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Control polarity supplied from external source to motor based on supplied polarity?

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rickz2012

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Hey all,

First off, hi to everyone - First time on this forum.

Ok, I am building a robot and I have a slight problem.

I have a LEGO NXT brick and on the brick there are motor ports. The ports allow for a motor to be connected and rotated cw or ccw. Obvioulsy the built in circuit allows for the polarity swap somehow.

I also have an external power source and a motor. What I want to do, is control the motor but the motor is connected to the external power source. This way I save energy being used up on my NXT brick.

How can I configure a setup so that when the polarity from the NXT motor port changes, the power supplied to the motor from the external source is of the same polarity? I also need to have an "off" state.

I have drawn up a little diagram for some visual aid.

Rich.

**broken link removed**
 
Hi.

Welcome here :)

[OT]
I'd say you're one lucky guy if you really do have an eternal battery avaiable. I'd also like to have one.

I'm not clear about the poblem. I haven't heard of a "motor port nxt brick". Is this a speed controller?
 
Your motor characteristics matter. So does your battery or power sources available.
What is the characteristics of the driving signal?
Also it would be nice to know if the motor is PWM modulated.

I could just answer "An H-bridge + some support circuitry" because its the best we can do with the available info.

That out of the way, look at this datasheet which is a popular H-bridge IC: https://www.st.com/stonline/books/pdf/docs/1773.pdf and look at the truth table

A simple way with lots of limitations would be to sense the motor direction with a couple of optocouplers and drive the bits accordingly. The first 2.1 volts of so wouldn't do anything.

It would be better, if you could find the control inputs and work from there. The simplest control is direction and (Not enable). More complex is (Brake, coast, fwd, rev). PWM can be applied to the (not enable input)
 
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