C++, Arduino, Electrical Engineering, and worked with a mechanical engineer on the design execution.
Here is the story, skip to the bottom if you want to see the youtube video-
Started this year working on a finance App, this one needed to be perfect. React Native, Laravel PHP mysql. The app is almost finished, and is worthy of a note in its own right. This 6 month project taught me more about programming than the last 9 years. In the process I started automating my Electrical Engineering job in python. Word got out, and I got head hunted by a stranger at work. They were looking for Embedded Systems, I applied, and was denied the position because I didn't have experience.
So I needed experience in Embedded Systems, and I've hated doing dishes. This was a problem that could teach me Embedded Systems and grow my Electrical Engineering skills(B.S. was in Chem engineering, M.S. Industrial Engineering). And it did.
Since there was nothing to copy from, I would spend 2-3 days researching EVERYTHING there was to know about resistors. Then when I knew which resistor I would need, I'd have to spend 2-3 days on capacitors. Since the HN thread might care about relevant details- Learned everything about stepper motors, ICs, I2Cs, controlling electricity, and voltage. Since I wanted to learn embedded systems, I didn't use delays, instead I used a state system with interrupts.
It took ~2 months start to finish.
Here is the video of the working prototype-