I wanted to have data send to my computer but also power the teensy externally. I already solved it by following @nominals first reply and getting an isolator + soldering on two schottky diodes. But I still did not fully understand when a solution like this is necessary and when it is not. Now, I have more of an understanding of why and when I need to take into consideration the different grounding potentials of multiple power supplies which will help me with projects in the future.@Nominal Animal I think you are complicating/confusing what @live424 needs/wants.
I am sure that he is new to Teensy and electronics and has become confused by all the complexities.
If he can come back to us answering Pauls question in #18 I am sure that we can help him, but it does not help when we bamboozle him with technicalities which are unnecessary and over his head.
Sorry @live424 is I am putting you down I don't mean to. We have all been where you are at the moment!