Between 2015 and 2016 I undertook a project to build a simple 8 bit processor using Transistor-Transistor Logic (TTL) chips. I designed all the hardware and instruction set and built the computer by hand over a few months. The final machine was simple and very slow but was Turing-complete using a RISC based architecture. I received a Gold CREST award from the British Science Association for the project, one of only two people from my school to do so.
You can read my write up for the project here.