June 20, 2026
Big drama in a tiny brain
Guide to the TD4 4-bit DIY CPU
Tiny DIY brain has people dreaming of games and swooning over old-school switches
TLDR: A maker posted a guide to assembling and understanding a tiny DIY computer kit that uses little switches instead of modern software. In the comments, people were split between nostalgic awe over the retro setup and ambitious joking about pushing the cute little machine into running games.
A hobbyist’s guide to building the TD4, a tiny homemade 4-bit computer kit from AliExpress, should have been a sweet little nerdy how-to. Instead, the comments turned it into a mini love letter to ancient-school computing with a side of "wait, you really program this thing by flipping switches?" chaos. The builder says the kit took two soldering sessions, the USB connector was annoying, and "soldering all those diodes sucked"—which honestly only made the whole project sound more heroic to readers.
The biggest reaction? Pure fascination that this adorable little machine has only 16 bytes of program space and is programmed with dip switches—yes, actual tiny physical toggles. That detail sent commenters straight into nostalgia mode. One reader basically gasped, "Truly old school," summing up the mood of everyone suddenly imagining computers before screens, apps, and copy-paste existed.
But the real spicy twist came from the dreamers: one commenter immediately asked whether this tiny CPU could be stretched far enough to run 4-bit games, even dropping a YouTube link like they were pitching the underdog sports movie version of computer engineering. That’s the whole vibe here: half the crowd is charmed by the educational toy aspect, and the other half is already trying to make it do something ridiculous. Tiny computer, giant ambitions, and a comment section fully ready to egg it on.
Key Points
- •The article presents a practical guide to building and understanding the TD4 4-bit DIY CPU kit, which includes two registers, LEDs, and 16 bytes of program ROM.
- •Assembly required two soldering sessions, with the author highlighting diode orientation and USB connector soldering as the main hardware challenges.
- •The TD4 stores its entire program in a bank of 16 DIP switches, with opcode bits and immediate-value bits encoded on each switch.
- •Opcode bits select instructions such as ADD, MOV, IN, OUT, JNC, and JMP, and combinational logic determines data selection and which register is loaded.
- •The article explains the roles of address decoder IC11, ROM buffer IC12, and command decoder logic in IC8 and IC10 in implementing the CPU’s control behavior.