I thought you couldn't connect anything to the doorbell?

I think when I want it to work without connecting the wires to the doorbell, it will take more time to get it to work, than just connecting it.

The HA11AA1 would do the job. Connect the input with a 390Ω resistor in series across the bell, and then use a 10kΩ pull-up resistor to +5V to get your logic output signal.

Is it possible to get 3.3v? Or do I need a voltage divider to get this signal?

 

Not my idea, will need modding to suit, but should give you a start

View attachment 114960

Credit goes to: http://www.anderswallin.net/2006/08/optoisolator-cards-for-mesa-5i20-servocard/#more-77

This is fine but use one of the AC optocouplers as that single LED will not like some 11V reverse voltage which it would get from 8V AC input.