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I want to see if it is possible to drive a Power Functions motor both with variable speed and forward/reverse with my own power source. I am planning to use my own power source, an IR receiver and a motor.

[Power Source] -> [IR Receiver] -> [Motor]

As I understand it, the pin out for Power Functions is as follows:

  • GND
  • C1 +/-9v
  • C2 +/-9v
  • +9v

Which of the above pins would I need to power to achieve this?

Do I need to provide a constant 9v to the IR receiver for it to function?

Can I rely on the IR receiver to vary the speed of the motor?

What would happen if the IR receiver was powered with less than 9v?

My power source may not always spin fast enough to develop 9v so I am wondering what would happen in this case.

P.S. I am returning to this hobby after 25 years so not entirely up to speed on how all of the new "stuff" fits together!

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Which of the above pins would I need to power to achieve this?

You would apply the (-) side of your power source to GND and the (+) side of your power source to +9v.

Do I need to provide a constant 9v to the IR receiver for it to function?

Generally, yes. However, lower voltage or suddenly losing power will not harm the part.

Can I rely on the IR receiver to vary the speed of the motor?

Yes. The IR receiver handles the speed regulation.

What would happen if the IR receiver was powered with less than 9v? My power source may not always spin fast enough to develop 9v so I am wondering what would happen in this case.

I haven't tested this, but I would expect it to lose range, provide less power to the motor, and at some voltage level under 9V completely stop functioning. This shouldn't cause any damage to the part.

It sounds like you are powering this from some type of generator. Make sure that you have protection in place to ensure that the IR reciever isn't power by significantly more than 9V.

Welcome back to the hobby! If you're looking to simply interact with PF motors at a low level, the control lines are simply attached directly to a 9V motor, so you can use those much like the old 9V motors. The basic motors aren't steppers or anything complicated.

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  • Thanks for the fast reply. From what you are saying the C1 & C2 are not used in this case. I guess the IR receiver pulses the received current (PWM) to drive the motor. I was wondering if it needed a constant 9v to do the pulsing. Seems not. So in theory I could provide a 6v input, and provide both variable speed and direction control to the motor with the setup I have described?
    – Remotec
    Commented Sep 28, 2016 at 12:58
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    The IR only needs power. It will generate the C1 & C2 signals. IF (and that's a big IF) the RF works on 6V, and 6V drives the motors, then yes, it would work. Also, please check out Philo's excellent documentation on the Power Functions protocol and characteristics.
    – Phil B.
    Commented Sep 28, 2016 at 13:43

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