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.


Parts Required

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


Tools Required

Small "+" screwdriver

Small "-" screwdriver

Hobby knife

Side cutters

Needle nose pliers

Soldering iron



Modification Procedure

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

resistors join.

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.


Advanced Tip

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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