My passion for athletics is inside me since I was a little boy. Several times in the life of an athlete, in particular of a sprinter, it could be useful to have a working automatic stopwatch which allows to count the time of your trials during your training sessions. And many times I have thought to build one on my own in some way.
In recent year computer vision has spread out and today it is not hard to build a system able to recognize the passage of the athlete over the finish line. I fastly modified a code used for vehicle counting, available here, to detect my passage over the finish line.
The idea is that a complete stopwatch system could be developed integrating a method to start the time and using computer vision to stop it when the athlete is passing over the finish line. One method could consist in using two devices (also smartphones) synchonized on an external clock provided by webservices such as NTP. The time could be started through pressing a button and the information could be send via network to the other device. Then, to compute the time of the run it will be enough to compute the difference:
time(run) = final_time – start_time;