We have been working on getting a new encoder this year for our robots after last years hall-efect ones started breaking through out the year.  The new odometer will have have a resolution that will have more than one tick per revolution of the wheel so we no longer have to worry about are odometer not working at low speeds.

The basic priceable is that we have photo transistor to see the difference between white and black and send ether a high or low signal to our micro controller.  The LED is used to show what the out put is to avoid having to have a programme running to do basic circuit debugging.