The shaft decoder is a system that we used to count revolutions of our rotating display. We built it by inserting a bright red LED (light-emitting diode) into our rotating display cylinder along with a battery to run it on. This LED would then temporarily shine through slits made in the black cardboard and would be detected by a light sensor facing the LED, on the outside of our cylinder.
This is a picture showing the red LED and the slits in the black cardboard that kept count of how much the display turned. The light sensor cannot be seen; it is behind the red LEGO plates and has the orange wires attached to it.
The code we wrote for the Crickets to follow included "looking for the edge" which means detecting when the light from the LED hits the light sensor and increasing a counter by one. The motor will stop when the counter has counted as many slits necessary. It knows how many slits to count depending on the GSR output, which assigns a certain number to the Cricket to tell it to which message to turn. The shaft decoder did work when run separately, but somewhere in between the data processing and motor moving, our Cricket Number Two kept crashing. It is a code problem, not a mechanical one, but we still haven't found the solution :(