Run Arduino on USB and at the same time BreadBoard on higher voltage? I am trying to design a bicycle LED lighting system, and for that I bought an Arduino yesterday (so very beginner with Arduino).

My final system will operate at 6V, so I plan to put everything on a breadbord and tune everything (blinking times, resistor values, etc.) and for that I want to keep my Arduino plugged to USB during development.

So my question is:

How should I connect USB-powered Arduino to a higher-voltage-powered breadbord (between 6 and 12V) safely?
I think as far as i understood the problem, the system or whatever you call is will run on what ever power as long that power is not fed to the arduino that will not be a problem, just connect the two grounds together and power the arduino through the computer and connect the pins to the system accordingly. i think this will solve thus the trivial problem.