We recently got hold of a “Crazy Cart”, a seriously cool cart to race around with. As we only have one, I did want to have something to make the driving a bit more competitive, so I build an Arduino Based Lap Timer. The Lap timer uses an “Infrared light bridge” (Velleman PEM10D) to detect when something crosses the line. The nice thing about this light bridge is that it only needs on one side electronics. The other side just needs a passive reflector. If you do not want to buy this light brigde, you can use an laser bean on one side and a photodiode on the other side, but that will require “electronics” on both side.”