Most of the servos that are available at reasonable prices are “standard” servos. That means, the output shaft of the servo can be positioned in the position from 0 to 180 degrees. It can be set by the duration of the pulse sent to the control wire (yellow, orange or white) of the servo. Other two wires are power (red) and ground (black or brown). In Arduino, there is Servo library, which serves this purpose.
Standard servo is useful when you want to control angle. However sometimes, you need continuous rotation in specified direction. It is easy to modify standard servo to become continuous servo. In such case, degree of 0 means full speed in one direction, 180 – full speed in other, while middle value means no motion (it is around 90 – more about calibration later).
This tutorial will show you, how to perform such easy modification. You will need to remove the potentiometer and replace it with two 2,2k Ohm resistors.