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Stepper Motor/Driver Tester
I'm in the middle of building a 3D printer from scratch, and found that something like this would be useful to check the mechanics without wiring in a full control board. I've lashed something together with an Arduino Uno, a DRV8825 board, and a breadboard...you can think of this as a more permanent version.
I also had a driver board in another printer crap out recently; something to verify proper operation of the driver board would've been useful.
I plan on having this device support pretty much any Pololu-compatible stepper-motor driver board, but with an emphasis on TMC2130 support so that you can determine the optimal settings to burn into your printer firmware.
With nothing but through-hole components, this should be simple for anybody to build. The only somewhat specialized bit of equipment you'll need is a USBasp to program the microcontroller (or maybe you can lash up an Arduino to do the job).