32-slot wheel, 80 mm in diameter, hole for 5 mm shaft with M3 setscrew. This has beveled vanes so it is closer to a knife-edge, than a simple 90-degree slot edge. All dimensions, number of vanes, etc. are parameters in OpenSCAD file. I wanted the somewhat large size you see in the photo, but for other purposes you might want a smaller one. Measuring the output timings of this wheel in motion with an opto-interrupter circuit, it looks like the total variation in the width of the openings is about 0.2 mm (5% of the ~4mm slot), with my printer (Ultimaker 2).
Remixed from "open-source chopper wheel" thing:28121 by jpearce.