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8.11 Write data to bluetooth

In this project, the Raspberry Pi Pico 2 W acts as a peripheral device in a Bluetooth Low Energy (BLE) network. It provides a custom BLE service with a characteristic that supports both reading and notification. A central device, such as a phone, can connect to the Pico W to receive text messages sent via BLE.

The onboard LED shows the connection status: it lights up when a central device connects and turns off when the device disconnects. You can customize the device name or let the system generate it automatically based on its MAC address. The script continuously advertises the BLE service and lets users input text through the terminal, which it then sends to all connected central devices.

1. Build the circuit

Required Components

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name

ITEMS IN THIS KIT

LINK

Pico 2 W Starter Kit

450+

Pico 2 W Kit

You can also buy them separately from the links below.

SN

COMPONENT

QUANTITY

LINK

1

Getting to Know Pico 2 W

1

2

Micro USB Cable

1

This project does not require building any additional circuits. Simply use a USB data cable to connect the Raspberry Pi Pico 2 W to your computer.

2. Run the code

Copy the following code into your IDE. Alternatively, you can find it in our repository at the path: pico-2w-kit/micropython/iot/8.11-write_to_ble/8.11-write_to_ble.py.

Note: This code depends on the ble_advertising.py file. Make sure to upload it to the Pico board before running the script.

import bluetooth
import struct
import time
import machine
import ubinascii
from ble_advertising import advertising_payload
from micropython import const
from machine import Pin

_IRQ_CENTRAL_CONNECT = const(1)
_IRQ_CENTRAL_DISCONNECT = const(2)
_IRQ_GATTS_INDICATE_DONE = const(20)

_FLAG_READ = const(0x0002)
_FLAG_NOTIFY = const(0x0010)

# Custom Service and Characteristic UUIDs
# Modify these as needed.
_SERVICE_UUID = bluetooth.UUID("3ec837af-b0c6-4e7e-a8c5-4b31311d98cf")
_CHAR_UUID = (
    bluetooth.UUID("945c4d90-825d-452f-820a-0d8b0cc74a12"),
    _FLAG_READ | _FLAG_NOTIFY,
)

_SERVICE = (
    _SERVICE_UUID,
    (_CHAR_UUID,),
)

led = Pin("LED", Pin.OUT)

class BLEText:
    def __init__(self, ble, name=""):
        # Initialize the BLE interface and register the custom service
        self._ble = ble
        self._ble.active(True)
        self._ble.irq(self._irq)
        ((self._handle,),) = self._ble.gatts_register_services((_SERVICE,))
        self._connections = set()

        # If no name is provided, generate one based on the MAC address
        if len(name) == 0:
            name = 'Pico %s' % ubinascii.hexlify(self._ble.config('mac')[1], ':').decode().upper()
        print('Device name: %s' % name)

        # Create the advertising payload with the custom service
        self._payload = advertising_payload(
            name=name, services=[_SERVICE_UUID]
        )
        self._advertise()

    def _irq(self, event, data):
        # Handle BLE events
        if event == _IRQ_CENTRAL_CONNECT:
            # A central has connected
            conn_handle, _, _ = data
            self._connections.add(conn_handle)
            print("New connection", conn_handle)
            led.value(1)
        elif event == _IRQ_CENTRAL_DISCONNECT:
            # A central has disconnected
            conn_handle, _, _ = data
            self._connections.remove(conn_handle)
            print("Disconnected", conn_handle)
            led.value(0)
            # Start advertising again to allow a new connection
            self._advertise()
        elif event == _IRQ_GATTS_INDICATE_DONE:
            # Indication confirmation received (not used here)
            conn_handle, value_handle, status = data

    def send_text(self, text):
        # Write the given text to the characteristic value
        self._ble.gatts_write(self._handle, text.encode('utf-8'))
        # Notify all connected centrals about the new value
        for conn_handle in self._connections:
            self._ble.gatts_notify(conn_handle, self._handle)

    def _advertise(self, interval_us=500000):
        print("Starting advertising")
        # Start BLE advertising with the given interval
        self._ble.gap_advertise(interval_us, adv_data=self._payload)

    def is_connected(self):
        return len(self._connections) > 0

def demo():
    # Create a BLE instance and a BLEText peripheral
    ble = bluetooth.BLE()
    ble_text = BLEText(ble,"pico2w")

    # Continuously read input from the terminal and send it via BLE
    while True:
        if ble_text.is_connected():
            line = input("Enter text to send via BLE (Ctrl+C to exit): ")
            ble_text.send_text(line)


if __name__ == "__main__":
    demo()

3. Read data from bluetooth

To interact with the services and characteristics defined in this code, use a generic Bluetooth® Low Energy central app, such as LightBlue (available for iOS and Android) or nRF Connect (for Android).

This section uses LightBlue as an example to demonstrate how to control the Pico 2 W’s features via Bluetooth.

  1. Install LightBlue

    Download the LightBlue app from the App Store (for iOS) or Google Play (for Android).

    ../../_images/lightblue.png

  2. Connect to Pico 2 W

    Launch LightBlue and enable location and Bluetooth permissions if prompted. On the Peripherals page, search for “pico” in the search bar, and tap to connect to the Pico 2 W device.

    ../../_images/11-1-connect-pico.png

  3. Read data form BLE

    After connecting, LightBlue displays detailed information about the Pico 2 W Bluetooth device. Scroll down to locate the Service (3ec837af-b0c6-4e7e-a8c5-4b31311d98cf) and Characteristic (945c4d90-825d-452f-820a-0d8b0cc74a12).

    Tap the characteristic 945c4d90-825d-452f-820a-0d8b0cc74a12. The app shows the properties of this characteristic: it supports reading and notifications.

    ../../_images/11-2-new.png

    In the top-right corner, select “UTF-8 String” as the data type.

    ../../_images/11-4-new.png

    Tap Read to retrieve the current value. Since no data is defined yet, the value displays as “No Value”. Next, return to the computer and enter “hello” in the terminal. Switch back to LightBlue and tap “Read” again. The message “hello” now appears, sent from the Pico 2 W to the phone.

    ../../_images/11-6-new.png ../../_images/11-6-micropython.png

    To continuously monitor updates, you can also tap “Subscribe” to subscribe to this characteristic. When you send new characters from the terminal, they will automatically update and display on your phone.

    ../../_images/11-8-new.png