Dual power supplies on one board

I have a project i'm working on that uses power off of a serial port, but
there's enough current to only power the microprocessor. The problem is,
every once in awhile a piezo buzzer has to go off, and it ends up being too
quiet. I figured I could use a 9-volt battery to power the buzzer, but I
need a way to keep the two supplies seperated so that I don't discharge the
battery while the buzzer isn't going off, and avoid sending current back
into the serial line. What would be the best way to do this? Thanks,

-- Jeff

 

I need a way to keep the two supplies seperated

Not a problem.

Buffer.