HEDC B002

dotclock

An ESP32-based alarm clock with a dot matrix LED display, rotary encoder interface, MP3 audio playback, and a web configuration UI.

Features

• 32x8 dot matrix display (4x FC-16 / MAX7219 modules) with custom 3x6 pixel font
• Rotary encoder for all on-device interaction (press to cycle modes, turn to adjust)
• MP3 alarm tones stored on-chip (LittleFS, ~12MB available)
• Web UI for settings, time sync, WiFi config, and file management
• 25-step brightness (8 software PWM + 16 hardware levels)
• Persistent settings via NVS

User Interface

The encoder button cycles through modes. The knob adjusts the value for the current mode. After 15 seconds of inactivity, the display returns to time mode.

When the alarm is ringing, the display flashes inverted at ~1.4Hz. Press button or turn knob to dismiss.

Web Interface

WiFi AP: dotclock / password dotclock1 / IP 192.168.4.1

• Sync time from phone/browser (with timezone)
• Set alarm time, enable/disable, select alarm sound
• Adjust brightness and volume
• Connect to a WiFi network (AP+STA mode, enables SNTP)
• Upload, rename, and delete MP3 files

Audio

• MP3 files, decoded via libhelix-mp3
• 8-bit DAC output on GPIO25 through a PAM8403 class-D amplifier
• GPIO25 driven low when idle to minimize speaker static

Project Structure

• HEDC-F001/ — Firmware (ESP-IDF v5.3.1 project)
• HEDC-B002/ — Hardware (board schematic, see hardware reference below)

TODO

• Read encoder (debouncing, velocity detection)
• Set display (writing, brightness control)
• Custom 3x6 pixel font with 8x8 mode icons
• MP3 audio playback
• Alarm system with looping playback
• Web interface (settings, file management, time sync)
• Persistent settings (NVS)
• WiFi AP + STA with SNTP
• Fine-tune font glyphs
• RC filter on DAC output for cleaner audio

HEDC-B002 Hardware Reference

Board Overview

The HEDC-B002 is an ESP32-based controller board with USB-C programming, SPI LED output (5V level-shifted), a rotary encoder with push button, an audio amplifier, and a 32.768kHz RTC crystal. The crystal, however, doesn't work right, it seems.

Power

ESP32 GPIO Map

Used GPIOs

Unused GPIOs

Components

U3 — CH340C (USB-to-Serial)

SOIC-16 USB-UART bridge. Internal oscillator (no external crystal). Connected to USB-C connector J1.

Q1 — MMDT3904-7-F (Auto-Reset)

Dual NPN transistor (SOT-363) implementing the standard ESP32 auto-programming circuit. Cross-coupled: RTS drives IO0 (BOOT), DTR drives EN, so a DTR/RTS sequence enters the bootloader automatically.

U1 — 74HCT244 (Level Shifter)

TSSOP-20 octal buffer. Powered from +5V. Converts 3.3V ESP32 SPI signals to 5V for J3 output. Bank 1 active (OE tied to GND), bank 2 active but unused.

J3 — 5-Pin LED Output Connector (FC-16 footprint)

This is the SPI output for driving an LED matrix or display panel.

SW1 — PEC09-2320F-S0015 (Rotary Encoder)

Bourns horizontal-mount rotary encoder with push button. All signals pulled up to 3.3V with 5.1kΩ resistors.

U2 — PAM8403DR (Audio Amplifier)

SOP-16 class-D stereo amplifier, powered from +5V. Left channel input (INL) driven from ESP32 IO25 through a coupling capacitor. Speaker output on SPK1 header (marked DNP — do not populate).

Y1 — CM315D32768DZFT (RTC Crystal)

32.768kHz crystal connected to IO32/IO33 with 22pF load capacitors (C7, C8). Provides a precise timebase for RTC functionality.

J1 — USB-C Connector (UJ20-C-V-C-1-SMT-TR)

USB 2.0 Type-C receptacle (14-pin). CC1/CC2 have 5.1kΩ pull-downs (R6, R7) for device identification.

Code Quick-Reference

// --- Pin Definitions ---

// SPI LED output (directly usable with hardware SPI: VSPI)
#define PIN_LED_CS    17  // SPI chip select
#define PIN_LED_CLK   18  // SPI clock (VSPI CLK)
#define PIN_LED_DIN   23  // SPI MOSI (VSPI MOSI)

// Rotary encoder (active LOW, external 5.1k pull-ups)
#define PIN_ENC_A     26  // Encoder channel A
#define PIN_ENC_B     27  // Encoder channel B
#define PIN_ENC_SW    14  // Encoder push switch

// Audio output (PWM/DAC → PAM8403 amplifier)
#define PIN_AUDIO     25  // DAC1 output (IO25 is DAC channel 1)

// UART (directly to CH340C — used by USB serial, avoid reassigning)
// #define PIN_TXD0    1
// #define PIN_RXD0    3

// RTC crystal (do not use as GPIO)
// IO32 — XTAL 32.768kHz XIN
// IO33 — XTAL 32.768kHz XOUT

Notes for Firmware

• SPI: IO17/IO18/IO23 map to the default VSPI bus (CLK=18, MOSI=23). IO17 is used as CS but is not the default VSPI CS0 (IO5), so configure it manually.
• Audio: IO25 is one of the ESP32's two DAC outputs (DAC1). You can use dacWrite(25, value) for simple waveforms, or configure I2S for higher quality audio output to the PAM8403.
• Rotary encoder: Use interrupt-driven decoding on IO26/IO27. The push button on IO14 should be debounced in software.
• RTC crystal: IO32/IO33 are dedicated to the 32.768kHz crystal. Use the ESP32's internal RTC with this external crystal for accurate timekeeping. Configure via rtc_clk_32k_enable() or the ESP-IDF RTC API.
• Boot strapping: IO0 is used for boot mode (auto-reset handles this). IO2 is NC. IO12/IO15 affect boot — if using them, ensure they're not pulled to unexpected states at power-on.