Tic-tac-toe is perhaps the most basic board game imaginable, and as such, it should be easy to simulate with a computer or microcontroller. Warren Toomey (AKA DoctorWkt), however, took things several steps forward — or backward depending on how you look at it — implementing a one-player game with only TTL logic. No CPU is in use whatsoever, instead different moves are stored on a 28C256 32Kx8 EEPROM chip. Only 8K of the onboard space is needed here, and a bunch of other components are required to make things work, including a 555 timer to generate the necessary clock signal.
To get started, the human first selects whether the not-computer should go first, then inputs moves via an array of buttons. Human moves are portrayed as green LEDs, while the device’s moves show up in red instantly (or nearly so) after the human presses a button. Once the game is over, a red LED is shown for ‘EEPROM wins,’ or a green if there’s a tie. Notably, the scenario where the human actually wins isn’t accounted for, as the board never looses.
You can see the project demonstrated in the video below on a rather large breadboard (or perhaps several next to each other). If you comb through the Toomey’s GitHub page, you’ll find that he has also come up with a PCB version that appears to be much more robust.