yzytz17/1.5-joint-module

Project overview

This is an open-hardware design for a very cheap robotic joint: a roughly $1.5 four-layer FOC driver board sized to fit inside a 1503 brushless gimbal motor, paired with a fully 3D-printed reduction gearbox. The aim is low-cost actuation for multi-joint robots such as a 6-DOF arm, a quadruped or a pan-tilt head, while keeping a usable reduction ratio.

The repository publishes PCB Gerbers and a manufacturing zip, ESP32-C3 + SimpleFOC firmware (English and the original Chinese source), SolidWorks CAD and print-ready STLs, plus documentation. Design details and measured dimensions are recorded in the English README, with the Chinese original kept as authoritative where both languages state the same fact language note.

Key design points

• Board: about 17.6 × 16.4 mm, four copper layers, three motor-phase pads placed 120° apart on a 6.0 mm radius. It must use the MP6540GU, not the pin-compatible MP6540HGU; the two share a QFN-26 footprint but differ in voltage and current ratings driver chip notes.
• Firmware: pure open-loop angle control with no sensor feedback, no initFOC() and no PID. The motor does not move at power-up; a stall or lost step makes the internal angle wrong, so reboot to re-zero.
• Angle sensor: the magnetic sensor outputs PWM, read through the ADC on GPIO0. There is no I2C in this firmware.
• Gearbox: module 0.6 gears, a 27-tooth internal ring and 30-tooth external ring, with an intermediate stage. The author's video states a single-stage 10:1 reduction, but the kinematic scheme is not fully described in the released material.
• Serial control: 115200 baud; D reads the encoder with the motor de-energised, a 90 moves to 90°, R2 spins at 2 rev/s.

Getting started

The documented flow is: order the PCB from hardware/pcb/gerber_gen1_adc_closedloop_2026-10-21.zip via JLCPCB or PCBWAY and buy an MP6540GU; print the gearbox from mechanical/stl/print/ following the hardware list in the BOM; flash firmware/openloop/; then open a serial monitor at 115200 and send D, a 90 or R2 getting started. Detailed assembly steps are in the assembly guide.

The firmware will not compile as released: openloop.ino includes config.h, which was not part of the material set. A reconstructed config.h.example lists all 17 macros, but six values are still guesses — MOTOR_POLE_PAIRS, OPENLOOP_DIRECTION, VELOCITY_LIMIT, VOLTAGE_RAMP_MS, ARRIVE_TOL_RAD and HOLD_VOLTAGE_RATIO — and must be replaced with real numbers before flashing config notes.

Constraints and gaps

The project is explicitly work in progress. Before a stranger can reproduce it, several items are still missing: config.h, closed-loop firmware (the Gerbers are named “ADC closed loop” but only open-loop code is released), a schematic or editable PCB source in-repo, a real BOM with part numbers, still photos, print settings, and a full kinematic description missing-material list. The cost table is a skeleton and does not break down what the $1.5 covers.

The firmware output is voltage-limited to 5 V × 15 % = 0.75 V, and at power-up the motor stays still. The design relies on the MP6540GU specifically; substituting the H variant is a documented mistake. Three CAD parts have no STL export. The design is licensed under CERN-OHL-W-2.0, which keeps the joint design open but allows a larger product embedding it to remain closed license terms.

HotRepo's take

The repository entered HotRepo's new-project list on 2026-11-05 with 13 stars gained over a roughly six-hour growth window, reaching 113 stars and a single fork. There is no recorded GitHub daily rank. That pattern — some early attention, almost no forks or derivative work — is consistent with a freshly published hardware design that has not yet been reproduced by others. The README's own repeated TODO(author) markers and its missing-material list make the unfinished state the main caveat for anyone considering a build; the value of the repository is the released Gerbers, firmware, CAD and STLs, not the still-incomplete documentation around them.