I am building a breadboard circuit to be controlled by Arduino. The circuit consists of about 20 bright leds simulating a vehicular lighting system (ten leds for headlight, 4 in taillight, 3 for each directional light, or something like that).
Since these leds would draw up to 500mA @ 6V, I would like to power the breadboard from a wall wart.
But, at the same time, I would like to run my Arduino from USB power, so that I can keep updating my logic until everything works fine.
It is important to note that I plan to drive the LEDs with transistors, their bases connected (via resistors) to Arduino output pins.
So my question is: how should I ground and connect both circuits so that 6V (or more if I change my mind) from the breadboard source does not "reflow" to the Arduino while it is being powered by USB?
It is possible, isn't it? Should I take any special care?