SoundCube

A battery-powered DIY music box with a Class-D amplifier, USB-C charging, MP3 playback from a USB stick, an Aux input and a small OLED display. Schematic and PCB designed in KiCad.

Status: Work in progress. Schematic and PCB layout are under active development; firmware and enclosure will follow.

Features

• Class-D amplifier based on the TI TPA3118 for high efficiency on battery power
• Single 21700 Li-ion cell (Samsung INR21700-50S, 5000 mAh) with BMS protection
• USB-C charging via TI BQ25606 buck charger with power path, so the box can play while charging
• MP3 playback from a FAT32 USB stick via an MP3 decoder module
• Aux input (3.5 mm jack) that mechanically disconnects the MP3 source when a cable is plugged in
• Analog volume control with a logarithmic stereo potentiometer
• OLED display (SSD1306, 128×64) and push buttons for play/pause, next and previous
• Microcontroller for user interface and player control

System overview

flowchart TD
    BAT["21700 Li-ion cell<br/>3.7 V / 5000 mAh"] --> BMS["1S BMS<br/>over-discharge & short-circuit protection"]
    USBC["USB-C<br/>5 V / 3 A"] --> CHG
    BMS --> CHG["BQ25606<br/>charger with power path"]
    CHG -->|"SYS 3.0–4.4 V"| SW["Main switch"]
    SW --> LDO["AP2112K LDO<br/>3.3 V logic"]
    SW --> BOOST["TPS61088 boost<br/>12–15 V"]
    BOOST --> LC["LC filter"]
    LC -->|"PVDD"| AMP

    LDO --> MCU["Microcontroller"]
    MCU -->|"I²C"| OLED["SSD1306 OLED"]
    BTN["Buttons"] --> MCU
    MCU -->|"UART"| MP3["MP3 module"]
    USBA["USB-A<br/>(USB stick)"] --> MP3

    MP3 -->|"Line out"| JACK["Aux jack<br/>switching contact"]
    AUX["Aux in 3.5 mm"] --> JACK
    JACK --> POT["Stereo potentiometer<br/>(log)"]
    POT --> COUP["Input coupling<br/>DC block + RF filter"]
    COUP --> AMP["TPA3118<br/>Class-D amplifier"]
    AMP --> OUTF["Output LC filter"]
    OUTF --> SPK["Speaker"]

Loading

Hardware

Block	Component	Purpose
Battery	Samsung INR21700-50S	3.7 V nominal, 5000 mAh energy storage
Protection	1S Li-ion BMS	Over-discharge and short-circuit protection
Charger	TI BQ25606	USB-C charging with power path (NVDC)
Logic supply	AP2112K-3.3	3.3 V LDO for microcontroller and display
Amplifier supply	TI TPS61088	Synchronous boost converter, 12–15 V for the amplifier
Amplifier	TI TPA3118	Class-D power amplifier
Display	SSD1306	128×64 OLED via I²C
Audio source	MP3 module + USB-A	Playback from FAT32 USB stick

Design notes

• Separate supply rails: The amplifier runs from a boosted 12–15 V rail to get more output power from a single cell, while the logic runs from a clean 3.3 V LDO.
• Supply filtering: An LC filter after the boost converter reduces switching noise on the amplifier supply.
• Grounding: Digital ground (MCU, display), analog ground (audio path, potentiometer, Aux) and power ground (boost converter, amplifier) are kept separate and joined at a single star point near the charger's main capacitor to keep switching currents out of the audio path.
• Amplifier fault monitoring: The TPA3118 fault output (open-drain) is read by the microcontroller using its internal pull-up.

Repository structure

SoundCube/
├── PCB/            KiCad project (schematics, layout)
│   └── lib/        Custom symbols, footprints and 3D models
├── LICENSE
└── README.md

The schematic is split into several sheets: power supply, amplifier and microcontroller.

Opening the project

1. Clone the repository
2. Open PCB/PCB.kicad_pro in a recent version of KiCad
3. Custom libraries are included in PCB/lib/ and referenced relative to the project, so no extra setup is needed

License

This project is licensed under the MIT License. See LICENSE for details.