This line follower has 3 light sensors to work with ("Sensor Claus" came to town). Three sensors mean that it can follow a line very smoothly and detect also the end of the line or its loss.

There is a summary on different line follower configurations at http://www.wrighthobbies.net/guides/linefollower.htm that is quite instructive.

The implemented algorithm of this line follower is straightforward: the difference of measurement of the two outer light sensors is added to a mean power to drive the motors.

If the middle light sensor does not see the line (raw value is greater than 500), both motors stop. (Of course one could add here a more sophisticated algorithm to regain the line, but this robot assumes that it has reached the destination.)

If the robot is placed again on the line it starts moving until an end is reached.

The follower has only two big wheels. Instead of having a small shopping cart wheel at one end, it is simply balanced so that sliding on the ground works fine.