This is actually an experiment. It shall proof that even a very simple designed rover with a simple micro controller, program and sensors is able to drive at least 500m outdoors without getting stuck or destroyed. Maybe I'll make a poll and everybody can guess how far the rover will travel.
Sor far the rover has only two sensors --- a SRF05 and the rear wheel, a steel ball. On the steel ball axle is a magnetic disc mounted and the roation of the steel ball is monitored by a hall sensor.
Added circuit diagram and first test code.