SERVOMOTORS
makezine.com/19/primer
Metal stop
(remove this)
Plastic stop
(remove this)
J
K
L
M
First, open up the servo. The HTX500 servo case
is made of 3 plastic pieces press-fit together. You
can use a small blade screwdriver or similar thin
wedge to separate them. From the top, pull off the
gears (note which ones go where). From the bottom,
carefully pull out the servo’s circuit board (Figure I).
There are 2 mechanical stops. Remove the metal
stop at the shaft base by bending it with needlenose
pliers (Figure J). Remove the plastic stop on the top
case with diagonal cutters (Figure K).
Replace the 5kΩ pot with 2 fixed resistors that
add up to near 5kΩ. A pair of 2.2kΩ resistors works
well. Unsolder the 3 wires from the servo’s pot and
solder them to the 2 resistors as pictured in Figure L.
Wrap this new assembly in electrical tape or heat-shrink tubing (Figure M). Tuck it and the rest of the
electronics back into the servo case and snap it all
back together.
That’s it for the hack. Calibrate your finished
continuous servo by finding where the zero point is.
If the 2 fixed resistors were exactly identical, a 90°
angle sent to the servo would make the motor stop.
Your servo’s value will likely be a bit off. You can use
N
Disc
Squaring
circuit
Photosensor
LED
the previous sketch to experiment in finding what
angle does stop the motors. Remember that value,
because it’ll be different for each servo.
Hobby servos use potentiometers to measure
shaft rotation. Servos used in larger systems like
industrial robots and CNC machines use rotary
encoders. Optical rotary encoders attach a disc with
slits and count the number of slits that go by with an
LED and photosensor (Figure N). This is also how
mechanical ball computer mice measure movement.
146 Make: Volume 19
