I love to modify hobby servos into continuous rotation gearmotors. Not only are the very useful, but I feel like I’m getting away with something sneaky. A modified servo in a radio controlled vehicle does the job of a motor, a gearset, and an electronic speed controller, and when cheap servos can be had for just a few dollars that’s a real bargain.
I always use cheap servos, but in my experience they are a hassle to modify. The potentiometer is often part of the structure, so it can’t be removed, but rather the wires to it have to be cut and matching resistors soldered in to replace its variable resistor. If you want it to have a fine adjustment trimpot you have to buy one and solder that in too.
Recently I bought some $3.50 HobbyKing 15138 servos to see how easy they would be to modify. It turns out that they are incredibly easy. Aside from turning some screws the modification consists only of removing a stop nub and drilling a hole. Here’s how to do it.
Here’s the servo you’re looking for:
Remove the four screws on the bottom.
Remove the top and bottom of the case.
Gently pry the circuit board out of the bottom with a small screwdriver.
Remove it and the attached motor. It will be attached by three small wires.
Remove the small screw securing the potentiometer inside the case.
Remove the white gears, but don’t forget how they go back together, then firmly press the black thing under the final output gear into the case. This is the potentiometer shaft.
Now the circuit board, motor, and potentiometer are free of the case.
Have a look inside. We want to drill a hole on the potentiometer side of the servo case so that we can press the potentiometer shaft bushing into it from the inside. I have found that the bushing is just slightly larger than 3/16″, so I drill this hole with a 3/16″ drill bit and then wobble the bit around in the hole to open it up a bit more. When you drill the hole make sure that the potentiometer body won’t hit the circuit board or any obstructions inside the case.
Press the potentiometer into the hole from the inside. I like to use a larger screwdriver to pry it in using interior features to leverage against. It is important to get the bushing to seat into the hole firmly. I have noticed that it will take more force to rotate the potentiometer shaft after the bushing has been pressed into this hole, and might require needle nose plyers depending on how much you opened your hole up past 3/16”.
Replace the motor and circuit board and gently press them back into the case.
Remove the stop nub from the final output gear. I like to clip it off with wire cutters, and then sand it down flush with the gears and shaft. It is important that nub remainders are sanded or filed down flat. Make sure you don’t damage the teeth. Wash the gear off, or blow it with compressed air to remove debris from sanding.
Reassemble the gears while leaving out the gear opposite the output gear so that you can check for remnants of the stop nub interfering with the top center gear. It must be sanded or filed down flush with the gear.
Now you can plug the servo into your receiver, center your transmitter trim, and then turn the potentiometer until the output shaft stops moving. At this point you should have a perfectly functioning continuous rotation servo.