Interference from motor on analog input pin?

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Dejay

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Not sure if this is the right place to ask. I've done this tutorial with the Teensy 3.1 from a starter kit using the L293D chip with a DC motor and a potentiometer I read on an analog input pin. It's all powered through the USB port (Vin pin of the teensy).

It works but I noticed that the range of the potentiometer changes. If I turn the potentiometer way down the motor is supposed to turn full speed backwards, but the potentiometer only reads about 10% instead of 0% (or 1%). It seems either the motor or the L293D H bridge is interfering with the analog pin. It seems rather obvious really that something like a motor could do this (maybe 5V voltage drops a little? Or electromagnetic interference through induction or something?).

But I'm an electronics noob. How would I go about to avoid such interference? Thanks.
 
Dejay said:
It seems either the motor or the L293D H bridge is interfering with the analog pin. It seems rather obvious really that something like a motor could do this (maybe 5V voltage drops a little? Or electromagnetic interference through induction or something?).
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Most likely, the 0V voltage rises a little due to current from the motor and non-zero wiring resistance. This is why high-current devices like motors, bid LEDs, etc typically have their own power supply. Also, a motor can become a generator - for example if it has been spinning fast, the mechanical inertia keeps it spinning for a while, and now it is making electricity rather than consuming it.
 
Thank you! I'll try first with a separate power source.

But I still connect both ground leads of both power sources to the same ground, yes? (Because there is only one ground connector for the L293D)
 
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