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.


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!<br><br>Somewhat off topic question...<br><br>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 <br>http://www.seattlerobotics.org/encoder/200009/S3003C.html <br>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 <br>also needed is 5k pot if making high torque servo <br>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 <br>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;<br><br>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
Does that mean they are brushless? One could be a ground... Or maybe it is used as a speed controller... Have you tried running power through different wire combos to see which make the motor spin? Are the wires different colors? If one of them is a ground, then you won't need it, but if it is a control wire it won't work. <br><br>I need to take apart a servo to test myself, if we could get something working it would be awesome! But if not, we always end up better educated.
Hmm... A quick google ended with this; <br><br>Modern servos are controlled by three wires; Power (red), Ground (black), and Signal (yellow). It appears all three are required to run the motor.<br><br>Maybe the circuit could be modified to control a normal DC motor. I don't know weather the signal motor is required to run a motor. I surmise it is only used for controlling speed and degrees rotated.
that's not the three wires i was referring to. the motor itself, not the unit. those 3 wires are how a servo normally operates. i have a little writeup here: http://www.ez-robot.com/Add-Ons/ <br> <br>i'm referring to the actual motor in the case. it has 3 wires. i have not attempted to put power or even look at it. i might another time. tonight i built a robot instead lol
Ahh, I see. Well it was worth a try. I think I'll go build a robot too. Lol
220-ohm? The POT in majority of Futaba servos, are 5K.. a pair of 2.2K (red-red-red) would be a better choice.. I've re-routed the stem from the POT out the side of the case, just above the wires, and &quot;Z&quot;-bent the leads to the POT. Sometimes, I've run across a few different microcontrollers, which seem to use different values for the all stop, so I leave the POT in circuit, to allow trimming.
Hey, would the resistor value change the speed of the servo? <br>If so, couldn't you just short those two resistor connections and get much faster transit time (or would this overload your receiver/motor)?
The resistance simply needs to be equal on both sides. That way, the voltage being read will be half of the input voltage. If you change the size of the resistors, you will merely generate heat and create a short -or- you will not provide enough current to the input line. (Depending on the direction of ohms you switched too) <br> <br>The speed of the servo is dependent on the servo itself -and- the pulse width. I wrote a little blurb about how a servo works on this page here: http://www.ez-robot.com/Add-Ons/
Great, thnx!
to get the servos rotating at a faster rate, you simply send the signal for the extreme direction. (I.E.. 1 to 255, 128 being stopped (centered), ) (or, arduino servo.write, 0 to 180, 90 being centered (stopped) ) You're still going to be limited by the step-down gearing of the servo. But, NEVER increase the voltage driving the servo. Most use the same voltaghe to drive the motor and the controller board inside, and going outside the maximum range will surely fry the controller.<br>
Cool instructable, with both step-bystep and video it's really nice. One question: why you put two resistors? If i remember correctly, parallel resistor connection decreases it's resistance value, can't you put a resistor with less value?<br>Thanks for sharing.
a variable resistor (potentiometer) has 3 leads. in order to simulate the position of the potentiometer being in the middle, you need to resistors. the resistors run from the first lead, to the second lead. and the second lead to the third lead. <br> <br>
Wow, you're fast! Super cool!
lol thanx! have to burn off the holiday calories somehow :)
Thanks, cool :)<br> <a href="http://www.buildhaus.ru">_</a><br>

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Bio: I build robots to encourage others to do the same. I believe the future is in robotics and playing a part for the future is ... More »
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