Building the TD4 4-Bit CPU

A maker built the TD4, a tiny homemade computer from old-school chips, and the internet instantly turned the comments into a mini fan convention with just a hint of chaos. The project itself is delightfully retro: a working processor you can power by USB, program with little switches, and watch blink through LEDs like it’s 1979 at a science fair. The builder even had to translate a Japanese book by hand to finish it, then made a web tool to help convert simple code into switch positions. In other words: part electronics project, part language quest, part patience test involving a small army of diodes.

But the real action? The crowd reaction. One commenter immediately cut through the awe with the most relatable question possible: where’s the full instruction list? That set the tone perfectly—people weren’t just impressed, they wanted receipts. Another commenter pulled the classic internet move of saying it would be “remiss” not to mention Ben Eater’s 8-bit computer and Nand2Tetris, basically turning the thread into a polite-but-firm “don’t forget the other legends” moment. It’s not exactly a flame war, but it is peak comment-section energy: one cool project appears, and suddenly everyone is nominating their favorite educational computer celebrity.

The funniest part is how wholesome the whole thing stays. There’s no angry pile-on, just a charming mix of curiosity, nostalgia, and geeky one-upmanship. Even the simple “Nice!” lands like a stamp of approval from the back row. Tiny CPU, big comment-section vibes.