Modify a Servo for Continuous Rotation





Introduction: Modify a Servo for Continuous Rotation

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.




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    1 Questions

    now i have a 40kg servo " cys s8218" and i need to know what resistance should i put the 220 ohm or a bigger one ???

    my pot says b5k-220 so that means i will use the 220 ohm ?
    and where should i solder them


    After removing the POT it became possible to read the specs of the POT and mine says B5K on it. So I'm thinking this means it's a 5k ohm POT and should be replaced by an equivalent amount of resistance, hence two 2.2k resistors.

    How is the resistor value determined? I will be modifying a different type of servo and was hoping there is a general equation or specs that determine which resistors to use.

    Wow. You do that quickly. Nice ible!

    Somewhat off topic question...

    Can the servo control circuit be used to control a similarly sized motor? Is there a tolerance range for the motor size?

    interesting question! i don't see why not. if you want to steal the circuit to have a pwm/hbridge hooked up to a dc motor gearset, then ur golden. that's a great idea too :)

    here is some info on a cheap futaba servo
    with schematic of circuit,on the BAL6686 H-BRIDGE CHIP can handle about 1 amp if need more just use a h-bridge circuit needed for motor you are driving
    also needed is 5k pot if making high torque servo
    to build servo gear set,an get gears from servo city

    i made many like that ,i buy a cheap futuba s3003 from china,since the have h-bridge easy to remove and replace with h-bridge with higher current and add a 5k pot to my gearset,may need a very simple logic circuit to convert the pwm signals on some h-bridge ,cd4001 and cd 4011 is needed
    on the pot mostly any value will work ,just both need to be the same to center the motor

    I was wndering if it could be used to replace the circuit board in someething like an rc car;

    You could have the motor controllers hooked up to a micro controller or RC control device, and with an adequate control interface, you could modify almost anything for either robotic or RC use. (assuming you have the appropriate sized motors, which usually goes along with the size of the device...) It would make for a great ible... I don't have the resources right now, but someone really should do that.

    I have an original dc motor gearset and another tamiya bulldozer. I'll throw it together and power the dc motors with the circuit from 2 servos. if it works, i'll throw up an instructable with props to you

    Cool. I think it could be used with the same EZ-B micro-controller too couldn't it? (Just use the same setup as your fast and cheap bot and throw it onto anything with a differential drive.)

    it would have been great. i took a servo apart. the motors have 3 wires... won't work sadly