Timing iOS app triggered by motion?

I need to time an object that stays still on a wall but may fail and fall off the wall after some time. Instead of sitting there for the fall that may never occur I would like to use a smartphone camera to sense motion as a trigger for a stopwatch. The app ideally would take snaps from the start of the timing and at the end so as to see what motion triggered the smartphone camera to stop timing... Any ideas?

P.S. Did some research that comes up with one rudimentary app for timing laps of runners by using a smartphone camera.

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