PARTS Mini-Sumo Robot Kit Mark II: Servos

Modify your RC Servos using these four links as a guide.

Darl Sandberg's Method - requires no electrical changes, just mechanical; easy to do on Futaba servos; not appropriate for Hitec HS300 servos

Kevin Ross' Method - also for Futaba FP148, but works for most servos

Mondo-tronics' Method - most appropriate hack for Hitec HS300 servos

Another Futaba Hack

The robot uses two Futaba 3003 or similar radio control servos. Click here for an explanation of how servos work. These servos are designed to rotate about 180 degrees and need to be modified to rotate continuously.

Servos are ideal for small robot use because they contain a motor, gearbox and motor drive electronics. And, servos can be controlled with only one I/O pin, which is very nice when using a microcontroller with limited I/O as in the Basic Stamp I used in the kit.

NOTE: Servos are not included in the kit. You must purchase them separately.

At least one vendor,, sells pre-modified, continuously rotating servos, which is a much easier route for begineers. However, this is not an endorsement of or its products.

Optional Additional Servo Modification #1:

Note: if this is not done, be sure to use the version of the program available for download which is designed for unrotated (identical) servos!

One servo should be modified to rotate the opposite direction of the other servo. This is because the servos are placed facing opposite directions of each other on the robot base.

This will make programming the software easier because using the same pulse width value for both servo's, will drive one servo clockwise and the other servo counter clockwise. This will move the robot forward or reverse.

To perform this modification on one servo you will need to remove the motor and circuit board from the case. The motor is attached to the circuit board by two solder tabs. To remove the motors, remove the solder from the solder tabs using a solder sucker, solder wick, or tapping the circuit board until the heated solder falls off.

Once the solder is removed and the motor is separated from the circuit board rotate the motor 180 degrees and reinsert the motor and solder it back into place.

Not necessary if your program accounts for the difference in direction between the two motors

Additional Servo Modification #2:

Both servos should be adjusted to the same neutral value. Click Here
Using the same neutral value for both servo's makes coding the software much easier because the same value can be used for both servo's.

If you use Daryl's instructions for modifying a servo, step 11 describes how to zero the pot; you should not do what follows.

This adjustment can be accomplished by using the BASIC STAMP program SERVADJ.BAS to send a pulse width of 1.50ms to each servo. Adjust each servo's potentiometer to it's neutral position. Put a drop of superglue on the potentiometer, then cut off the shaft of the potentiometer with bolt cutter or push it down into the servo case.

Not necessary with Daryl's servo hack

Next Step

  last updated 3/2/2001