SWC

SWC — Steering Wheel Controls Adapter

A programmable interface between a vehicle's factory steering-wheel buttons and an aftermarket Android head unit, so the buttons can be re-mapped, combined, or given press behaviours the factory wiring never had.

Status: design complete and routed; 0 DRC errors, 0 unconnected, 0 schematic/PCB parity errors. Not yet ordered.

Top — ESP32-S3 module, four left-edge screw terminals, USB-C	Bottom — connector pinouts silkscreened on the back

Why this exists

A steering-wheel button is not a digital switch. It is a resistor dropped across a single analog KEY line; the head unit pulls that line up and identifies the pressed button from the voltage it reads. There are usually two such lines — this board calls them SWC1 and SWC2 — with the buttons split between them.

That arrangement is cheap and robust, and also completely fixed: the resistor values, and therefore the head unit's behaviour, live in the factory button pod. An aftermarket head unit wants a specific resistance it was taught, or one the user can program. This adapter supplies it by reading the ladder on one side and servoing the head unit's KEY line on the other, so the mapping becomes software. It can additionally send commands to the head unit over USB when a button is pressed, for actions the head unit's own steering-wheel input cannot express.

Architecture at a glance

Two identical channels. The factory ladder is read by the ESP32-S3's ADC; the head unit's KEY line is driven by a closed-loop servo, so the output tracks what the board actually senses rather than what it assumes.

  SWC1  J2.3 ─R1 10k─┬─ /SWC1_ADC ─────────► U3 ADC1
  SWC2  J2.2 ─R2 10k─┴─ /SWC2_ADC ─────────► U3 ADC1
        (R15/R16 10k pull-ups to +3V3 · D4/D5 BAT54S clamp · C3/C4 100nF)

  AUX1  J5.4 ─R23 1k─┬─ /AUX1_F ───────────► U3 ADC
  AUX2  J5.3 ─R24 1k─┤   /AUX2_F
  AUX3  J5.2 ─R25 1k─┴─ /AUX3_F
        (R17/R18/R19 10k pull-ups to +3V3 · D8/D9/D10 BAT54S · C15–C17 100nF)

  U3 ESP32-S3 ══ I2C, 3.3 V, R5/R6 10k pull-ups ══ U4 MCP4728  (VDD = +3V3,
       │                                              VREF = VDD, 0–3.3 V FS)
       │                                              ┌ A ─ signal, ch 1
       │                                              ├ B ─ gain mode, ch 1
       │                                              ├ C ─ signal, ch 2
       │                                              └ D ─ gain mode, ch 2
       │
       └─ per channel:  U4.VOUTx ─R46 100k─► U6x− ─(C24 100nF fb)─► U6x out
                                          U6x+ ◄─ R58 82k ─ V_buf
                                              ◄─ R61 100k ─ V_ADJ (VOUTB/D)
                        U6x out ─R52 1k─► Q4 gate ─► Q4 drain = KEY line
                        KEY ─R36 1M─► U6y buffer ─┬─► R54 10k / R50 10k ─► /SENSEx ─► U3 ADC
                                                   └─► R58 (feedback above)

  12 V ─ J1 ─ F1 PPTC ─► /+12V_SW ─► D1 SMBJ18A ─► U1 XL1509-5.0 ─► L1 47µH ─┐

The three ideas that make it work:

• The ladder is read, not re-created. 10 kΩ series (1 kΩ on the AUX inputs), a BAT54S clamp to +3V3/GND, a filter, and a pull-up — so a bare switch-to-ground button works as well as a resistor ladder.
• The output is a servo, not a buffer. V_KEY = (1 + R58/R61)·V_DAC − (R58/R61)·V_ADJ, so the gain is a resistor ratio and the offset is a second DAC channel. That gives runtime range switching between 3 V and 5 V head units, and it is why the ESP32 needs no DAC of its own (the S3 has none).
• Idle is inherently high-impedance. The sink FET only pulls down; a command above the head unit's own idle voltage turns it off and the line floats back up through a 1 MΩ sense resistor.

Full description, including the analog detail, the power path, the USB front end, and the reasoning behind each component choice: DESIGN.md.

Repository guide

The live project is SWC.kicad_sch / SWC.kicad_pcb / SWC.kicad_pro / SWC.kicad_sym / the two library tables. 3dmodels/ and footprints/logo.pretty/ are required project-local assets; tools/ holds the sanctioned direct-write scripts; SWC-backups/ and SWC2-backups/ are the restore path; old/ is the 2022 RP2040 design, kept for reference only. The full annotated layout, including what is git-ignored, is in AGENTS.md.

Toolchain and credits

This board was developed with a small set of external tools, all worth citing:

• KiCad 10 — the EDA suite. All file manipulation in this project goes through an MCP server rather than hand-editing.
• mcp-server-kicad (uvx --from mcp-server-kicad mcp-server-kicad) — the MCP server providing byte-preserving schematic, PCB, ERC/DRC and manufacturing-export tools. It is the reason every change here is scriptable and repeatable.
• pcbparts-mcp (https://pcbparts.dev/mcp) — component search, JLCPCB stock and cross-reference, reference-board lookup, and PCB design rules. Used for the part-selection and stock-availability pass.
• KiStack by American Embedded — a human-written set of KiCad skills (schematic, PCB, BOM, export, gerbers, footprint, symbol, panelize, product render) that standardised the workflow. Installed with npx skills add American-Embedded/kistack.
• JLCPCB component skills — jlcpcb-component-finder (part search by keyword, package, stock and price) and jlcpcb-bom-generate-from-kicad (KiCad BOM/position → JLCPCB PCBA order format), with its database refreshed from jlcparts.

The per-tool notes and the working rules for this repository live in AGENTS.md.