MAKE contributor and robotics author Tod Kurt put up this short video and how-to on hacking tiny, cheap servo motors for continuous rotation. Anyone who’s done a lot of miniature robotics building probably knows the hardware hack to turn a regular servo motor into a continuous-rotation gearmotor, by removing the mechanical stop and the servo control PCB. Here, Tod is removing the stops (two here) and swapping out the potentiometer for 2 resistors on the PCB. What you end up with is a full-rotation, bi-directional gearmotor, with positional control, sort of like a low-precision stepper motor.
Hacking tiny servos for continuous rotation