# Build three now, reuse them in the 2×7

The prototype is the first three reusable modules. There is one mechanism design, one 38-symbol alphabet and one set of calibrated motor identities. No smaller prototype drum, different leaf size or fixed dash is used.

| Item | Prototype installed | Final installed | Add for expansion |
| --- | ---: | ---: | ---: |
| Complete moving mechanisms |3|14|11|
| Two-sided leaves |114|532|418|
|5V28BYJ-48 motors and ULN2003 drivers |3each|14each|11each|
| Hall sensors and 3×1mm magnets |3each|14each|11each|
| ESP32-C3 controller |1|1|0|
| MCP23017 expanders |1|5|4|
| BSS138 translator |1|1|0|

Use the C3/MCP build from the beginning. The optional direct-GPIO WROOM bench setup proves a mechanism, but its controller wiring is a different path and is not the default upgrade plan.

## Label and keep the completed assemblies

Label the first three mechanisms01,02,03 (firmware IDs0,1,2), including their motor, Hall and driver connections. They occupy the first three positions of the final top row. Keep each 38-leaf drum assembled, with its marked magnet polarity and original driver. Do not shuffle motors between calibrated modules. Save the controller configuration before moving the harness.

The three modules retain the same shell, motor plate,38-slot drum, two gears, Hall retainer,114 leaves, twelve M3×30 screws and twelve module nuts. Both layouts use37.9mm column pitch. Qualification leaves that pass are used in module 1: five sample leaves plus33 remaining leaves make its 38; the next two modules each need one complete 38-leaf set.

All five prototype casing parts transfer unchanged: both ends, rear cover, power spacer and removable power tray. Keep their seven inserts, four M3×16 cover screws and three M3×6 tray screws. Add another five-piece set, four 2-character extensions, four rear wings, four hidden row links and one unused-inlet plug. The full board has23 installed case parts; the upgrade prints18. The four 124.7mm prototype metal rods are replaced by eight 276.3mm rods; retain the short rods as spares. No qualified printed part is discarded. Exact shared casing inventory and rod requirements are defined by the current [mechanical manifest](../hardware/redesign/dimensions.json). Use the **upgrade_from_three** sequence in the [P2S print plan](../hardware/print-prep/note/manifest.json), rather than printing the full-from-zero plan after completing the prototype. Do not mix old case revisions.

## Preserve calibration when adding the remaining eleven

Add expanders at 0x21–0x24, and retain the original expander at 0x20. Original module IDs0–2 stay on its slots0–2. Follow the board-specific pull-up/address changes in [ELECTRICAL.md](ELECTRICAL.md). Inspect the harness with power disconnected, then increase the active count in the existing controller UI. Its saved offset and steps-per-revolution entries remain attached to the same IDs.

The host command is:

```sh
python3 scripts/splitflap.py --url http://CONTROLLER-IP --token-file /path/to/token expand 14 --max-active 4
```

Use the token setup described in [SOFTWARE.md](SOFTWARE.md). This command reads current device settings and changes only module count and the requested concurrency limit. **Do not load the fresh `flight-panel.json` profile over a calibrated prototype**; that would replace measured values with defaults. Re-home after expansion and calibrate the eleven newly added modules. Re-check the original three for alignment after moving/clamping them; their stored calibration is preserved, but a mechanical change can still require adjustment.

## Print only what has passed the previous step

1. Fit coupon and five-leaf sample, using the loaded black Generic PETG and no AMS.
2. One mechanism and the33 remaining leaves, plus the other two shell/gear-plate housings and prototype casing. Clamp the normal three-position stack; load the RIGHTMOST drum/motor against the right-cheek bearing for the initial powered test, wired as ID0. Verify fit, paint, free movement, homing and all38 symbols.
3. Finish the two already printed housings with their drums/gears/retainers and76 leaves. Reorder the complete tested module into the leftmost position (still ID0), keeping its own shell and motor plate; re-clamp, re-home/recheck, then qualify the complete three-character prototype. Do not reprint those housings.
4. Eleven more mechanisms and 418 leaves, plus only the expansion case pieces listed by the print plan.

The geometry and software can be checked digitally. PETG shrinkage, paint thickness, sensor polarity/gap, motor variation and repeatable motion must be checked on the first physical assembly before batch printing.