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Motor controller boards tend to be pricey, L298's a bit of hassle for the occasionally hobbyist, and both take up 2 IO pins per motor.

I've had the idea to hack off the control pcb from inside a servo, replace the pot with a trimpot, and solder this to a DC motor in situ on a device I want to control (consider the device would be hard to mechanically modify to accommodate a servo).

Servo Innards Block Diagram - Ground and Vin implied

My understanding of servo innards are shown here. vIn and Ground are implied only. The signal is compared with the signal from the potentiometer (motor position normally), and the generated error code is used for the PWM driving a H-bridge around the internal motor. What I am proposing is taking the internal motor out, and connecting an external one there.

The intent is that you'd use 1 I/O control pin for a bidirectional servo, and a speed controlled servo modified for continuous rotation. It already has FET's needed to handle motor current. Then taking the controlled motor outside of the servo casing or substituting it with a similar motor.

The motors in question are small toy motors - nothing large, or with major loads.

This sounds cheaper and easier to connect than a motor driver. Is this going to work?

Opening my first servo, a working supertec, the servo motor is soldered into the PCB. Desoldering it would be possible.

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  • \$\begingroup\$ I've found this forum thread here - forum.pololu.com/viewtopic.php?f=15&t=1785 - where the same thing is being proposed. \$\endgroup\$ – Danny Staple Jul 24 '12 at 13:20
  • \$\begingroup\$ This may make more for an instructible than a question. Opening my first servo, a working supertec, the servo motor is soldered into the PCB. Desoldering it would be possible - but I am going to put it back together, and try a Hitec servo instead. I am a bit reluctant to use the pricier Futaba servo's I have. \$\endgroup\$ – Danny Staple Jul 25 '12 at 14:52
  • \$\begingroup\$ The servo still works after reassembly. I may need to look inside a few to find ones that are suitable - which may make this far less useful than I thought. \$\endgroup\$ – Danny Staple Jul 25 '12 at 15:03
  • \$\begingroup\$ The board from a cheap PMDC hobby servo could likely drive another DC motor of similar specifications, but only in the same manner as it functions in a hobby servo or as a hobby servo can be modified to, and that may not fit your application need. You may also be able to get in between the servo error amp chip and the actual H-bridge driver which are often separate ICs. It's also worth noting that high performance hobby servos may use other motor types - but as those are pricey you'd be less likely to choose one to dissect. \$\endgroup\$ – Chris Stratton Jan 19 '15 at 15:11
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Servos usually use a simple protocol based on pulses. Servo boards are designed to generate these pattern(s). DC-Motors are usually driven using a H-bridge (wikipedia), which uses a (constant) controlled current flow. So technically it is possible, but not really practical to use a servo board to drive a DC motor.

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  • \$\begingroup\$ Hmm - not quite what I was expecting. When driving a speed controlled motor via a H-bridge, you would possibly use PWM to do so, being aware of which duty-cycle's a DC motor is not going to stall with. I've clarified my question as I think you think I meant a board external to the servo - I mean the board inside the servo. Perhaps a block diagram would help. \$\endgroup\$ – Danny Staple Jul 24 '12 at 9:24

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