Adding external power supply to any arduino connected component I have an Arduino Pro Mini 3.3v and the voltage is too small for my buzzer to ring loudly. I want to add a 9v battery to power the buzzer.

This is my current circuit:
Arduino (pin 9) => Buzzer +ve
Arduino Gnd => Buzzer -ve

I want to power the buzzer using an external 9v battery but I have no idea how to implement it in the circuit. All I know is that the ground should be common.  

What next?
You can use an NPN transistor for this purpose.

All you have to do is connect it this way

Obviously use a buzzer instead of the motor and pin 9 instead of 4 :D

Arduino (pin 9)==>1k-10k Resistance==>Transistor base

Buzzer +ve ==> + 9V

Buzzer -ve ==> Transistor collector

Transistor Emitter ==> battery -ve ==> Arduino GND.

So now whenever you set the pin 9 to HIGH the transistor would allow current to pass through it from collector to emitter and your buzzer would work just as you wanted.