Hi Ricardo, hello sweety. I meddle to clarify to Chinito that Ricardo's drawing is all inside the bell box.
This connects directly to 220 volts permanently and two cables (would be low voltage) are taken to the front where you have the metal switch. EYE WITH CARRYING 220 VOLTS TO THE SWITCH OF THE CARTON BOX, IT IS NOT ISOLATED TO THE METAL.
I would never take it as an intrusion :) and also clarify that there are 12v bells to which a transformer must be added (an example are baby bells, widely used in electric gatekeepers) Greetings