This is an old revision of the document!
MP3 Player
Author: Necula Mihail
Group: 333CA
Introduction
What does it do?
The project is an MP3 player based on the AVR ATmega2560 architecture. It allows for audio playback, track navigation and smart brightness adjustment via an ambient light sensor.
Other key technical feature is the multi-step speed control (x0.5, x0.75, x1, x1.25, x1.5). The speed adjustment is available only if the audio files are processed and uploaded to the microSD card using my software. The app pre-encodes the tracks at various speeds, allowing the system to switch between files seamlessly.
Additionally, the system features persistent memory. Using the internal EEPROM, the device saves the current volume level and the last played song. This ensures that at restart, the player resumes exactly where it left off.
What is its purpose?
The goal is to provide a convenient, offline and distraction-free listening experience.
What was the initial idea?
The inspiration for this project was nostalgia for the dedicated MP3 player which I used during my childhood.
General description
Block Diagram
Hardware Modules Description
The hardware is divided into four main functional blocks:
- Central Processing Unit (ATmega 2560): Acts as the system “Master.” It orchestrates all operations by managing the I2C bus for the display and light sensor, the UART interface for the MP3 module and GPIO pins for user inputs and status feedback.
- User Interface & Sensing:
- Light Sensor (GY-302 BH1750): Communicates via I2C to provide lux values for the “Smart Brightness” feature.
- Rotary Encoder & Buttons: Provide the primary user interface for navigation, volume control, speed control and playback commands.
- Audio Subsystem:
- MP3 Decoder (DFPlayer Mini): Offloads the heavy task of MP3 processing from the MCU. It accesses the MicroSD Card via SPI.
- MicroSD Card: Stores the mp3 files.
- Amplifier (PAM8403): Takes the low-level analog output from the decoder and drives the Visaton K50 speaker.
- Speaker: The final output transducer of the audio chain. It converts the amplified analog electrical signals from the PAM8403 into audible sound waves.
- Visual Feedback:
- 1.3” OLED (SH1106): Displays metadata and system status via I2C.
- Status RGB LED: Provides immediate visual cues (e.g. playback state or errors) using PWM.
Software Modules Description
The software is organized into logical modules to ensure maintainability:
- Track Manager App: A PC-side utility used to pre-encode audio files at different speeds (x0.5, x0.75, x1, x1.25, x1.5), allowing the hardware to switch playback speeds without digital artifacts.
- Audio Manager: When a track is uploaded via the app, the user can select the option to generate five synchronized versions of the audio file at different speeds (x0.5, x0.75, x1, x1.25, x1.5). This allows the hardware to switch playback speed instantly by jumping between files, avoiding the digital artifacts. Moreover, the app includes the functionality to delete a song and manage playlists.
- Input Manager: Implements a hybrid input strategy (Interrupts for rotation, Polling for buttons).
- Display & UI Engine: Manages OLED rendering and the RGB led.
- EEPROM Manager: Ensures persistence by saving the current settings into the MCU's non-volatile memory.
Hardware Design
Bill of Materials
| Component | Amout |
|---|---|
| MEGA 2560 Development Board (16U2) | 1 |
| DFPlayer Mini MP3 Player Module (TF-16P) | 1 |
| GY-302 Digital Light Sensor (BH1750, I2C) | 1 |
| 1.3” White OLED Display (SH1106, I2C) | 1 |
| Visaton K50 Speaker (50mm diameter, 8 Ω) | 1 |
| PAM8403 Stereo Audio Amplifier Module (2x3W) | 1 |
| 8GB MicroSD Card | 1 |
| MB-102 Breadboard (830 Points) | 1 |
| AC/DC Power Adapter (5V, 2A, 5.5×2.5mm) | 1 |
| Female DC Barrel Jack to Screw Terminal Adapter | 1 |
| Tactile Switch Buttons (12x12x7.3mm) | 3 |
| Encoder Module | 1 |
| 5mm Common Cathode RGB LED | 1 |
| 220 Ω Resistor (1/4 W) | 3 |
| 1 kΩ Resistor (1/4 W) | 2 |
| 10 kΩ Resistor (1/4 W) | 2 |
| 1 μF Electrolytic Capacitor (50V) | 2 |
| 100 μF Electrolytic Capacitor (25V) | 2 |
| 1000 μF Electrolytic Capacitor (25V) | 1 |
| 100 nF Ceramic Capacitor (THT, 50V) | 4 |
| 10 cm Male-to-Male Jumper Wires | ? |
| 10 cm Female-to-Female Jumper Wires | ? |
| 20 cm Male-to-Male Jumper Wires | ? |
| 20 cm Female-to-Male Jumper Wires | ? |
Software Design
Input Strategy: Interrupts vs. Polling
A critical design choice was made to balance CPU responsiveness with input accuracy. The system distinguishes between high-speed dynamic signals and slow mechanical transitions.
- Encoder Rotation (Hardware Interrupts):
- Why interrupts?: Fast rotation generates pulses in the millisecond range. If handled via polling, these pulses would be missed whenever the MCU is busy with I2C communication (OLED) or UART (Audio). Interrupts force the MCU to pause its current task and register every “click” of the rotation instantly.
- Debouncing: To avoid using CPU-freezing delays during rotation tracking, we implement a Quadrature State Machine. This software logic only validates a step if the transition between signals follows the correct Gray Code sequence (e.g., 01 → 11 → 10).
- Tactile Buttons & Encoder Click (Software Polling):
- High-Frequency Sampling: Although handled via polling, the button is sampled thousands of times per second. Since a human press is very slow (~100ms) compared to the MCU loop speed (< 10ms), it is impossible to miss a press.
- Why not interrupts?: Polling is “cheaper” for the CPU. An interrupt forces a full context switch (saving registers, jumping to ISR), which is overkill for a slow button. Mechanical switches bounce violently. Using interrupts would force the MCU to stop and restart dozens of times for a single click, causing audio glitches or UI lag.
- Debouncing: We use a time-based “lockout” to ensure a single clean press is registered: if (current_time - last_press_time > DEBOUNCE_THRESHOLD). This ensures that after the first detection, any subsequent bounces are ignored for 50ms, while the MCU continues to drive the OLED and Audio without any freezing delays.
Results obtained
Conclusions
Download
Journal
- 24.04.2026: Project Theme Selection and Approval
- 03.05.2026: Components Order
- 05.05.2026: Documentation Page Creation
Bibliografy
