Servo modification for 360 degree rotation.
We recently purchased a few R/C servos from Tower Hobbies, model STD-TS53, for under $10 each. They can be modified for 360 degree continuous rotation by following the steps below. By attaching small wheels to a pair of modified servos, it can serve as a simple motorized base for a small robot without needing an H-Switch motor driver circuit. Other model R/C servos can be used since they are mostly the same construction.
Remove the four screws on the bottom of the servo and take off the bottom plate. If you have a control horn attached to the top of the servo, remove that also. With a small screwdriver, carefully pry out the PC board motor control unit from the plastic case. It will be tight so go slowly.
Once the motor unit is removed from the casing, you will need to desolder the 5K feedback pot and replace it with a couple of resistors. There are surface mount parts on the printed circuit board so be careful not to loosen them when desoldering the 5K pot.
I used 2.2 K resistors soldered together as a drop in replacement for the 3 terminal pot. 2.7 K resistors will also work.
By replacing the pot with the two resistors, it will keep the feedback loop of the servo mechanism at the "centered" value.
Remove the top cover of the servo to reveal the gear box. Slide off the gear with the plastic mechanical stop on it and file it off. I used a X-acto knife to shave off the plastic. Replace the gear and make sure everything rotates smoothly.
Reassemble the servo and test it with the Handy Board servo routine. It should continuously rotate forward or reverse when you output a pulse width from the 68HC11. The speed of rotation can also be set by changing the pulse width value. Speeds range from very slow to about a revolution a second with pretty good torque. To stop the servo, send out a pulse width of about 1.5ms. Experiment with different values to find out what range works best with your servo.
STD-TS53 connection diagram:
White Pulse width input signal