MP3player

MP3 Player

50x75mm mp3 player with am and fm radio

Why I built this

seems like fun also radio stuff

Why the STM32H743: it has the RAM to buffer audio enough peripherals that almost every feature ended up on its own bus and 480MHz means I never have to care about optimization.

Current status

Working on splitting this into two boards. The main board keeps the MCU, SD card, buttons, and OLED; the audio/power board takes the DAC, radio, microphone, charging, and the analog rails. It's early in the split: the single-board layout below is what's designed so far, and I'm still shuffling what goes where between the two. Might need to fix power stage to make it a 1-sided board

Pictures

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Features

Audio

• CS43131 headphone DAC with a built-in amp
• Low-noise supply rail just for the audio path
• 3.5mm headphone output
• Si4735 AM/FM radio receiver
• ICS-43434 I2S microphone for recording

Storage and controls

• microSD card over a 4-bit SDMMC bus
• OLED display
• Seventeen buttons: a 3×3 matrix for navigation, the rest are individual function buttons
• APA102 RGB LEDs

Sensors

• LSM6DSL 6-axis IMU
• BME680 (temperature / humidity / pressure / VOC)
• ISL29035 ambient light sensor
• DS3231M real-time clock

Power

• USB-C charging through a BQ24090DGQ Li-ion charger
• TPS613222ADBV boost converter makes the 5V rail
• Audio DAC gets its own clean supply

What the sensors actually do

Strictly speaking none of these have anything to do with playing music. They're in the design because a player that reacts to its surroundings is more interesting than one that doesn't:

• LSM6DSL IMU: motion and orientation. Shake to skip a track, flip over to mute, rotate the UI, or let the LEDs follow movement.
• BME680: temperature, humidity, pressure, air quality. Environmental readouts on the OLED, and it can log air quality over time.
• ISL29035: ambient light. Dims the OLED automatically so it stays readable in daylight and doesn't glare at you in bed.
• DS3231M RTC: keeps time even when the player is off. Clock display, timestamps on recordings, alarms someday maybe.

Hardware

Power

Everything runs off a single Li-ion cell. There are three rails downstream of it: one for digital logic, one for the LEDs, and a quiet one for audio. The BQ24090DGQ takes USB input, manages charging, watches pack temperature through an NTC thermistor, and drives the charge status LEDs. From the battery, a TPS613222ADBV boost converter produces 5V for the LEDs and peripherals, a TLV70233 LDO makes the 3.3V digital rail, and a TPS7A4701 low-noise LDO feeds the audio section on its own.

Power flow:

USB-C
  |
  +-- USBLC6 ESD protection
  |
  +-- BQ24090DGQ Li-ion charger
          |
          +-- Li-ion battery
                  |
                  +-- TPS613222ADBV boost converter
                  |       |
                  |       +-- 5V rail (LEDs and peripherals)
                  |
                  +-- TLV70233 LDO
                  |       |
                  |       +-- 3.3V digital rail
                  |
                  +-- TPS7A4701 low-noise LDO
                          |
                          +-- audio rail

Why three rails? the boost converter switches at a few MHz and I really did not want that showing up in my headphones. Same reason the DAC gets its own regulator.

Schematic

The design is split across hierarchical KiCad sheets:

PCB

Six layers, 1.6mm thick, roughly 50×70mm. I kept the analog audio away from the digital stuff and the switching regulators as much as I could. DAC sits right next to the headphone jack to keep those traces short. I was careful about return paths under the charger and boost converter to keep the switching noise contained.

Assembly

Designed for standard SMT assembly. The order I use:

1. Apply solder paste with the stencil.
2. Place the small parts first: 0603 resistors and caps, ferrite beads, crystals.
3. Then the big ICs: STM32H743 (LQFP-100), CS43131, Si4735, power management, APA102 LEDs, sensors.
4. Reflow.
5. Inspect the fine-pitch parts under magnification for bridges.
6. Install connectors: USB-C, headphone jack, microSD socket, SMA antenna connectors, battery connector, debug headers.
7. Install the user-facing stuff: seventeen tactile switches and the OLED connector.
8. Electrical checks: continuity, shorts, verify each rail comes up at the right voltage.
9. Flash initial firmware over SWD.

Firmware

Lives in firmware/, PlatformIO project on the STM32 HAL. Right now it brings up the hardware: 480MHz clock from the 8MHz crystal via PLL, GPIO, the I2C/SPI/UART buses, SD card interface, control pins. Playback, the UI, radio control, recording, and the LED effects don't exist yet. That's the next chunk of work.

Build and flash

You'll need PlatformIO (CLI or VS Code extension) and an ST-Link, CMSIS-DAP, or J-Link debugger.

cd firmware
pio run

Flashing over SWD:

pio run -t upload

Bill of Materials

BOM.csv has supplier part numbers, links, and pricing for everything above.

Fabrication

Gerbers are in kicad/production/MP3player.zip

Boards and stencils come from JLCPCB.

PCB specs:

Stencil specs:

Known Issues

• Single-board design is being split into main + audio/power boards; that work is early.
• Audio performance hasn't been measured with real firmware yet.
• Battery runtime is untested.
• RF side needs work: antenna matching was calculated, not measured.
• No thermal testing under load yet.
• Enclosure isn't designed.

Firmware currently only does hardware init. Still to write:

• Audio playback pipeline
• SD card filesystem support
• Audio decoding
• OLED user interface
• Button handling and debounce
• Radio control
• Recording
• Sensor drivers
• LED spectrum visualization
• Battery monitoring

Credits

• Board, schematics, and firmware all my own work.
• STM32 HAL by STMicroelectronics.
• PlatformIO for builds.
• KiCad libraries for standard symbols and footprints.