[Ben Eater] posted some videos of an 8-bit computer with no CPU chip that he built completely on a breadboard a few years ago. After being asked for schematics, he finally admitted that he didn’t have any. So, instead, he decided to rebuild it and keep a video log of each step in the process. You can see his kickoff video, below, but you can also find 30 more recent videos covering topics from the ALU design and troubleshooting to the decimal LED display. He even uses an Arduino to program a EEPROM that he uses to replace a lot of logic.
You probably want to wait until you have some free time as there are around eight hours of videos so far. The videos start off with a simple 555 timer and work up from there. Each piece gets a test separate from the whole, so with luck you won’t have an impossible job trying to troubleshoot the whole thing at the end.
Projects like this are decidedly impractical, but if you ever want to really understand how a CPU works, building one is a great way to develop that understanding. We’d suggest learning Verilog or VHDL and building on an FPGA, but the breadboard computer has a certain street cred and certainly has a nice array of blinking lights.
The CPU design follows a design in the book “Digital Computer Electronics” so if you were serious about recreating this, you could follow along with that, too. The book is out of print, but these days finding out of print books isn’t very difficult.