The following steps will, despite their manufacturer's best efforts, demonstrate the modification of sub-micro servos for continuous rotation with position feedback. We leveraged this process for a 4 axis positioning system, but it may be useful in your other projects as well. Anyway, our build used the Hitec HS-55 servos, but other servos of similar size should also be modifiable with similar steps. Note that if another type of servo is used for the aquarium project, the CAD models for the 3D printed parts may need to be changed as well. This was part of a project for the spring 2012 Things That Think class at CU Boulder.
1 Hitec HS-55 sub-micro servo
2 2.2K Ohm 1/8 Watt resistors
Approx. 1 ft 30 AWG wire
Wire cutters, wire strippers
A lot of patience
Step 1: Remove the Backside Casing
On the back of the servo, remove the four easy to loose screws and the back cover, placing them safely to the side. Note that these four screws hold on the front cover and gear train and, as such, care should be taken to make sure the front cover does not fall off.
Step 2: Remove the Frontside Casing
Face the front of the servo upward and lift off the front cover to expose the gears. After noting the arrangement of these small plastic pieces so that they can be re-assembled later, carefully remove the gears with tweezers, setting them in a safe location. However, be sure to leave in place the brass gear attached to the motor shaft.
Step 3: Remove Upper Physical Stop
On the inside of the front cover, at the top between the output hole and the top edge, there is a small plastic peg that prevents the output gear from turning all the way around. Using diagonal cutters, a knife, sandpaper, or whatever works best for you, cut, scrape, or sand this peg down until it is level with the rim of the output hole.
Step 4: Remove Potentiometer Physical Stops
Pull out the circuit board on the back of the servo body. Remove any screws holding the potentiometer in and push the potentiometer out the back of the servo.
On the front of the potentiometer, you should see some bumps on the inside of the potentiometer housing and a small metal plate around the shaft that together act as physical stops for the potentiometer. First, use a knife to cut out the bumps on the housing. Then, taking advantage of a pair of needlenose pliers, pry the metal plate off of the shaft. However, exercise caution in this precision demanding step and be sure not to damage the potentiometer wiper underneath.
Step 5: Remove Potentiometer
Unsolder the potentiometer from the three wires connected to it, keeping track of which wire was connected to the middle pin.
Step 6: Attach New Resistors
Cut the leads on the 2.2K Ohm resistors as short as possible while still allowing for soldering. Then, after this careful application of wire-cutters, solder one side of each resistor to the middle potentiometer wire. Finally, solder the other end of each resistor to one of the wires that was previously connected to the outside pins on the potentiometer.
Step 7: Re-Attach Potentiometer
First, cut two pieces of 30 AWG wire (each about 1 in. long) and strip a small part on each end. Solder these to the power and ground pads (red and black wire respectively) where the input cable meets the circuit board. Then, solder the rest of the 30 AWG wire to the middle pin of the potentiometer. This is the signal out line from the potentiometer. Finally, cut a notch in the servo housing so that this wire can be routed out of the servo.
Step 8: Re-Assemble Potentiometer
Push the potentiometer back into place and replace the retaining screw.
Step 9: Fold Circuit Board
While being careful not to cause a short, fold the circuit board and the resistors into the back of the body of the servo.
Step 10: Replace Cover / Wiring
On the front of the servo, replace the gear assembly and front cover. Then, replace the back cover, reinstalling the four back screws. Now your servo should be ready to go for continuous rotation with potentiometer feedback!