Matthias Wandel Builds Auto-Turret Machine — Unleashes It on His Kids

You likely know Matthias Wandel for his ingenious woodworking exploits and plans, but he’s been looking to possibly get back into high tech. To test out whether he still enjoys these pursuits, he did what anyone else would, and built a cap shooting crossbow, with two steppers used to aim it at a target, and a third to reload and fire the mechanism.

As we’d expect from Wandel, it’s a clever mechanical project, pulling the shooting rubber band back and releasing it in one smooth motion, letting a cap fall into place from a stack on top in the process. The servos use rack and pinion assemblies to move everything into place, and notably, the hole spacing on at least the tilt servo (which appears to be a NEMA 17 model, common on 3D printers), conveniently fits LEGO parts with only some minor modification.

The device is controlled using a Raspberry Pi, which controls the steppers via a driver board and tracks objects using a 120-degree fisheye camera. In order to compensate for lens distortion, he even made a LEGO jig and modeled everything in a spreadsheet to get targeting spot-on. Aim/shooting at a target is shown at around 7:40 in the video below, followed by a trial run with his own daughter wearing a modified bicycle helmet for protection. According to Wandel, “The kids enjoyed this less than [he] anticipated.”