8.2 Modification for Full Rotation

As mentioned, R/C servos are designed to rotate from -90 degrees to +90 degrees, and the control signal is meant to control the angle of the output shaft. This means that R/C servos are not useful for wheeled robots that require gear motors that rotate 360 degrees.

As it turns out, most R/C servos can be easily modified for full rotation. Regardless of the brand and basic design, here's the general method:

• open the case, disassemble the gears
• on the output (final) gear, remove the tab that stops it from free rotation
• gain access to the printed circuit board
• desolder the potentiometer, you have two options to go from here:
  • replace the potentiometer with a series of two 5k Ohm resistors. The mid point connects to the pad/through hole of the center tap of the removed potentiometer, while the two ends connect to the pad/through hole of the two poles of the removed potentiometer.
  • use wires to connect the pad/through hole to the removed potentiometer so that the potentiometer is external to the R/C servo for tweaking.
• reassemble

After you perform this modification, send 1.5ms pulses to the control signal. If you bring the removed potentiometer out with wires, you can tweak the ``center'' position until the motor stops spinning.

With the modification, the R/C servo is tricked to think it is physically at the center position (0 degree). When you send pulses of 1.75ms, for example, the R/C servo control circuit tries to correct the ``error'' by turning on the DC motor to spin one way. When you send control pulses of 1.25ms, on the other hand, the R/C servo control circuit rotate the DC motor the other way to try to correct the error.