1,400 Transistors for One Perfect Game of Tic-Tac-Toe

Who needs integrated circuits? Certainly not Philipp Schilk, who has designed and built a circuit from nothing but discrete transistors that plays Tic-Tac-Toe. However, while it is an impressive feat, you might not want to give up on integrated circuits just yet. Using discrete components increases the complexity, size, and cost of devices by orders of magnitude. But it sure does look cool, and we are glad that Schilk went through the exercise.

Schilk created a fully functional implementation of the classic game, complete with both player-versus-player and player-versus-computer modes. The system can detect all win and draw conditions and even reject invalid moves. What makes it different, however, is not what it does, but how it does it: every piece of logic is realized using individual transistors.

The project began in simulation, where Schilk first developed the design using a graphical logic tool. Initially, the computer opponent relied on a ROM-based lookup table, mapping board states to optimal moves. While effective, this approach proved inefficient, as only a small fraction of possible inputs represented valid game states. Seeking a more elegant solution, Schilk replaced the memory-based system with a purely combinational logic engine capable of perfect play.

The final hardware design uses 19 flip-flops to track the game state and active player, along with a network of logic gates constructed from basic transistor-level building blocks. In total, the system comprises approximately 1,400 transistors, with the game engine alone accounting for over 1,000. Each logic function — from simple NOT gates to more complex decision-making circuits — was designed hierarchically, mimicking professional integrated circuit workflows.

Physically, the board is divided into two sections: a main area handling user input, clocking, and display, and a secondary area dedicated to the game engine. The board is densely routed in a Manhattan-style layout, reflecting the sheer scale of implementing even simple logic without modern conveniences.

To make decisions, the system uses a chain of decision gates, each corresponding to a specific board condition. These gates evaluate scenarios in priority order — winning moves first, then blocking the opponent, preventing forks, and finally selecting optimal positions like the center or corners. Once a condition is met, subsequent gates are disabled, effectively creating a hardware-based if-else structure.

Assembly was one of the biggest challenges of the build. Hand-placing and soldering thousands of components led to multiple revisions, with early prototypes suffering from reliability issues due to PCB warping. Ultimately, professionally assembled boards were required to achieve consistent operation.

While wildly impractical by modern standards, Schilk’s transistor-based Tic-Tac-Toe machine is a great way to learn digital logic fundamentals. It strips computing down to its bare essentials, offering a tangible reminder of the complexity hidden inside the chips we take for granted every day.