Tic Tac Toe is a classic board game. Two persons take turn to fill in a 3x3 board. The winner is the first to create a line of the same colour. As the board only has 9 slots, there is no strategy to win the game every time. An extended version of Tic Tac Toe is the Cross & Nought game, where the board has no size restriction.
In the early of 2007 during my first year undergraduate, I made my original Tic Tac Toe Robot. Two motors were used to carry a 3x3 game board, while another motor drops a game piece into the board. Since the Lego motor has a circular motion, I used a worm gear, with a moving shaft to transform the rotary motion into a linear motion. A cheap light sensor was used to recognise the piece's colour.
This Robot won the First Prize in the Cool Computing Competition 2009 at University of London. A performance of the robot can be seen here http://www.youtube.com/watch?v=o9LxPQSkcno
I re-designed this robot in the summer of 2010. The game board was fixed. The worm gear system was replaced with an infinite conveyor belt. The light sensor was replaced with two colour sensors for better colour detection. The whole robot can fit nicely in a 30 cm x 30 cm standard Lego base.
A performance of the newer version can be seen here