The TD4, a 4‑bit CPU detailed in Kaoru Tonami’s *How to Build a CPU*, sits on a 5‑V board that can be powered through a Micro‑USB port or a 2.54‑mm header. The design relies on a handful of 74HC series ICs, with DIP switches encoding a 16‑byte program. Building it required translating the Japanese text and sourcing a few parts online.

Source files for the PCB live on BG5DIW’s GitHub, and the author’s own assembler tool translates text into DIP configurations, speeding up development. The CPU’s internals feature two 74HC161 counters for registers and the program counter, a 74HC283 ALU, and multiplexers from 74HC153. A 74HC14 oscillator drives the clock, letting users step through instructions manually.

Soldering demands attention; the board uses only four SMD LEDs and a Micro‑USB socket, the latter requiring only pins 1 and 5. Most diodes are individual 1N4148 parts because the 1S1588 network is hard to source. These diodes form a diode‑matrix ROM that isolates switch states, preventing back‑feeding across the logic gates.

Once powered, the TD4 runs without firmware, exposing every instruction path through discrete gates. The assembler tool and a logic probe let developers trace data flow from a DIP switch, through multiplexers, into registers. This hands‑on experience demystifies hardware‑software coupling, making the TD4 an ideal teaching aid for embedded‑systems enthusiasts.