Currently I am working on perfecting a CdS cell optical encoder for tank treads. Tank treads can be difficult to work with sometimes, but I have found a way to use the gaps between the treads and a simple voltage divider using a CdS cell with a White LED to "flood" the tread piece and have the CdS cell to read the corresponding brightness. when the tread rotates to the joint, most of the light is sent inbetween and hits the idler cog, creating a dimmed effect. This can easily be seen by the "flashing" or "strobe" effect of the reflection. This will be better explained once it is complete.