Powerful DC motor to Servo motor

I have a few car window glass motor which I wish to make a servo.
Similar servo reaches 100$....while window motor costs 10$ or less from junk Yard.
Recently I modified a 9g servo into a geared motor. Question is
Can I use the servo circuit salvaged from the 9g servo to run a powerful 12 Volt motor ? I assume I need
1- to use a bigger size pot that reads position
2- add power transistors at the output to drive the motor
3- add a 12-to-5 volt regulator to feed the servo controller small IC

did anyone experiment on this matter ?
I also found an IC M516601 ( posted on Hs-311 servo control circuit )
as a second option. the circuit below maybe can add higher power transistors to drive the big motor

If your serious about doing something like this it should already occurred to you to look for power transistors that can handle the current of the window motor. They will probably need heatsinks. The main problem I see is that pins 4 and 12 probably do not have enough drive for the power transistors so you will need to use the existing transistors as drive transistors for the power transistors.

Search on youtube for giant servo and monster servo to see similar projects. The 9g servo should be able to drive a large h-bridge like below. The 9g servo pot could be used, but it is physically small so a larger pot might be easier to work with.


Look at something like the Pololu high current H-bridges for
driving the motor, then you need position feedback (an encoder?)
and then the Arduino to implement the PID loop to control the whole
thing. Start with P-loop, then PI, then PID if necessary.

I don't really see how you can repurpose the pot in the 9g servo for
a car window motor, its too small and fiddly - an encoder seems much more
sensible, or if only needing less than a turn you can use a full size pot as

A popular setup, but the servo does not necessary need to be physically attached to the wiper arm, just the pot will do.

A popular setup, but the servo does not necessary need to be physically attached to the wiper arm, just the pot will do.

A couple of more links:

Homemade Radio-Control Mowers (scroll down a bit - part of the article details "monster servos")


Basically, you take a servo, and remove the motor from the control board - then you solder wires from the control board to the control lines of your (larger, high-power) h-bridge; provided that the control lines on the bridge are 5 volt tolerant, and you supply the servo with 4.8-5 volts - it should all work fine. Then just remove the remaining gear train from the servo, but leave the potentiometer in place. Attach a servo horn and connect that horn to your moving bit. A little tuning and debugging, and it should be good to go.

While zoomkat is right in that you don't need to keep the servo intact - you do need to keep the potentiometer (or one of the same type and size) connected to the servo board for this to work. That said, I would personally keep everything in the servo case, for a number of reasons:

  • The case will protect the electronics and potentiometer (especially if it was a "waterproof" servo)
  • The case will provide the support for the board and potentiometer
  • The case has convenient (and standard) mounting holes (and convenient/standard servo horn)
  • And the biggest one: The potentiometer shaft and such will be supported by some form of bearing to prevent side-loads from the horn being rotated externally from causing additional mechanical wear

@cr0sh: link you gave are quite impressive, I learned alot..Although I still find installing the read pot on the motor a bit tricky.
@zoomkat/raschemmel/MarkT: I will give it a try, adding 2 power transistors on top of the small transistors, wish me luck.

I will give it a try, adding 2 power transistors on top of the small transistors, wish me luck.

Probably better to get a large H-bridge like below and drive it with the servo internals.


looking at that diagram it appears all the motor current has to pass through pin 7 of the ic.
can it handle the extra current ?

I looked and I don't think so.
you will need an external H bridge
or risk blowing the chip that way

The datasheet just shows the output driver circuit as a block
its possible that pins 6 and 10 have current sensing in which case it may not function when an external H bridge is used.
Just a possibility but you wont know until you try it