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MODIFYING A STANDARD SERVO FOR CONTINUOUS ROTATION 261

As of this writing, there’s just a small handful of servos made for continuous rotation.
These include the GWS S- 35, the Parallax Continuous Rotation Servo, and the SpringRC
SM- S4303R. These are available from a variety of online resellers; see Appendix B, “Internet
Parts Sources,” for more information.

Modifying a Standard Servo

for Continuous Rotation

You can convert most any servo to continuous rotation. Once modified, they no longer are
capable of precise angular rotation, but they’re perfectly suited as drive motors for wheeled
and tracked bots.
There are a number of methods you can use to modify an R/C servo for continuous rota-
tion. The process involves removing the mechanical stops, and— for some types of servo
surgery— making a change in the electrical connections inside.

WAYS TO MODIFY A SERVO
There are many ways to modify R/C servos. In reverse order of difficulty:

• Power- train mod. For this modification, the control electronics are removed completely,
and the motor is driven by an external H- bridge. Any stops are removed from the power
train, and the potentiometer is either removed or its shaft is otherwise disengaged from the
servo drive. This is a lot of work, and most people don’t bother.
• Signal tap- in mod. In this conversion the electronics of the servo remain, but, as usual, the
mechanical stops are removed and the potentiometer is disengaged. In this variation, the
robot’s computer or microcontroller is connected to the H- bridge chip within the servo’s
circuitry. Not all servos are so adaptable, however. Several Internet sites detail this
tapping- in process for the BAL6686 H- bridge IC, used in certain Futaba models and other
servos. Do a Web search for “BAL6686” for a number of useful sites that describe this.
• Potentiometer mod. Here all the control circuitry is left in place, but the potentiometer is
either removed completely (and replaced with a couple of resistors) or disengaged from the
power train. This is the easiest modification to make, and the most popular.
Of these methods I’ll talk only about the last one, as the others have far more restricted
uses and are much more difficult. The potentiometer mod that follows is relatively quick and
easy, requires no soldering, and needs basic basic tools.

BASIC MODIFICATION INSTRUCTIONS
Servo modification varies somewhat among makes and models, but the basic steps are the
same:

1. Remove the case of the servo to expose the gear train, motor, and potentiometer. This
is accomplished by removing the four screws on the servo case and separating the top
and bottom.
2. File or cut off the nub on the underside of the output gear that prevents full rotation.
This typically means removing one or more gears, so you should be careful not to

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