Modify the Hitec Hs-325 Servo for Continuous Rotation





Introduction: Modify the Hitec Hs-325 Servo for Continuous Rotation

Servo motors are designed to rotate a maximum of +/- 130 degrees. But they can be easily modified to make 360 degree turns. The hack is very well documented for various servo motor models. Here I use a Hitec HS-325HB servo purchased at ServoCity. This motor has a very good torque/size ratio and costs only around 10 dollars.

Step 1: Take the Motor Apart

Remove the screw holding the white wheel on the front of the motor.
Remove the four long screws on the back of the servo.
Remove the front cover of the servo (might require some pressure).
The gears are now exposed. Remove them, place them in a clean place making sure they don't get dusty.
Remove the back cover of the motor and save all the covers and screws in a safe place.

Step 2: Replace the Potentiometer

If you look on the back of the motor, you will see the PCB. Unsolder the two large metal tabs holding the PCB in place. Gently remove the PCB with pliers. This should not require force. If it does, make sure the metal tabas have been unsoldered properly and completely.
Unscrew the screw holding the potentiometer in place. Remove the potentiometer from the motor case (you might need to give it a little push from the front).

Cut the three wires between the PCB and the potentiometer (red, gree, yellow), but not too close to the PCB.
Get two resistors of 2.2K. tie a leg of one of them around a leg of the other, and put some solder to hold. Cut one of the legs you just twisted. You are left with a Y shape as seen below.
Strip the three wires you just cut off and insert small pieces of heat shrink tube in each one. Solder the center leg of the Y resistor ladder to the green wire, and the two others to the yellow and red wires. Heat the tubes over the joint to make sure the joint is secure.
Insert the PCB back into the servo cavity making sure the resistors don't short out any components on the PCB. Resolder the two tabs to secure the PCB.

Step 3: Cut the Stop

Now that you have replaced the potentiometer with a resistor ladder, you can cut the mechanical stop preventing the motor from rotating 360 degrees.
The biggest gear has a small rectangular plastic stop. Using an exacto knife, cut the stop off without damaging the gear teeth.
Return each gear to its place, screw the covers back on the motor. The servo can now rotate continuously.



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    20 Discussions

    I have modified my servo and now it moves continuously, but in one direction. In the position i have that servo i just need it to rotate in the opposite sense . How can i do that ? Is ok to switch the red and black (5 V with ground) wire to change the sense of rotation ? Thanks.

    2 replies

    I think this can help you:

    Hmm - this is a bit old - but no, you wouldn't want to switch them, just change the PWM value you give the servo. If no pwm value gives you reverse rotation, then the resistance you used to replace the pot was wrong. I'd strongly advise reading about normal servo operation - error values and the like.

    Why replace this and not just use the build in one as a trim pot?

    I'm confused. If I understand right, they limit the pivoting of the servos. Therefore, the servos are technically useless until modified. So why don't the companies just build them without the pin?

    3 replies

    because they have feedback control, these can be used in robotic arms... so it still knows where it is. this is because of the pot... when you cut the tab the pot stays in the center so it doesn't know where it is..

    the are here becaus the stearing of a car may not turn 360degres. and the trottle is onmly 45 degres. so it has more range than you actualy use.

    servos are intended to be actuators; they are suppose to swing , turn some levers as in model airplane surfaces or a gripper in some hobby robots. there are motors than fit your concern. one is a stepper motor. 360 degrees continuous turn or precision positioning in any direction.

    now one to modify a standard servo to rotate 180 degrees for r retract servo. i wanna put retracts on one of my planes, but the price of a good retract servo!!! jeez!

    1 reply

    many years ago, i had the same need (and the scarcity of hobby money) , i hack an old airtronics servo and played around with the fixed resistor values to increase the sweep angle to 180 degrees.

    ghicken, in robotics it does. there are a lot more applications out there where this trick can find use.

    servos are more precise and can accurately turn to any given angle precisely on command. Motors have more torque and higher speed but they don't stop on a dime and they aren't nearly as precise.

    I tried this with some fubuta servos. what i did was mount a separate pentomiter on the outside so i could adjust it. I didn't like the idea of just soldering two resistors in it's place. Your pictures are a little unclear though

    This was exactly what I needed. The servo wouldn't work properly, and I don't want to buy a new one. This helps me with a couple projects. I thought the servo was stuck as a 90 degree turn radius.

    Be very careful when dealing with the circut was broken when I attempted this for the first time.

    or use a pot without a limiter. they are very useful for drive synchos also. Did that about 20 years ago... cheers, chuck...

    yup, they work well. only minor problem is that they rotate fairly slowly but for most purposes this is a non issue. if you need to ensure they match, add a 100k or so resistor in parallel with one of the two resistors you replaced the pot with, until they both stop at the same time. -A