USB Servo Controller

USB-C 12-Channel Servo Controller

12-channel USB-C servo controller, Pololu Maestro compatible, with an MCP server so AI agents (Claude, etc.) can drive servos. $18 on [Tindie](https://www.tindie.com/products/lll7/usb-servo-controller-12-channels-usb-c/) · [product page](https://ucandevices.github.io/servo.html).

▶ 2-minute video: wiring, current monitoring, GUI and the controller driving a real MG995

Open-source alternative to the Pololu Mini Maestro 12 that speaks the Maestro serial protocols, so Maestro libraries work unchanged — plus USB-C, servo-rail current and voltage sensing, on-board scripts. Built for JLCPCB turnkey assembly, ≈ $4.43 per board at 100 units against $29.95 for the Maestro.

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AI agents (MCP server)

tools/servo_mcp.py is an MCP server that lets an AI agent move servos, run poses and sequences, and read rail current/voltage, with guardrails: per-channel limits, ramped moves, overcurrent stop. pip install -r tools/requirements.txt, then:

Claude Code

claude mcp add servo -- python /path/to/UsbServoController/tools/servo_mcp.py

Claude Desktop (claude_desktop_config.json)

{

"mcpServers": { "servo": { "command": "python", "args": ["/path/to/UsbServoController/tools/servo_mcp.py"] } } }

Add "--port", "COM5" to pick a port, or "--mock" to try it without hardware. Close the GUI first: only one program can hold the serial port.

The server is also on PyPI, so any MCP client can run it without cloning the repo: command uvx, args ["ucan-servo-mcp"] (plus --config path/to/servogui.json to use the limits, poses and sequences saved by the GUI).

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1. Quick start

pip install -r tools/requirements.txt

python tools/servogui.py # GUI (or --mock with no hardware) python tools/servoctl.py set 0 1500 # CLI cd embedded-ch32 && make flash # build + flash over USB, ~3 s, no jumper

Downloads: firmware images and the prebuilt GUI for Windows and Linux are on the Releases page. To build the GUI yourself: python tools/build_release.py → tools/dist/.

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2. Why the CH32V203C8T6

The hard requirement was a USB bootloader in ROM, so a blank chip can be flashed over USB-C with no programmer. That rules out the STM32F103 the design started on (USART-only bootloader). Of LQFP-48 parts that have one and are stocked at JLCPCB:

Cheapest part with real inventory, a third the price of the STM32F072, and pin-compatible with the STM32F103 layout (32 of 33 connected pins identical).

Costs of the choice:

• RISC-V firmware, WCH HAL, no CubeMX.
• An ST-Link will not work. Debug is WCH 2-wire SDI on J9 — needs a WCH-LinkE. Routine flashing is over USB, so the probe is only for debugging.
• PB2 is BOOT1 on this chip, so it is tied low by R17 (5.1 kΩ).

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3. Hardware

                            U4 INA180A2 (Kelvin across R18) ─► PA5 I_SENSE

J2 Servo V+ ── R18 5mΩ ──┬────────► servo header V+ rail (unfused) (VSRV_RAW) ├─ R19..R22 ÷4 ─► PA4 V_SENSE └─ JP1 ──D2──┐ ├─► HT7533-1 ─► 3V3 ─► CH32V203, LED USB-C VBUS 5V ─────────────D1──────────┘ GND: one continuous plane, common to both supplies

• Two supply domains, one ground. D1/D2 are a diode-OR so neither feeds the other. Every servo amp crosses R18, so the current reading covers the whole board.
• JP1 (2-pin header, open by default): fit a shunt cap to run the logic from the servo supply with no USB — the Maestro's VSRV=VIN block.
• JP2 (2-pin header): BOOT0 high, for first flash / recovery only.
• Logic draw ≈ 15 mA, so the HT7533-1 (100 mA, 2.5 µA quiescent) dissipates 0.07 W even from 2S LiPo.
• USB-C: CC pull-downs R14/R15, SRV05-4 clamps D+, D−, CC1, CC2.
• Servo signals are 3.3 V through 220 Ω (R1–R12). Headers are SIG / V+ / GND.

Channel map

Reserved: PA4/PA5 rail sensing · PA11/PA12 USB · PA13/PA14 SDI · PD0/PD1 crystal · PC13 LED · PB2 BOOT1. LQFP-48 has only 10 ADC pins, and PA4/PA5 went to sensing, so analog is 8 of 12 — the Maestro 12 has all twelve.

Rail sensing

High side keeps the ground plane unbroken. It measures total rail current, not per channel. The ADC reference is VDDA = the LDO output (no VREF+ on LQFP-48), so readings are good for thresholds, not calibrated metering. The firmware trips all channels off after 5 bad readings (~50 ms): 5 A default overcurrent, and undervoltage chosen automatically (2S → 6.4 V, 3S → 9.6 V, anything else off). Details in USER.md §4.4.

Power limits

The 24 A terminal is not the limit; the copper behind it is.

PCB

2 × 1 oz, 1.6 mm, 0.2 mm min trace/space, 0.3/0.6 mm vias. GND is a solid pour on both layers; SERVO_V+ is carried by three priority-ordered top-layer zones. Headers along the bottom edge, USB-C right, screw terminal left, MCU centre.

⚠️ F8 (Update PCB from Schematic) deletes H1–H4 — they have no schematic

symbol. Untick *Delete footprints with no symbols*, or re-add them at

(16, 4), (61.5, 4), (16, 33), (61.5, 33).

⚠️ HT7533-1 is not pin-compatible with the AMS1117 it replaced (SOT-89:

GND / VIN+tab / VOUT). The tab is VIN.

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4. Bill of materials and cost

Class: B Basic · P Preferred (both fee-free) · E Extended ($3.07 feeder fee if SMT).

J9 (SDI) is not fitted. Shunt caps (C100114, two per board) cannot be fitted by JLCPCB — order them loose, ≈ $1.80 per 100.

Excludes shipping and duties. Prices are LCSC qty-100; stock checked 2026-09-19/20 (CH32V203 the thinnest at 8,643) — re-verify before ordering.

What drives cost:

• Through-hole soldering is 57 % of assembly. J3–J8 cost $0.25 in parts and $0.59 in hand soldering. SMT-only with loose headers would cut assembly to $0.55/board.
• Unique Extended SMT parts, not unit price. Hence 220 Ω doubles as the LED resistor, 5.1 kΩ serves CC1/CC2/BOOT0/BOOT1/divider, one SRV05-4 replaces discrete TVS diodes, and JP1/JP2 share one header part.

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5. Firmware (embedded-ch32/)

RISC-V, WCH ch32v20x HAL and USB device library, xPack riscv-none-elf-gcc 15.2 (make GCC_PATH=... to override). Version USBServoController 0.4.

Layout lives in User/flash_layout.h and both Link.ld files.

make                 # build/USBServoController-ch32.{elf,hex,bin}

make flash # BOOT command -> bootloader -> write, CRC-32 check, restart (~3 s) make hosttest # on the PC: sense.c protection (21 supply scenarios), maestro.c protocol, ramps and limits (11) python tools/verify_pinmap_ch32.py # schematic <-> pin table <-> firmware

Blank chip or recovery: make full, fit JP2, plug in USB, wchisp flash build/USBServoController-ch32-full.bin, remove JP2, replug. Jumping to the ROM bootloader from firmware does not work on this chip (tested), which is why the board has its own.

Timing: 96 MHz timers, PSC 47 → 0.5 µs ticks, ARR 39999 → 20 ms frame. 0.25 µs would need a 4 MHz tick, which only fits 16 bits at ≥ 100 Hz.

Protocol: one ASCII c