Servo-motor Converted to Motor (with Speed & Direction Control)



Introduction: Servo-motor Converted to Motor (with Speed & Direction Control)

The purpose of this Instructable is to convert a servo-motor 9g to a motor while conserving the servo-driver to control the rotary speed and direction. No electronic modification is required for this Instructable. No need of hot soldering iron!

The last step is optional. It is dedicated to Lego adaptation in continuity with my main project (see Servo-motor adapted to Lego for more explanations).

Step 1: Materials and Components


  • cutter
  • hot-melt glue applicator
  • precision screwdriver (adapted to servo-motor screws)
  • precision flat screwdriver (1,2 to 1,5 mm wide)


  • servo-motor 9g
  • Lego gear 16 tooth with Clutch*
  • Lego liftarm 1 x 3 Thick (dark bluish gray)*

*required only for Lego adaptation (see optional last step)

The Lego components can be easily purchased from Lego parts suppliers such as ToyPro.

Step 2: Fixing the Potentiometer Angle

Firstly, unscrew the four packaging screws and remove the upper side of the servo-motor. Then, remove all the small gears to access to the potentiometer.

Some explanations:

The potentiometer is a variable resistance. It gives to the servo-driver a voltage level that is the image of the angular position of the gear train. This position is compared to the angle to reach and the motor speed is directly proportional to the difference between the both. The motor turn quickly (in the good direction) when the gear train is far away from the targeted angle, it slows down when it gets closer and stops when the angle is reached.

The principal of this Instructable consists in fixing the position of this potentiometer at the middle angle. When we will send a full left command to the servo motor, it will turn quickly in this direction (big difference with the potentiometer angle). When we will send a full right command, it will do the same thing in the other direction. And when we will send a middle angle command, it will stop (no difference with the potentiometer angle). For intermediary commands, we will obtain intermediary speeds.

Due to the fixed position of the potentiometer, the angle difference for a given command will be always the same and the rotation speed will be constant.

To fix the position of the potentiometer, plug the gear train and move it at the middle position, as shown on the picture. When it is done, use the hot-melt glue applicator to stick it in order to be sure that it will never move.

Step 3: Adapting the Gear Train for Free Rotation

The purpose is to remove the mechanical parts that limits the rotation of the gear train.

Firstly, cut the mechanical stop.

Then, destroy the flat spot inside the hole by using a precision screwdriver with a flat-blade. This is to make the rotation of the gear train independent from the potentiometer.

You can manually test that nothing limit the rotation of the gear before repackaging the servo-motor.

Step 4: Adapt the Motor for Lego

The gear must be cut as seen on photos, then refer to the following Instructable : Servo-motor adapted to Lego (note that the last adaptation in the Step 2 is not required in this case).

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