Hello Kiwi
Attached inline is a write up of how I did the project:
View attachment 10189
You are right about the internal pull-ups, direct input should never be used when interfacing to the outside world. It's a cruel place out there and the Arduino will last about a day. All that wire is nothing but a big antenna and a local FM station will look like data to be processed! The one thing about using a normally-closed contact for the sensor output/s (high true as an Arduino digital input) is that a failure of the sensor wiring (like has been cut) will cause an alert. This is stand burglar alarm good practice even if I did not follow it.
Have fun, Eric