Door sensor: a micro:bit, two Swift apps and a hard lesson in networking

My mom tends to walk into my room unannounced, and store-bought door sensors couldn't make my Mac react. So I built my own from a spare BBC micro:bit.

An ultrasonic ranger confirms the door opening over four readings in a row. Over Bluetooth, a Mac app closes chosen tabs and an iPhone app by the bed switches to a clock.

When raw TCP sockets kept failing on the iOS 26 beta, I rebuilt the link with Apple's MultipeerConnectivity.