Bell Detector with Arduino
Here's your detection circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

The output will pulse 4.7V at the same frequency as the source when the bell is being triggered, which is appropriate for a 5V Arduino. Use a different zener diode if your Arduino is running at a different voltage. Note that R1 should be a 1/2W device or higher since 200+mW of power will be dissipated through it.

EDIT:







simulate this circuit

This new schematic connects to an input with the internal pull-up enabled. It will pulse low at half the frequency of the source.