control all your lights from your electric board, depending on the maker of your device, they can be controlled by 5V, 12V or 20V impulses. So one device on the electric board and you’re all set. But those switches need a redesign of the light’s circuit in your home…
solved Home automation for software enginners using Arduino [closed]