A modified servo can be purchased from various online hobby stores. There are two kinds of servos, Standard and Modified. A modified servo has continuous rotation.

There are positive advantage to using servos for robot drivetrain VS DC motor gearsets. The DC motor would require a transistor, resistor, more space, and doesn't have very good speed control. Even with PWM, a DC motor lacks low speed torque.

All of my robots (except the Attacking Spider), use servos instead of DC motors.

What you need:

1 x Acoms AS-12 Servo (or similar servo by GWS, Parallax, or Tamiya, etc)
2 x 220 ohm resistors
Soldering iron
Small side cutters
Small jewerlers screwdriver set

Watch this video before reading the instructions. The video will give you an idea of what to expect as you follow the steps.

Step 1: Take Servo Apart

There are 4 screws on the back of the servo. Take them out.

The back will come off easily.

The front will come off easily.

*Note: when taking the front off, notice the location of the gears. There are only 4 gears, but it could be a puzzle if you don't remember how they go together.

Step 2: Regonize Notched Gear

The main gear that sticks out the top of the servo has a notch on it. The notch pervents the gear from turning 360 degrees.

You will need to cut that notch with a pair of side cutters.

Step 3: Remove Potentiometer

The potentiometer measures the angle of the servo. This will need to be removed. There will be two tabs holding it in. Break the tabs with the side cutters.

Cut the wires to the meter and remember where they attached on the circuit board.

Step 4: Solder Resistors

Solder the 2 x 220 ohm resistors in place of the potentiometer wires.

Step 5: Voila! Modified Servo!

Put it all back together and now you have a modified servo.

These steps aren't nearly as detailed as the video. So please make sure you watch the video!

For more robot ideas with your newly modified servo, be sure to check out my robots and my ez-b projects.