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](/reference/bluetooth/bluetooth-pairing). Once paired, the other device may connect to the micro:bit and exchange data relating to many of the micro:bit's features.
The [Bluetooth UART service](start-uart-service.md) allows another device such as a smartphone to exchange any data it wants to with the micro:bit, in small chunks.
With the Bluetooth UART service running, this block allows a micro:bit to send data to a Bluetooth connected device.
```sig
bluetooth.uartWrite("");
```
### Example: Starting the Bluetooth UART service and then sending "HELLO" whenever button A is pressed and another device has connected over Bluetooth
```blocks
let connected = 0;
bluetooth.onBluetoothConnected(() => {
basic.showString("C");
connected =1;
});
bluetooth.onBluetoothDisconnected(() => {
basic.showString("D");
connected = 0;
});
input.onButtonPressed(Button.A, () => {
if (connected == 1) {
bluetooth.uartWrite("HELLO");
}
});
```
### Video - UART service guessing game
https://www.youtube.com/watch?v=PgGeWddMAZ0
### Advanced
For more advanced information on the micro:bit Bluetooth UART service including information on using a smartphone, see the [Lancaster University micro:bit runtime technical documentation](http://lancaster-university.github.io/microbit-docs/ble/uart-service/)
