ABK-20 showcase variants

ABK-20

8-Bit Algorithmic Audio Synthesizer with 5×4 NeoPixel Matrix Display

The ABK-20 is a modernized desktop hardware console redesign of the legendary 8Bit Mixtape. Powered by a Microchip ATtiny85 running an 8-bit algorithmic sound engine (bytebeats, wavetable, granular, and punk console synths), it features an illuminated 20-LED (5×4) visual display screen, tactile physical controls, and instant sound sketch loading via the AudioBoot bootloader.

Hardware Highlights

• Microcontroller: Microchip ATtiny85 running at 16 MHz (Internal PLL).
• Visual Display: 20x WS2812B / SK6812 RGB NeoPixels arranged in a 5-column × 4-row matrix for expressive animations, meters, and UI feedback.
• Illuminated Slide Fader: Bourns PTL20 20mm slide potentiometer with an integrated lever LED tied to the status/data line (PB0).
• Tactile Controls:
  • Illuminated Rotary Potentiometer: 10kΩ vertical potentiometer (POT2-CV1 on PB4, RV09 / Alpha RD901F compatible) with illuminated transparent knurled shaft.
  • Master Volume Control: Side-mounted thumbwheel potentiometer (POT3-Vol1).
  • Navigation Switch: MTS-102 sub-miniature toggle switch (SW3) on PB3 for menu navigation and mode selection.
  • Illuminated User Button: Highly KS01-BL-13 round illuminated push button (SW2) on the PB3 resistor ladder.
  • Hardware Reset Button: Dedicated tactile push button (SW4) pulling PB5 (Reset) to ground.
• Audio Circuit:
  • High-speed 8-bit PWM audio synthesized via Timer1 64 MHz PLL.
  • On-board PAM8002 audio amplifier optimized for unity gain ($R_{14} = 22\text{k}\Omega$, $C_5 = 1\mu\text{F}$, $C_8 = 100\text{nF}$) to prevent clipping and preserve deep sub-bass bytebeats through the internal speaker.
  • 3.5mm stereo output jack (switchable line-out / headphones with volume control).
• AudioBoot Programming: Upload new sound sketches directly through an audio cable or web browser—no external programmer or USB serial driver required.
• Power Circuit:
  • USB-C 5V Input: Modern 6-pin USB-C receptacle (J5) with dual 5.1kΩ pull-down resistors (R17, R18 on CC1/CC2) for universal compatibility with USB-PD smart chargers and MacBook Type-C ports.
  • Diode OR-ing Protection: Automatic seamless power switching via Schottky diodes (SS14) across USB-C (5V), 7805 DC In (7–12V), and 3x AAA Battery input with reverse-charge protection.
  • Master Power Switch: Latching push switch (SW1) with power status LED (D-POW1). Cycling SW1 provides the exact power-cycle timing required to activate AudioBoot sketch loading.

Schematic

The ABK-20 schematic is organized into a modular KiCad 9 hierarchical architecture:

Page 1: Main Architecture & Sound Engine

Controls, ATtiny85 microcontroller, audio filtering, power regulation, and hierarchical sub-sheet block.

Page 2: 5×4 NeoPixel Display Matrix

Dedicated hierarchical sub-sheet (hardware/LED_Matrix.kicad_sch) with the raster-scan daisy-chained 20-LED array.

📄 Download Complete Multi-Page Schematic PDF (A4)

PCB Layout & 3D Model View

The compact 75×80 mm rounded-corner desktop PCB design features the centered 5×4 NeoPixel display matrix, framed by the Bourns illuminated slide fader, clear-shaft illuminated rotary potentiometer, 3-way toggle switch, tactile buttons, thumbwheel volume pot, audio jacks, and side expansion headers:

Hardware Variants & Comparison Showroom

The repository includes a dedicated standalone showroom board (ABK-20_showcase_variants.kicad_pcb) for mechanical verification, 3D rendering, and rapid component swapping across various hardware options:

• Audio Jacks: PJ-317 vertical, PJ-342 SMD gullwing, and CUI SJ1-3525N horizontal jacks.
• Faders & Pots: 20mm and 30mm Bourns slide pots, Alpha RD901F vertical, and 16mm back-pad pots.
• Power & Battery: 3x AAA on-board battery clip holders, barrel jack, and USB-C.

Component Staging & Exploration

Earlier component staging layout used during initial mechanical fit checks, 3D VRML model validation, and footprint library curation:

Frontplate & Enclosure Ecosystem

The repository provides fully parametric OpenSCAD designs and production deliverables in production/:

1. Parametric 3D-Printed Frontplate (production/ABK20-case/)

A dedicated OpenSCAD front panel (ABK-20_frontplate_3dprint.scad) designed for standard 16 HP ($80.96 \times 92.28\text{ mm}$) Eurorack rails or the desktop laser-cut chassis:

• Integrated 5×4 Baffle Grid: Extends downward to the PCB to completely isolate each NeoPixel and prevent optical light bleed between cells.
• Selectable Matrix Styles (via OpenSCAD Customizer):
  • diffuser_grid: Integrated $0.20\text{ mm}$ translucent 3D-printed diffuser skin (single-extruder M600 filament swap: white on bed, colored panel face).
  • open_frame_grid: Crisp open apertures with elegant top edge bevels.
  • window_recess: Stepped frame recess for an acrylic window insert.
• Mounting & Cutouts: M3 Eurorack mounting slots with counterbore recesses, underside PCB corner standoffs, and precise cutouts for the slide fader, pots, switches, buttons, and expansion headers.
• Pre-Rendered STL: ABK-20_frontplate_3dprint.stl and configuration presets in ABK-20_frontplate_3dprint.json.

2. Laser-Cut Frontplates (production/frontplate/)

• Vector Artwork Panels (production/frontplate/ABK_frontplate_02.svg, output.pdf): High-detail laser cutting and surface engraving artwork with silkscreen labeling and control indicators.

3. Parametric Enclosures (production/ABK20-case/)

• 3D-Printable Snap-Fit Enclosure (ABK-20_case_3dprint.scad, .stl, .3mf):
  • Two-part direct-fit enclosure (top_shell + bottom_cover) designed for supportless FDM 3D printing with integrated 5×4 NeoPixel baffle grid.
  • Prototype Hardware Support (Preset: "perfect Proto"): Specifically tailored for early prototype boards with top-mounted PJ-317 audio jacks. Adds beveled rectangular window cutouts ($14.5\times 15.5\text{ mm}$, $R=1.4\text{ mm}$) centered over each jack and widens lateral board clearance to $1.5\text{ mm}$ for effortless PCB insertion around the side thumbwheel volume pot.
  • Standard Production (Preset: "ABK-20_diffuser_grid"): Solid continuous top face with rear-exiting audio jack ports.
• Laser-Cut Desktop Console ("Chistli") (ABK-20_case.scad):
  • Parametric finger-jointed desktop chassis in 3mm acrylic or wood with an ergonomic $12^\circ$ playing tilt.
  • Extended side cheeks with custom corner radii and Eurorack rail mounting options.
• Enclosure Documentation: production/ABK20-case/README.md & CASE_DESIGN_NOTES.md.

4. Acoustic Optimization & Waveguide Engineering

The repository includes a comprehensive acoustic engineering architecture to solve the acoustic dipole short-circuit on rear-mounted micro-speakers and dramatically enhance bass/mid punch on physical hardware:

• Master Engineering Plan: production/ABK20-case/ACOUSTIC_OPTIMIZATION_PLAN.md
  • Downward Acoustic Chimney Waveguide: Integrated directly into both the 3D-printed case ceiling and Eurorack faceplate. Tapers downward to seal against the PCB acoustic holes, transforming the $8.1\text{ mm}$ airspace into an acoustic horn with $+3\text{ to }+6\text{ dB}$ higher acoustic projection and zero internal sound spill.
  • 8 Classic Vintage Sound Grill Styles (selectable via speaker_grill_style):
    • horiz_slots: Classic Dieter Rams / Braun T3 tiered horizontal slits.
    • horiz_4slots: 4 thin horizontal slits (retro 70s/80s Hi-Fi cassette deck).
    • soundwave: 7 graduated vertical slits forming an analog synth graphic EQ waveform.
    • braun_dots: 19-hole staggered precision dot matrix (Braun ET66 / T41).
    • chevron: $45^\circ$ angled Japanese synth louvers (Roland TR/TB style).
    • radial: 1950s transistor radio radial dial spoke starburst.
    • slots: Classic 5 vertical Eurorack slots.
    • aperture: Open framed window for real speaker cloth or brass wire mesh.
  • Modular Acoustic Kit (production/ABK20-case/ABK-20_speaker_acoustic_kit.scad):
    • Parametric sealed and Helmholtz bass-reflex back-box cups (Standard $2.4\text{ cm}^3$, XL $5.1\text{ cm}^3$, and MAX $6.3\text{ cm}^3$) with extended snorkel ducts tuned to $280-360\text{ Hz}$.
    • Support-Free Printing: Continuous flat PCB support keel at $Z = \text{depth}$ with $\le 45^\circ$ transitional chamfers prints 100% support-free when oriented flange-down.
  • Precision Centering: Added panel_alignment = "symmetric" to resolve legacy SVG viewBox offsets, achieving exact $2.98\text{ mm}$ symmetrical PCB margins and balanced top jack outer walls.

ATtiny85 Pin Mapping

Firmware & Web Flasher

• ABK-20 Online Web Audio Player & Flasher (hosted directly from web/index.html)
• Software & Sketches Suite (PlatformIO build environment, hex2wav pipeline, and 30+ sound sketches)
• Also compatible with the 8Bit-Vintage Firmware Repository

TinyAudioBoot Bootloader (ABK-20 Custom Edition)

The ABK-20 features a dedicated, highly optimized build of TinyAudioBoot for ATtiny85 running at 16 MHz internal PLL, fully documented in software/tools/bootloader/:

• Skipper Edition (TinyAudioBoot_Skipper_ABK-20.hex, 960 bytes):
  • Instant Boot: Starts the active synthesizer sketch in $< 25\text{ ms}$ upon normal power-on.
  • Upload Mode on Demand: Enters a 30-second audio upload window only when holding button SW2 during power-up or reset.
• Standard Edition (TinyAudioBoot_Standard_ABK-20.hex, 874 bytes):
  • Provides an automatic 5-second audio upload window on every power cycle.
• Silent Standby on PB1:
  • Actively clamps PB1 to GND during bootloader standby so there are zero pops, clicks, or 1 Hz ticking chirps in headphones or speaker.
• Page-Write Bi-Color Visuals:
  • Alternates Green $\leftrightarrow$ Red on every 64-byte flash page write for real-time visual progress monitoring.
• Flash Budget:
  • Occupies $\le 1\text{ KB}$ at address 0x1C00, maximizing flash memory to 7,168 bytes ($0\text{x0000} \to 0\text{x1BFF}$) for user sketches.
• ISP Flashing & Fuses:
  • Quick one-liner AVRDUDE flash commands and fuse reference (lfuse: 0xe1, hfuse: 0xdd, efuse: 0xfe) are detailed in software/tools/bootloader/burn_bootloader.md.
• Architecture & Evolution:
  • Comprehensive breakdown of all circuit and firmware differences compared to the original v3.0–v3.2 bootloader in software/tools/bootloader/BOOTLOADER_EVOLUTION_AND_DIFFERENCES.md.

Repository Structure

ABK-20/
├── .agents/                          # Antigravity agent rules & 8Bit Mixtape skills
├── web/                              # Web AudioBoot player & sound app catalog (GitHub Pages)
│   ├── index.html                    # Online Web Audio Flasher & synthesizer player
│   ├── apps.json & apps.js           # Sound track catalog metadata & registry
│   ├── wav/                          # Production 8-bit bootloader audio streams (.wav)
│   ├── audio/                        # 30-second audio preview MP3s
│   ├── gifs/                         # Animated 5x4 matrix LED previews (solid clean mode)
│   └── manuals/                      # Individual user manuals for each sound application
├── docs/                             # Project documentation, hardware notes & datasheets
│   ├── hardware/                     # Circuit notes, engineering summaries & schematics
│   │   ├── HARDWARE_NOTES.md         # Hardware & circuit design notes
│   │   ├── REV03_ENGINEERING_SUMMARY.md # Engineering breakdown
│   │   ├── AMP_AUTO_SHUTDOWN_CIRCUIT.md # LM4871 amp shutoff specification
│   │   ├── DESIGN_VARIANTS_GUIDE.md  # Alternative layout variants
│   │   └── ABK-20_schematic.pdf      # Multi-page A4 schematic export
│   ├── datasheets/                   # Component datasheets (Bourns faders, switches, pots)
│   ├── fabrication/                  # DIY etching & drilling guides (ABK-20_DIY_DRILL_HANDOUT.pdf)
│   ├── history/                      # 8Bit Mixtape lineage & Coconut doctrine (HISTORY_AND_ARCHITECTURE.md)
│   ├── software/                     # Software developer guides (WORKFLOW_GIF_GENERATION.md)
│   └── img/                          # Images, 3D renders & diagrams
├── hardware/                         # KiCad 8 EDA design source files
│   ├── ABK-20.kicad_pro              # KiCad project file
│   ├── ABK-20.kicad_sch              # Main hierarchical schematic (Page 1)

License

Licensed under the CERN Open Hardware Licence Version 2 – Strongly Reciprocal (CERN-OHL-S-2.0).