Let's Make Robots!

Continuous rotating/normal servo - all in one

Rotates continuous or moves to desired position

This is a short tutorial how you get a servo for continuous running AND normal servo mode for a few bucks. You can switch between the modes in setting an additional pin high or low.

1) Open the servo and desolder the PCB. Remember the connections for motor and potentiometer

2) Remove the mechanical stop on the gear

3) Remove the potentiometer and modify it according to picture below. At least Hitec servos have this kind of potentiometer. Assemble the potentiometer and put it back on its original place

4) Prepare the servo as following

5) Make a small additional perfboard according to this circuit diagram and connect everything accordingly

I am using a TAKAMISAWA A5W-K PCB relay. They are very tiny, just like a 10 pin DIL chip and they are DIL socket compatible:

This is how my first prototype looked like:

6) You are done. Using a Sharp IR or ping on your servo, your robot can now not only look to the front, left and right, it can also look to the back!

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If you can make this work you might have a very marketable product on your hands.

I'll build a prototype and test it the next days. But I am quite sure it works :)


The question is what happens when you switch back from continous to positioned?

The ideas are what the possible answers give you. An answer I like a lot is that it aligns by accepting whatever the current position is as the right place to position using whatever the current PWM is. Another reasonable answer is that it aligns and accepts the current position as exactly mid range and adjusts if you aren't currently set at 90. I don't like that answer quite as well because it could be a little jerky. The possible correct answer (as far as what it will actually do) is that it would adjust exactly like it would have when first powered on.

The reason I like the first 2 answers better is for using it for dozer style tracks or similar drive mechanism. I would want to be able to pivot precisely without knowing exactly where the motors are currently turned.

I've never seen potentiometers that can turn endless.. do they exists?

I know rotary encoders, but thats will give problems if you move the servo by hand, I think.

By the way, this tread reminds me of this:

If your servo rotated continiously then it would break your potentsiometer.. Pots used in servos normally doesn't survive more than they are designed for. Other way would be to find pot that you can modify to continious rotation and then put it in servo.

The micro servo Tower Pro SG90 has for example a potentiometer without endstops. I am just building the prototype with this servo.

could be if the potentiometer is 'between the legs'  - I would assume even continuous pots have a break between the legs.  Would switching to normal servo mode then be a problem as there would be no feedback loop.


No necessarily. I assume, the motor would move as long as it get the feedback back.

Here is one example of a continous rotating potentiometer: http://www.potentiometers.com/SP22G.cfm

Datasheet: http://www.potentiometers.com/pdf/SP22G.pdf



All standard size servos have a potentiometer with a 5mm axle. At the time I was looking into this, all I could find available for sale by piece were pots with 6mm axle. I wanted to make a smart servo, that had a tiny85 inside and a 8 pin H-bridge IC replacing the servo's electronics, and use TWI to communicate with the master microcontroller. Since then, there were better solutions to this idea, see the link for the node that presents "hack your servo v2". 

Cool! Will follow your updates. Good luck with this one!