Ok, I might start off without encoders. But the project above (MURtle Drawing Robot) seems to use IR sensors as encoders. Can anybody confirm this?
The below seems to indicate this. I've seen simple bot setups that used continuous rotation servos with large plastic jar lids attached for wheels. Encoder patterns were printed and attached to the wheels and the Ir distance chips used for rotation detection. A rubber band can be used around the lid to act as a thin gripping wheel tread.
The IRS01A IR Sensors are connected to 2 analog input pins of the controller board. No additional hardware like Schmitt-Triggers has been used.