A few weeks later we discovered just how much fun swings can be. Taking a picture of a kid on a swing can be tricky. The best points for triggering the camera obviously are the highest points since the swing is not moving so there's no risk of motion blur. 

Clearly, this is something that can be automated!

Since whatever system comes out of this project should be able to work on-the-go, and should require little setup and calibration time anything using a through-beam or reflective photoelectric sensor is ruled out.

A rotation rate sensor or an accelerometer seem to be a practical solution. Since the motion on a swing is typically sinusodial with a frequency in the order of 1 Hz it should be pretty easy to identify the turning points with a low-cost sensor and some microcontroller (no, I am not going to mention the A-word here.