Hacking Tiny Servos
Thought I would blog about hacking the 9g servos from SparkFun and converting them to continuous rotation since I’m performing said hack for an upcoming project. I’m sure there are more and better instructions on how to do this, but I’ve looked at tutorials before for larger servos and noticed these little guys are a bit different.
What I’m doing is changing a servo that normally has around 90-120 degrees of motion to have full 360 continuous rotation. Instead of the servo reacting to the standard PWM signal as a position signal, it will now act as a velocity signal with full forward/stop/reverse control! Best of all, when people tell you the servos on your YT-1300 look like a piece of junk, you can tell them you’ve made a few special modifications yourself.
Apology in advance for the images – I promise to never use my cell phone cam for blogging again! It’s abysmally unfit for close up pictures of electronics.
Take the servo apart
Remove the sticky label (goo-be-gone is needed) and remove the screws. You’ll likely need a jewelry screw driver. Pull all three sections apart, remove the gears, and remove the white plastic cover over the potentiometer. Easy peasy.
Cut out the stops
I’m running on memory, so feel free to correct me in comments if I’m wrong. I recall this as the part that’s different from the standard sized servos. There are stops on the potentiometer and you practically have to cut out half of the pot to get it to rotate properly. This is because the pot shaft is used as the axis for the gears and external drive shaft. Boo to the man that designed it this way, unless it made it significantly cheaper. In that case, yay… but boo.
Warning: The plastic on the pot is EXTREMELY brittle. If the pot case crumbles, your servo is trashed.
Proceeding onward. You’ll see two plastic stops in the pot – cut it out in small pieces. Once that’s gone, you’ve got to remove the potwiper. The copper-colored middle terminal of the pot should come out with a little force at this point. There are a couple of “C” shaped metal tabs that will come out with it. Now cut the entire terminal off and leave the wire hanging for now.
On one of my servos, I cut it out without pulling the shaft out. On another, the shaft hole cracked cand the whole shaft came out which made this easier – but I’ll have to report later as to whether it still works well or not.
You also need to cut out the springy piece. You can’t remove it completely, it appears to be connected with the shaft itself, so you’ll need to cut it flush against the remaining metal flange.
The final stop that has to be cut is on the actual case, just inside the hole where the shaft exits the case. Flush cutters make quick work of this, but an Xacto knife could probably work too.
Solder resistors in place of pot
So that the potentiometer has a reference point for its control loop, we need to solder resistors in place of the pot. I used two 10K resistors, which will make the signal that would have centered the servo be the stop position. Positions signals right from the signal will be forward, and positions left from the signal will be reverse.
Solder the two 10K resistors (surface mount or 1/8W or smaller should fit) in series between the two remaining posts. Then solder the currently-floating middle terminal wire in between the two resistors.
Put the servo back together
Pretty straightforward. Do step one in reverse. The gears may feel like a puzzle, but there’s only one way they fit. Lastly, be careful with the screws – they strip out pretty easy.
Shazaam! Full control of a motor with great torque/gear ratio with only a half hour’s worth of effort.