I bought a really neat wrought-iron door knocker. It's basically a little hammer, about 6" long, that comes down on a plate to make noise.
I'd like to convert this to a doorbell. Because the hammer is pretty heavy, I just don't think I can embed a regular switch in the plate. I'm thinking a hall sensor and a magnet.
Something like this: https://www.sparkfun.com/products/9312
I have 2 wires going to the doorbell, so whatever I do has to work off of 2 wires; if the arduino is at the doorbell then the two wires have to supply power as well as ring the bell, much in the same way as the existing lighted manual doorbell works.
(The light bulb in the doorbell is run in series with the solenoid coil. The bulb provides enough resistance so the solenoid does not activate. Pushing the doorbell shorts the bulb, and lets enough current flow to move the solenoid.)
The arduino would have to be powered in a similar way: a resistor to take place of the bulb, a relay across the resistor, a cap to store power while the resistor is shorted, and diodes to keep it all from shorting out. Plus some circuitry to provide 5VDC from the 16VAC the doorbell puts out....