What parts are needed for a closed door sensor?

Question

Do it yourself

There is an idea to put a sensor on the door that will send messages to the phone (via the Internet) that the door is open. But before that, I had never encountered such things and didn’t really delve into the electronics .. Tell me, what details will be needed for this?

Answer 1

Reed switch + magnet. Limit switch. Optocoupler. Choose whatever.
The sensor clings to the simplest board with MK, the GSM module clings to it.

Answer 2

Keyword - GSM alarm

Answer 3

arduino + ultrasonic range finder + gsm