What have people come up with for a good fridge door left open alarm? Do people use door contact sensors or temperature sensors inside?
Yup! I use Iris contact sensors on all my frig and freezer doors, on the inside too.
I’ve used the xiaomi aqara temperature/humidity sensor inside a Freezer.
You will need the Ultimate Lithium Chrome 2032 batteries mind, as regular 2032 batteries seem to fail with the cold temperatures of a freezer.
An advantage of a temperature sensor, is if your freezer compressor fails, you will get a warning, which wouldn’t be the case with a contact sensor.