3-278 - R/C Servo, HS-300B $19.95
Quality RCD servo motors as used in model planes & cars. Ideal for numerous
robot projects as described in Mobile Robots by Jones & Flynn (#3-098).
Easily interfaced to any microprocessor (BASIC Stamp, BOTBoard, Mini Serial
Servo Controller, etc.), they provide precise positional control, and are
modifiable for continuous rotation.
Torque: 42 oz/in, 0.19 sec.
Size: 1.4 x 0.8 x 1.6 in
Weight: 1.57 oz
Mondo-tronics' Robot Store Tech Notes
Servo Modification For Continuous Rotation
How to convert a standard R/C servo into
a continuously rotating drive motor.
This procedure results in a servo motor having continuous rotation in
both forward and reverse directions, and with some degree of speed control
from slow to full in both directions. BUT NOTE: the servo does not retain
any of its position control abilities.
NOTICE: This procedure is provided for information purposes only, and is
recommended only for experienced hobbyists. Caution: This procedure
involves making permanent, irreversible changes (that are not at all
undoable, even), and that will certainly and undoubtedly void the product's
warranty. Warning: Product details and designs may vary greatly between
manufacturers and even within the same product over time. Inspect your
servo's actual design and features carefully before proceeding with any
modifications. Mondo-tronics makes no claims to the accuracy or fitness of
this information for any particular use, or appropriateness for any
particular device. Proceed boldly at your own risk.
1 each Hitec HS-300 servo (or similar servo)
2 each Resistor, 2.2K ohm 1/4 Watt 5%
1 each Shrink tubing 3/8" or black electrical tape
Small "+" screwdriver
Small "-" screwdriver
Needle nose pliers
1) Open the servo housing by removing the 4 bottom screws.
2) Carefully remove the top and remove all the gears and retaining ring
making sure to note their positions for proper reassembly. Use care to not
loose any of the parts.
3) Locate the last and largest gear in the drive train, the one with the
shaft that extends outside the case.
4) Locate the stop molded into the top surface of the gear. Carefully
trim it off with a sharp hobby knife.
5) Use the needle nose pliers to unscrew retaining nut holding the
potentiometer shaft in the servo housing.
6) Remove the bottom cover from the servo and gently lift out the small
printed circuit board (PCB).
7) Press on the shaft of the potentiometer and remove it from the servo
8) Cut the three wires on the potentiometer in half. Then trim and tin
the ends of the wires still attached to the PCB with solder.
9) Twist the two 2.2K resistors closely together and solder them. When
cool, trim the ends to about 3 mm long.
10) Solder the red wire from the PCB to one end of the resistor pair,
the green to the other end and the yellow to the center where the two
11) Slide the resistors into shrink tubing and shrink it (or wrap in
tape) to prevent electrical contact.
12) Unsolder the wires remaining on the potentiometer .
13) Pry up on the tabs on the bottom of the potentiometer and remove the
14) Clip off the wipers inside the potentiometer .
15) Locate the stop notch on the top side of the potentiometer . Use a
side cutter to make two sharp cuts into the body of the potentiometer on
either side of the stop notch.
16) Bend and break away the potentiometer wall, and flatten the stop
notch, using caution to not damage any other part of the potentiometer .
Check that the potentiometer can now rotate freely. If not trim or flatten
any remaining obstruction.
17) Return the potentiometer into the servo body, and retaining it with
the washer and nut. Tighten securely.
18) Carefully pack the new resistor pair and shrink tube into the servo
body, and replace the PCB. Make sure the resistors do not touch or
interfere with the rotation of the potentiometer .
19) Carefully reassemble the gears and bearing ring, returning them to
their original positions.
20) Replace the top and bottom covers. Secure with screws.
Test the Modified Servo
1) Connect the servo to an appropriate R/C receiver or electronic driver
2) Apply power.
3) Send a "high" or "forward" signal. The servo should rotate in one
direction continuously at full speed.
4) Send a "low" or "reverse" signal. The servo should rotate in the
opposite direction at full speed.
5) If you are using a computerized driver circuit (like the Mini Serial
Servo Controller, #3-205) you should be able to find an exact value
(usually in the range of 150 to 220) that will cause the servo to stop
rotation entirely. Values slightly to either side of this number will
produce slowly increasing rotation in both directions, permitting some
degree of speed control.
6) Double check that all screws are secure.
7) Label the servo as modified for continuous rotation.
You may use 1% precision resistors for greater repeatability if you are
modifying more than one servo and want greater consistency.
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