pxt-calliope/docs/reference/bluetooth/on-bluetooth-disconnected.md

On Bluetooth Disconnected

~hint

For another device like a smartphone to use any of the Bluetooth "services" which the micro:bit has, it must first be paired with the micro:bit. Once paired, the other device may connect to the micro:bit and exchange data relating to many of the micro:bit's features.

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This block starts an event handler which in this case will run when a device which is connected to your micro:bit over Bluetooth disconnects.

You could use this event handler to display a letter "D" on the micro:bit LED grid so you know that the Bluetooth connection has been closed.

bluetooth.onBluetoothDisconnected(() => {
});

Example: Displaying "D" when a Bluetooth connection to the micro:bit is closed

bluetooth.onBluetoothDisconnected(() => {
    basic.showString("D");
});

Video - on Bluetooth disconnected

http://www.youtube.com/watch?v=HyBcsD9Eh6I

See also

Bluetooth SIG, Bluetooth on micro:bit resources

microbit-bluetooth