I pulled out my Basic Stamp 2 Board of Education to help with the hacking of some Hitec HS-55 Feather Servos (these servos can be bought here on Amazon). These probably wouldn’t have been my first choice for hobby servos, because I wanted something with metal gears, but I was able confirm online that these could be hacked for continuous motion, and was able to find several different sets of instructions on how to accomplish the task, here: one, two and three.

At first, I decided to follow the instructions in the first one pretty much all the way, except to avoid soldering the resistors directly onto the board of the servo controller. I’m not good at soldering tiny things onto other tiny things. Especially when they can be damaged by shorts caused by errant solder. I just won’t do it. So I soldered some resistors onto the ends of the wires that connect to the internal potentiometer. Not to get too deeply into the process here, because these three other articles covered it so well, the process is:

1. Open the servo.
2. Pull off the gears (make note of the order first!)
3. Unscrew the screw that holds in the potentiometer.
4. Pull the pot out and cut off the wires (make note of which go where!)
5. Hack the pot so that it has no stops and put back into the servo.
6. Put the screw back in.
7. Solder resistors to wires to replace potentiometer.
8. Cut off a stop built into the top of the servo case.
9. Reassemble everything, tucking the resistors back in to the servo.

So, as noted, I used resistors, and even found some in my electronics parts stash that were around the right values and had a 2% variance rating. After hacking them, I got out the Basic Stamp 2 and the Board of Education and tested them both. They worked fine, but to my dismay neither of them seemed to “center” where they should. These hobby servos (like most standard servos) are supposed to sit still at 1500 uSec. They both moved quite a bit. I also found that one was only moving in one direction, no matter what I changed the pulse width to. Of course, I’m sure this is due to my bad soldering skills and the sensitivity of the equipment!

As a result, I decided to try using an external potentiometer to replace the internal one, so that I could adjust them to center on 1500 uSec. I pulled out my potentiometer bag from my electronics stash and after quite a bit of trial and error, I settled on two 2 K ohm, linear potentiometers. I tried some larger standard pots, and also tried some with extra resistors in series and etc. Somewhere in the process I managed to fry one of the servo control boards. (Luckily I ordered an extra with the idea that hacking these might prove fatal to at least one).

The most painful lesson of the day: Don’t forget to attach all grounds when dealing with separate power supplies for the logic and motors!!!!

It seems kind of silly to attach so much gear to a servo the size of a quarter, and maybe later I’ll look for a cleaner solution, but for now these are working great. I’m able to adjust the potentiometers such that there isn’t any movement at 1500 uSec pulse width. Sadly, these two do not seem to be willing to move at the same speed, so sooner or later I’m going to need to implement a shaft encoder system of some kind. Part of me is not looking forward to that. The mad scientist part of me is delighted, however.

Of course, moving to a shaft encoder system is going to require a different microprocessor setup…