This Tic-Tac-Toe game connects to an Arduino UNO R3; I used an ELEGOO UNO R3. The ZIP file contains the game and a test script. Run the test first: all LEDs light green, turn off in sequence, then cycle blue, red, and green. Hold each button until its LED lights, then keep it lit for three seconds. The player can beat the Arduino, though you can modify the code so the best outcome is a draw, or CAT’s game. I previously coded Tic-Tac-Toe for an Apple computer, Microsoft Quick Basic 4.5, and a Java applet. The Quick Basic version still runs in DOSBox.

The board’s LED cathodes connect to Arduino pins 0–8. The two top LEDs indicate the current player and mode and connect to A0 and A1, respectively; the anodes connect to pins 9–11. During assembly, test the LEDs with a multimeter in diode mode: connect the positive lead to pin 9, 10, or 11 and the negative lead to any cathode pin.

At startup, Mode 1 pits one player against the Arduino. Press a keypad button to select a blue LED; after about two seconds, the Arduino selects a red LED. The program chooses almost instantly, but the interval constant adds the delay. After a win, the three winning LEDs blink in sequence while the others blink dimly; after a draw, all LEDs blink. Press the mode switch to cycle through Modes 1–4, or hold it for two seconds to reset. Variables and constants make the code easy to modify.

Modes: 1, Player vs. Arduino—player starts; 2, Player vs. Arduino—Arduino starts; 3, two players; 4, Arduino vs. itself.

The top LEDs show the current player (left) and mode (right). The mode LED is off for Mode 1, green for Mode 2, blue for Mode 3, and red for Mode 4.

How the Arduino chooses a square:

1. Detect Win: determine whether the player has already won.

2. Check for Win: complete a row in which the Arduino already occupies two squares.

3. Check for a Block: block a row in which the player already occupies two squares.

4. Pick a Random Square: because the same seed produces the same sequence, I used the millis() function, which returns the number of milliseconds since power-on, to seed the random generator when the user presses a button.

I used an acrylic mounting plate designed for the Arduino UNO R3, which includes mounting hardware and a small breadboard I didn't need for this project. For power, I attached a 9-volt battery holder with an Arduino barrel plug, an on/off switch, and double-sided foam tape. Because the switch is opposite the battery door, I mounted the holder with the door facing down and routed the power connector beneath the Arduino.

I developed the circuit and code in Tinkercad before designing the PCB: https://www.tinkercad.com/things/57Vc8MxJXxy-tic-tac-toe-with-tri-color-leds