The primary input expansion board. Uses the 74HC4067 analog multiplexer to provide 8 key inputs with NeoPixel LED feedback, plus a bridge connector for cascading to Extension Board 2.
Overview
Extension Board 1 is the first level of input expansion in the legacy OpenPipes architecture. It sits between the MIDI Controller Board and the physical keys or buttons, using a 74HC4067 16-channel analog multiplexer to read up to 16 input channels through just 5 signal lines (4 address + 1 common I/O).
In its standard configuration, 8 of the 16 MUX channels are populated with key inputs on this board, while the remaining 8 channels are routed to Extension Board 2 through a bridge connector.
Key Features
- 74HC4067 multiplexer — 16-channel analog MUX/DEMUX for efficient I/O scanning
- 8 key inputs — Directly mounted switches for stop tabs, pistons, or other controls
- WS2812 NeoPixel LEDs — Per-key RGB LED indicators connected in a daisy-chain configuration for visual feedback of active states
- Bridge connector — Pin header for attaching Extension Board 2, carrying the remaining 8 MUX channels and shared power/data lines
- Auxiliary debug pins — All channel states are broken out to test points for easy troubleshooting with a multimeter or logic analyzer
How the Multiplexer Works
The 74HC4067 is a CMOS analog multiplexer that connects one of 16 input channels to a single common output, selected by a 4-bit binary address:
| Address (S3-S0) | Selected Channel |
|---|---|
| 0000 | Channel 0 |
| 0001 | Channel 1 |
| … | … |
| 0111 | Channel 7 (on this board) |
| 1000-1111 | Channels 8-15 (on Extension Board 2) |
The Arduino on the MIDI Controller Board cycles through all 16 addresses at high speed, reading each key state in sequence. A complete scan of all 16 channels takes less than 1ms, ensuring zero perceptible latency.
NeoPixel Feedback
Each key position has a corresponding WS2812 addressable RGB LED. The LEDs are wired in a daisy chain, allowing the entire strip to be controlled from a single data pin. When a stop is engaged, its LED illuminates—typically in the console’s accent color—providing clear visual feedback to the organist.
Connection Diagram
MIDI Controller Board
│
│ Ribbon cable (address + data + power)
▼
┌───────────────────────┐
│ Extension Board 1 │
│ │
│ [Key 0] [LED 0] │
│ [Key 1] [LED 1] │
│ [Key 2] [LED 2] │
│ ... │
│ [Key 7] [LED 7] │
│ │
│ [74HC4067 MUX] │
│ │
│ ──Bridge Connector── │
│ │ │
└─────────┼─────────────┘
▼
Extension Board 2
Manufacturing
Standard 2-layer PCB. All components are through-hole or common SMD. Gerber files and BOM available on GitHub.