Senior year I did a controls final project where we levitated a nut underneath an electromagnet using a photoresistor, pretty simple except for some fiddly execution details. I remember really loving how controls theory can inform real-world phenomena, but haven’t had a chance to dig into it in a formal way since. A video of the system can be seen here. For more information you can check out the our final report, which my goodness have I forgotten a lot about.