Modifying Servos: No Soldering Required

R/C servos are intended for such applications as controlling the steering in a model car, or the ailerons in an model airplane. They don't need to turn more than 180 degrees, and they include mechanical stops to prevent them from doing so. But by removing the mechanical stops, and making a change in the electrical connections inside, it's possible for the output of the servo to turn continuously in either direction.

Modified servos are often used for the drive wheels of small robots because their use simplifies both the mechanics and the control electronics of the robot. The servo package includes motor, reduction gearing, and power drive electronics, and can be directly connected to a microcontroller, computer port, or other digital interface.

One common way to modify a servo is to remove or disengage the potentiometer from the power-train. If the potentiometer is removed, a new trimmer pot, or series of two 2.2K ohm resistors, takes its place.

Most potentiometer mods detailed on the Internet call for soldering in addition to any mechanical changes. However, the design of many servos made today allow for a far less-invasive modification technique, where no soldering or unsoldering is required. The potentiometer is left in place. Alterations are made only to the output gear of the servo.

The no-solder process involves these basic steps:

  1. Remove the four case screws of the servo
  2. Remove just the top case of the servo.
  3. Remove the output gear, and cut off the stop nub on its top side.
  4. Remove the potentiometer shaft clip from the underside of the output gear. Or, for servos that don't have a shaft clip (these use a molded-in socket that engaged with the potentiometer shaft), drill out the bottom of the output gear. This removes the socket so that the potentiometer rotates freely.
  5. Make sure the potentiometer is centered.
  6. Replace the top of the case, and the four screws.

The exact steps depend on the design of the output gear, and how the gear engaged with the potentiometer shaft: retainer clip or molded-in slot.

Modifying a servo with a retainer clip
Modifying a servo with a molded-in slot (to come!)

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