Although the handyboard is very slick, this type of robot can be done easily with the lego mindstorms in a few hours. there are probably even sites that have the logic and design for the project. The basic idea of a good line follower with one sensor is to memorize the last direction that you turned and go the opposite direction next time you "lose the line". In general, the line followers with one sensor will track across the line. Memorizing the location makes the problem really easy to solve.