With V1 and V2 being the points that I can easily hook into. The "Bell Actuator" (resistance 6.7Ω) is an electromagnet that physically hits the two chimes to sound. It's powered by four alkaline batteries. The blue highlight is what I think are the coil terminals and the white wire goes to the button.
I'd like to leave the existing circuit untouched so that the bell will work if the Arduino is out of power (e.g. batteries run down). I see no evidence of EMF protection.
I have some electronics experience but I'm a bit lost on a good starting point here (opto-isolator, relay, mosfet, 10MΩ resistor to the ADC?).
Thanks for the response, do you have any suggestions for a suitable component? All the ones I've found so far seem to assume that the signal will come from the low voltage side.
The switch will have 6V across it when off and 0V across it when pressed. If everything is battery operated with no "ground" you can safely connect the side with negative voltage to the Arduino ground.
The other can go to an Arduino input pin (with some voltage protection). Of course the logic is "reversed" (active low) since the input is normally high.
Add a over-voltage protection circuit to protect the Arduino from the 6V and from the negative inductive kickback from the solenoid inductance. (You can increase the current-limiting resistor to between 1K and 10k.)
Or, an opto-isolator also protects the Arduino but it also needs a reversed diode to protect the isolator's LED from inductive kickback.
Thanks @paulpaulson and @DVDdoug, I opted to use a reverse wired 1N4001 in parallel with a current limiting resistor and an opto-coupler (HCPL-817) and it's working really well.
