Thanks geekinthesticks, that's pretty much what I'm trying to implement (except all the timestamping will take place on one device, to minimize error). Concerning Linux freezing up for a tenth of a second, I should be able to minimize the issue by running nothing else on the board. And I guess, wors...

Ah, I understand your point. Well, in the case of wireless transmissions, this won't be a problem: the time taken for the software to interface with the hardware will be included in the average travel time. However, I can see how this could be an issue with the sensing at the finish line. The good n...

Pi sends 100 packets of data to Arduino, and waits for responses. Calculates the average one-way travel time between both devices. User presses button on Arduino to arm timer. Pi is notified. Pi (after a delay), sends the command to the Arduino to play a sound through a Piezo. The time at which this...

I've decided to go the wireless way, with two Pi's or a Pi and an Arduino. I imagine I could have communication between both devices before the run, and calculate the average transfer time; I could then just subtract this from the final speed. Thanks for suggesting a semiconductor light sensor as we...

Thanks for the suggestion. It sounds great, but I'm afraid I won't be able to use WiFi, or have any other connection to Internet. Any other ideas ?

I'm trying to create a system that will precisely time sprints on a track (100M probably the main distance being timed). The Raspberry Pi will emit a sound, and 100M later, a sensor will be set off by the runner. The difference in time between both of these events will then be recorded. However, the...