One of my original ideas was to use a pressure sensor and tubing at the finish line as a trigger, but questions around the reliability of using a pressure sensor as a trigger and the durability of the hoses in an outdoor sport like a mountain biking have come up.
I decided that a good way to test both of these concerns was to make a speed trap. I will be using two of the differential pressure sensors (MPXV7002DP) shown above by connecting them to analog pins and recording the times they are tripped.
Two 3 metre (9 feet) silicon tubes connected to the pressure sensors will be placed on the ground a certain distance apart. Using the known distance between the two tubes and the time it took for both sensors to be tripped, I can easily calculate the speed at which the rider was travelling.
Simply recording the speed isn't doing anyone any favors, so I will use a spare LED matrix I have to show the speed of last rider.
I plan to share the sketch, circuits, photos and video of this device in action this weekend, so check back here in the next few days.