Objective : Calculate the value of acceleration due to gravity by measuring the time between succesive bounces of a ball.

This can be done in a couple of different ways:

1) make a plastic ball bounce on a hard surface, and use a microphone with high gain so that the sound of the bounce clips. Connect it to the digital input, and measure time between rising edges.
2) make a laser beam bounce back and forth between two mirrors, and finally stop at a light sensor(photo transistor : G2 )

We're trying to make the second one : The design file for laser cutting is here

The mirror remains to be cut and assembled for the laser grid. However, the simple photogate featured in the center, works well enough for measuring RPMs of small fans, and time periods of pendulums.