TinyDrop20

TinyDrop20 is a small ATtiny85-based 5x4 charlieplexed LED matrix board. The repository contains the PlatformIO firmware project, Eagle design files, and manufacturing outputs for hardware revision V1_1.

Project background

This project was created in collaboration with @talofer99 as part of a series of free-time hardware and firmware projects we built years ago. I decided to move these projects to GitHub in the hope that they may be useful, educational, or inspiring to others.

This PCB revision was manufactured and tested.

Repository layout

Firmware/
  V1_1/
    platformio.ini      PlatformIO project configuration
    src/main.cpp        ATtiny85 firmware
    include/            Project headers, currently unused
    lib/                Project libraries, currently unused
    test/               PlatformIO test folder, currently unused

Hardware/
  V1_1/
    Eagle/              Eagle schematic and board files
    Manufacturing/
      GERBER/           Gerber package for PCB fabrication
      Schematic/        Exported schematic PDF

Hardware overview

The V1.1 board is built around an ATTINY85-20SU microcontroller and a 20 LED display arranged as a 5 row by 4 column matrix. The LEDs are 0603 green parts driven through five 100 ohm resistors from ATtiny pins PB0 through PB4.

Other notable hardware:

• 20 green 0603 matrix LEDs
• 1 red 0603 status LED
• ATtiny85 SOIC-8 microcontroller
• Five 100 ohm matrix resistors
• 10 kOhm reset pull-up resistor
• Micro USB connector
• MCP73831 battery charger circuit
• CR2450-size coin-cell holder footprint
• Six round test points for programming or probing
• 3.6 mm mounting pad

Power note: the schematic includes an MCP73831 Li-ion/LiPo charger and a battery holder labeled CR2450. Standard CR2450 cells are not rechargeable. Before connecting USB power with a cell installed, verify that the assembled board is intended for a compatible rechargeable lithium coin cell, such as the exact cell specified by the hardware build.

Firmware overview

The firmware is an Arduino sketch built with PlatformIO for attiny85 at 8 MHz. It drives the LEDs directly by changing ATtiny GPIO direction and output state for charlieplexing.

Display constants in Firmware/V1_1/src/main.cpp:

#define SPEED          35
#define DISPLAY_WIDTH  4
#define DISPLAY_HEIGHT 5

The sketch includes:

• A 4x5 bitmap font for uppercase letters, digits, space, !, #, $, and ^
• setLed() and setPixel() helpers for charlieplexed LED control
• renderCharacter() and renderString() helpers for drawing text
• Several visual modes and effects:
  • matrix_fill_test()
  • write_text()
  • scrolling_text()
  • screen_saver()
  • rainEffect()
  • rows_effect()

The current sketch displays myString, defined near the top of main.cpp:

char myString[] = "#$^";

In the checked-in firmware, lastState is hardcoded to 1, so setup() sets mode = 0 and the main loop runs matrix_fill_test(). The EEPROM-based mode toggle code is present but commented out.

Build firmware

Install PlatformIO, then build from the firmware directory:

cd Firmware\V1_1
pio run

Build output is generated under:

Firmware/V1_1/.pio/build/attiny85/

The main outputs are firmware.elf and firmware.hex.

Upload firmware

The PlatformIO environment is configured for a USBtiny-compatible ISP programmer:

[env:attiny85]
platform = atmelavr
board = attiny85
framework = arduino
board_build.mcu = attiny85
board_build.f_cpu = 8000000L
upload_protocol = usbtiny

Connect the programmer to the ATtiny85 programming pads or test points, then run:

cd Firmware\V1_1
pio run -t upload

The Micro USB connector on the hardware is part of the board power and charger circuit. The firmware configuration expects ISP programming with usbtiny, not USB bootloader programming.

Hardware files

Open the board in Autodesk Eagle/Fusion Electronics using:

Hardware/V1_1/Eagle/TinyDrop20_V1_1.sch
Hardware/V1_1/Eagle/TinyDrop20_V1_1.brd

Manufacturing files are already exported:

Hardware/V1_1/Manufacturing/GERBER/TinyDrop20_V1_1_2020_03_09.zip
Hardware/V1_1/Manufacturing/Schematic/TinyDrop20_V1_1.pdf

Development notes

• The charlieplexing code uses ATtiny pins 0 through 4, corresponding to PB0 through PB4 in the schematic.
• Only one LED is actively driven at a time. Persistent images are produced by repeatedly scanning pixels fast enough for persistence of vision.
• To show different text, update myString in src/main.cpp.
• To enable the text display mode, set mode = 1 or re-enable the EEPROM mode selection logic.
• Generated PlatformIO build files under .pio/ should not be committed.