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How to modify a micro servo for continuous rotation

make a cheap servo rotate forever!

Here is how I've modified cheap e-bay servo for continuous rotation,

Thanks to Oddbot for his explanations :) 


Servo is a SG90 ,  equivalent to HXT900... and many others 


Tools you will need : A sharp knife, a PH0 screwdriver, some sandpaper, soldering iron (or glue)

+ Nice to have : A microcontroller that continuously sets the servo to its center position  ( 1.5 ms ,  position 150 on picaxe)


below : the package I recieved from Hong-Kong    (about a week after the order was placed)




Step 1:  Cut the stickers and remove the 4 screws



Step 2: Gently remove all the gears  (remind that you will have to put them back !!!)  and Pull the circuit away



Step 3: Push the potentiometer out of its casing   (as you can see, the output shaft is the potentiometer itself)


Step 4:

Connect the servo to your controller

The motor will rotate until you put the POT to its exact center position. 

When the motor stops, you have found the good position,  DO NOT disconnect the controller for the moment


Step 5 : SOLDER (yes SOLDER!) the shaft of the POT, from the back

Oddbot recommends to use hot-glue

If you decide to use glue, you will have to remove the white plastic cover sheet of the shaft-side, then fill with some glue


The shaft will never be able to move again,  try to make sure...  and power-off the controller if OK.

Then put the POT back to its casing 



Step 6 : It's time to make the shaft thinner.   (the outer gear as to turn freely)

 I use sandpaper.



Step 7 : Cut the "stop finger" of the output gear




Step 8 : The finger is far away now,  identify the 'notch' that prevents the gears to turn around its shaft. 




Step 9 : I've used one of the screws to drill the notch out,

Then I've used the same screw to drill the plastic gear  (Not sloppy on the shaft --> just be sure that it can turn with ease)  



Step 10 : Put all the gears back to the shafts



Step 11 : You can put everything back together



Step 12: Write some code to test... and enjoy ;)

(note that servo0 is used to center the POT ,  and servo1 is used to test turning forward then backward for 3 seconds) 


servo 0,150

servo 1,100

pause 3000

servo 0,150

servo 1,150

pause 1000

servo 0,150

servo 1,200

pause 3000

servo 0,150

servo 1,150

pause 1000

goto main



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I take it that your output gear did not require the pot shaft to spin on? Has removing the pot shaft reduced the mechanical strength of the output?

I don't think so. It may be a feature of my particular (Acoms) servos, but the main drive gear is a sort of flange on the drive shaft. The center of this "flange" sits over a shroud on the motor casing (the shroud can be seen on the reassembled gearbox). The pot was only attached by 3 gobs of solder and didn't fit particularly well in the middle of the shroud anyway, so I don't think it was affording much mechanical strength.

If your servos gain strength from the presence of the pot knob, I suppose an aluminium or plastic piece could be turned on your lathe (ha ha) to fit.