Robots I have built:
This line tracker was built from a kit as a demonstrator for the line following event at the micromouse competition.
The standard kit (Hyper Line Tracker) is able to folow a black line on a white surface, but since the UK competition uses a white line on a black surface a couple of extra transistors were added which invert the phototransistor outputs if a plug is inserted one way round. With the plug the other way round it operates normally. The standard device just uses 6 transistors which are used to switch the motors on and off. A really nice feature is a bistable which remembers which side of the line it came off, so even if it loses the line it will circle back onto it.
In spite of its simplicity it is a very reliable line tracker.
By virtue of being the only senior demonstrator in it's first outing it won first prize.