EnergyOptPCB

Bluetooth Audio Receiver — ESP32

A Bluetooth audio receiver built on an ESP32-WROOM-32D in ESP-IDF C++. The system receives stereo audio from a phone over Bluetooth Classic, outputs the decoded audio through PCM5102A DAC, and displays the current song, artist, album, and playback status to an LCD Display in real time, all housed in a custom PCB.

Features

• Operates as a Bluetooth Classic A2DP sink
• Receives stereo audio from compatible phones and computers
• Outputs decoded PCM audio at 44.1 kHz, 16-bit stereo through a PCM5102A DAC over I2S
• Retrieves song title, artist, album, and playback status through AVRCP and Requests new metadata whenever the track changes. It displays media information on a 128 × 160 ST7735R TFT over SPI
• Uses a separate low-priority FreeRTOS task for the connection-status LED
• Includes a custom PCB design, schematic, and board renders

Hardware

Components and Estimated Cost

IMPORTANT: This project requires an ESP32 variant with Bluetooth Classic support. A2DP and AVRCP cannot run on ESP32 variants that provide only Bluetooth Low Energy (BLE).

Pinout

PCM5102A I2S Audio Interface

ST7735R SPI Display Interface

Status LED

PCB

KiCad source lives in the Hardware/

PCB Top

PCB Bottom

Build and Flash

Requirements:

• PlatformIO (VS Code extension or CLI)

The repository includes platformio.ini, ESP-IDF CMake files, and board-specific SDK configuration files. PlatformIO uses them to select the framework, board environment, dependencies, and required ESP-IDF features.

git clone <this-repo-url>
cd <project-folder>
pio run --target upload
pio device monitor

Once flashed, the board advertises itself over Bluetooth as "BT Audio" — pair your phone and start playing music.

File Structure

├── Hardware/              # KiCad PCB and Design files
├── lib/                   # Project-local libraries, if used
├── src/
│   ├── CMakeLists.txt     # ESP-IDF component registration
│   └── main.cpp           # Main Firmware
├── test/                 
├── .gitignore
├── CMakeLists.txt         
├── platformio.ini         # PlatformIO environment and dependencies
├── sdkconfig.esp32dev     # ESP-IDF board configuration
└── README.md

Code Architecture

Execution Contexts

Several pieces of the program can run independently:

The firmware explicitly creates the LED task with xTaskCreate(). Bluetooth tasks are created internally when the controller and Bluedroid stack are enabled.

1. Display Configuration (LovyanGFX Library)

Our LGFX class inherits from lgfx::LGFX_Device and owns the panel and SPI bus objects:

lgfx::Panel_ST7735S _panel_instance;
lgfx::Bus_SPI       _bus_instance;

The bus configuration selects SPI2_HOST, SPI mode 0, and a 20 MHz write clock. MISO is disabled because the application does not read data from the TFT.

The panel configuration defines the chip-select and reset pins, 128 × 160 drawing area, panel offsets, display inversion, and RGB order. The SPI bus is attached to the panel, and the panel is attached to the parent LovyanGFX device. The resulting global lcd object performs all drawing operations.

2. I2S Audio Configuration

setup_i2s() creates a transmit-only channel:

i2s_chan_config_t chan_cfg =
    I2S_CHANNEL_DEFAULT_CONFIG(I2S_NUM_AUTO, I2S_ROLE_MASTER);

The ESP32 acts as I2S master and generates the clocks for the PCM5102A. The standard-mode configuration selects:

• 44.1 kHz sample rate (CD quality)
• 16-bit samples
• Stereo slot mode
• Philips I2S timing
• No receive channel
• No master-clock output

i2s_channel_init_std_mode() applies the format and pins. i2s_channel_enable() starts the peripheral.

3. Bluetooth Initialization

app_main() initializes NVS before Bluetooth because ESP-IDF's Bluetooth stack depends on non-volatile storage. If NVS has an incompatible version or no free pages, the firmware erases that partition and retries initialization.

The application then:

1. Creates the default Bluetooth controller configuration.
2. Initializes the Bluetooth controller.
3. Enables Bluetooth Classic mode.
4. Initializes and enables the Bluedroid host stack.
5. Sets the device name to BT Audio.
6. Initializes the AVRCP controller.
7. Registers the AVRCP callback.
8. Registers the A2DP PCM callback.
9. Initializes the A2DP sink.
10. Makes the ESP32 connectable and discoverable.

4. A2DP Audio Path

A2DP carries the stereo Bluetooth stream from the source to the ESP32. After Bluedroid decodes a chunk of audio into PCM samples data buffer, it invokes bt_audio_data_callback which writes the audio samples to the DAC with i2s_channel_write

5. AVRCP Metadata and Playback State

AVRCP provides information about the media session. The callback receives an event identifier and a union containing the corresponding event data.

6. Deferred Display Updates

Title, artist, and album commonly arrive as separate AVRCP events. Drawing immediately for every event would cause multiple rapid full-screen redraws. Instead, the callback updates the appropriate buffer and raises need_screen_update

The main loop checks the flag every 50 ms. When an update is pending, it clears the flag and calls update_screen(). This groups closely timed metadata events and reduces visible flicker.

update_screen() opens one LovyanGFX write transaction, clears the screen, and renders the playback indicator, song title, artist, album, labels, and divider.

7. Bluetooth Status LED Task

Made BluetoothStatusLED() as a separate FreeRTOS task that has a 2,048-byte stack and priority 1 (low priority) which uses the is_connected flag (gets updated in bt_avrc_ct_callback) to update LED:

vTaskDelay() blocks the task between transitions instead of consuming CPU time in a busy-wait loop.

8. Main Application Loop

After initialization, app_main() remains alive as the UI service loop that polls need_screen_update and updates the screen