HC-05 Bluetooth Interfacing with Raspberry Pi Pico – Control Outputs

In this tutorial, we will learn about the HC-05 Bluetooth-to-serial port module and how to interface it with the Raspberry Pi Pico. This Bluetooth device operates on UART communication. In other words, it uses serial communication to transmit and receive data serially over the standard Bluetooth radio frequency. Therefore, we will use the TX and RX pins of the Raspberry Pi Pico to connect with the module.

In this tutorial, we will use an Android application to communicate with the Raspberry Pi Pico through the HC-05 Bluetooth module. We will demonstrate an example to control an LED from the Android application using Bluetooth wireless communication. The LED will be connected to a GPIO pin of the Raspberry Pi Pico, and we will be controlling that pin remotely from a smartphone.

Prerequisites

Before we start this lesson, make sure you are familiar with and have the latest version of Python 3 installed on your system, and that MicroPython is set up on your Raspberry Pi Pico. Additionally, you should have a working Integrated Development Environment (IDE) for programming. We will be using Thonny IDE throughout this tutorial.

• Getting Started with Raspberry Pi Pico using Thonny IDE

If you are using uPyCraft IDE, you can check this getting started guide:

• Getting Started with Raspberry Pi Pico using uPyCraft IDE

Bluetooth Terminal Application

Throughout this guide, we will use an Android smartphone to connect to our Raspberry Pi Pico. Make sure you have an Android phone available. We will also use a Bluetooth terminal application to pair the two devices together.

Go to the Play Store and download the application named Serial Bluetooth Terminal. This is shown in the figure below.

HC-05 Introduction

The HC-05 is one of the most commonly used Bluetooth modules that operates on the UART communication protocol. It stands out from other Bluetooth devices because of its versatile pin configuration and dual operating modes. The module normally communicates via UART serial at 9600 baud rate by default. However, it can be configured using AT commands and supports baud rates of 9600, 19200, 38400, 57600, 115200, 230400, and 460800.

One of the most important features of the HC-05 is its ability to work in both Master and Slave modes. In slave mode, the module waits for an incoming connection from a master device such as a smartphone or another Bluetooth module. In master mode, the module actively initiates a connection with another Bluetooth device. This makes HC-05 extremely flexible for a wide range of wireless communication applications.

The module operates in the 2.4GHz ISM band and uses the Bluetooth 2.0 standard with Enhanced Data Rate (EDR). It has a typical communication range of about 10 meters in open space, though this can vary based on obstacles and interference. The HC-05 module is based on the CSR BC417 chip and comes pre-configured with firmware that makes it simple to use in UART passthrough mode.

Its CSR Bluecore 04-External single chip is pre-configured to communicate with other Bluetooth devices through serial communication. It provides two-way communication, and the HC-05 can act as either a slave or master. The module is limited to short-range communication, but its speed and security make it a popular choice. One limitation is that it does not support media file transfer.

This module can be used for low-cost wireless communication between microcontrollers such as Arduino, TM4C123, PIC Microcontroller, Raspberry Pi, BeagleBone, STM32, and Arduino Mega. It can also be used for communication between a microcontroller and a PC or Android application. For this tutorial, we will use the ‘Serial Bluetooth Terminal’ application.

Recommended Reading: HC-05 Bluetooth Module

Pinout

The HC-05 module has multiple pins and status indicators that help control different operations. The pinout diagram below shows all available pins.

The table below describes the functionality of each pin.

HC-05 AT Command Mode

One of the most powerful features of the HC-05 is its built-in AT command interface. AT commands allow you to configure the module’s settings such as its name, baud rate, pairing PIN, and operating mode. To enter AT command mode, hold the button on the HC-05 module while powering it on. The onboard LED will blink slowly (approximately once every two seconds) to indicate that the module is in AT command mode.

Some commonly used AT commands for the HC-05 include:

• AT – Tests the communication link. The module responds with “OK”.
• AT+NAME=MyDevice – Changes the Bluetooth device name to “MyDevice”.
• AT+PSWD=1234 – Changes the pairing PIN to “1234”.
• AT+UART=9600,0,0 – Sets the baud rate to 9600.
• AT+ROLE=0 – Sets the module to slave mode (0 = slave, 1 = master).
• AT+RESET – Resets the module and applies new settings.

To send AT commands, you can use a USB-to-serial adapter connected to the HC-05 TX and RX pins, and use a terminal program such as PuTTY or the Arduino Serial Monitor at the correct baud rate (38400 baud for AT command mode by default).

Interfacing HC-05 with Raspberry Pi Pico

In this section, we will show you how to connect the HC-05 module to the Raspberry Pi Pico. The connection is straightforward since we will be using the UART serial interface, which only requires two data lines: TX and RX. The Raspberry Pi Pico has two UART channels: UART0 and UART1. The figure below shows the available RX and TX pin options on the board.

LED Control Example with HC-05, Raspberry Pi Pico, and Android App

For demonstration purposes, we will control a Raspberry Pi Pico GPIO pin with an LED connected to it. This will be done via the Android application through the Bluetooth module. We will toggle the LED by pressing buttons configured inside the Android application.

Required Components:

• Raspberry Pi Pico
• HC-05 Bluetooth Module
• 5mm LED
• 220-ohm resistor
• Connecting Wires
• Breadboard

Schematic: Raspberry Pi Pico and HC-05

Follow the schematic diagram below to connect all the components together.

The HC-05 module has two serial data pins. The TX pin of the HC-05 connects to the RX pin of the Raspberry Pi Pico (GP1), and the RX pin of the HC-05 connects to the TX pin of the Raspberry Pi Pico (GP0). We are using UART0 for this example. The VCC pin of the HC-05 connects to the 5V supply pin of the Raspberry Pi Pico, and both grounds are connected together.

The anode of the LED connects to GP2 of the Raspberry Pi Pico, and the cathode connects to GND through a 220-ohm current-limiting resistor. All ground pins are tied to a common ground.

The data is transferred at the default baud rate of 9600, but this can be changed using AT commands if required.

Setting up the Android App

In your smartphone settings, enable Bluetooth and scan for available devices. You should see the HC-05 in the list. The default device name is “HC-05”, and the default pairing PIN is either “0000” or “1234”.

After installing the Serial Bluetooth Terminal app, open it and tap the three horizontal bars in the top-left corner of the screen.

Now tap on the Devices tab.

In the ‘Available Devices’ section, select ‘HC-05’.

After selecting the HC-05 module, tap the ‘link’ icon at the top of the screen. You will first see a ‘Connecting to HC-05’ message, and after a successful connection, you will see ‘Connected’.

Now we will configure the buttons used to toggle the LED. First, click the M1 button as shown below:

Add the button details. Configure this button as an “LED ON” button and assign the value ‘1’ to it. Press the tick icon at the top right to confirm the changes.

Similarly, create another button M2 as the “LED OFF” button and assign the value ‘0’ to it.

You will now have two buttons to control the state of the LED.

MicroPython: Raspberry Pi Pico LED Control with HC-05 and Android App

from machine import Pin, UART
uart = UART(0, 9600)

Led_pin = 2
led = Pin(Led_pin, Pin.OUT)

while True:
    if uart.any():
        data = uart.readline()
        print(data)
        if data == b'1':
            led.high()
            print("LED is now ON!")
        elif data == b'0':
            led.low()
            print("LED is now OFF!")

How the Code Works

We begin by importing the Pin and UART classes from the machine module.

from machine import Pin, UART

Next, we create a UART object by calling UART(), passing the UART channel number (0) as the first argument and the baud rate (9600) as the second argument. This initializes UART0 to communicate at 9600 baud with the HC-05 module.

uart = UART(0, 9600)

We then define the GPIO pin number for the LED (GP2) and configure it as a digital output.

Led_pin = 2
led = Pin(Led_pin, Pin.OUT)

Inside the infinite while loop, we continuously check whether any data has been received on the UART channel using uart.any(). If data is available, we read it with uart.readline() and store it in the variable data.

while True:
    if uart.any():
        data = uart.readline()
        print(data)

When the ‘LED ON’ button (M1) is pressed in the app, the value b'1' is received. When ‘LED OFF’ (M2) is pressed, b'0' is received. We use if-elif statements to act on these values accordingly. The b prefix indicates a bytes object, which is what MicroPython’s UART returns.

        if data == b'1':
            led.high()
            print("LED is now ON!")
        elif data == b'0':
            led.low()
            print("LED is now OFF!")

You should first run the code and press each button individually to observe what byte values are printed in the Thonny shell. Adjust the comparison values in the code to match what your app actually sends.

Extending the Project: Controlling Multiple Outputs

The example above controls a single LED, but the same principle can be extended to control multiple GPIO pins or even more complex outputs. For example, you could add more buttons in the Serial Bluetooth Terminal app and map each button to a different GPIO pin. Below is an extended version of the code that controls three LEDs connected to GP2, GP3, and GP4.

from machine import Pin, UART
uart = UART(0, 9600)

led1 = Pin(2, Pin.OUT)
led2 = Pin(3, Pin.OUT)
led3 = Pin(4, Pin.OUT)

while True:
    if uart.any():
        data = uart.readline()
        print(data)
        if data == b'1':
            led1.high()
            print("LED 1 ON")
        elif data == b'2':
            led1.low()
            print("LED 1 OFF")
        elif data == b'3':
            led2.high()
            print("LED 2 ON")
        elif data == b'4':
            led2.low()
            print("LED 2 OFF")
        elif data == b'5':
            led3.high()
            print("LED 3 ON")
        elif data == b'6':
            led3.low()
            print("LED 3 OFF")

In this extended example, six buttons (M1 through M6) in the app send values ‘1’ through ‘6’, each mapped to turning a specific LED on or off. This approach can be adapted to control relays for home appliance control, motors, buzzers, or any other digital output device connected to the Raspberry Pi Pico’s GPIO pins.

Troubleshooting Tips

If you encounter issues with the HC-05 and Raspberry Pi Pico connection, here are some common problems and their solutions:

• HC-05 not detected by the phone: Make sure the HC-05 is powered correctly. The VCC pin must be connected to 5V (not 3.3V). The onboard LED should blink rapidly when the module is on and waiting for a connection.
• Connection drops frequently: This usually indicates interference or a weak power supply. Make sure your power supply can provide adequate current. The HC-05 can draw up to 30mA during active Bluetooth transmission.
• No data received in Thonny: Double-check that TX of HC-05 is connected to the RX pin of Raspberry Pi Pico and RX of HC-05 is connected to the TX pin of Raspberry Pi Pico. A common mistake is swapping these connections.
• Incorrect byte values received: Always print the raw received data first to confirm what the app actually sends before adding logic to control outputs. Different app versions or button configurations may send different byte sequences or include a newline character.
• Module not entering AT mode: Ensure you press and hold the button on the HC-05 before and during power-up. The LED blinking pattern changes from fast to slow when AT mode is active.

Demonstration

After connecting the HC-05 module and uploading the code, click the LED ON button in the app. The LED will turn on, and you will see a “LED is now ON!” message in the Thonny shell terminal. The value ‘1’ will also appear in the app’s terminal window.

Pressing the LED OFF button will turn the LED off and display “LED is now OFF!” in the Thonny shell.

Watch the video demo below:

Related Bluetooth tutorials and projects:

• HC06 Bluetooth Module Guide with Arduino Interfacing
• HC-05 Bluetooth Interfacing with TM4C123G Tiva C Launchpad – Keil uvision
• HM-10 Bluetooth Module – Interfacing Example with Arduino
• Introduction to Bluetooth Low Energy
• Control 2 DC Motors via Bluetooth and Arduino
• Bluetooth Based Home Automation
• Bluetooth based DC Motor Speed and Direction Control using Arduino
• HC-05 Bluetooth Module Interfacing with Arduino
• Bluetooth Module HC-05 Interfacing with PIC Microcontroller
• Bluetooth Controlled Robot using PIC Microcontroller