user20381: 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?

user29488: 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. 

20381: <p>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.</p>

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

<p>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.  </p>

<p>What next?</p>
 29488: <p>You can use an NPN transistor for this purpose.</p>

<p>All you have to do is connect it this way<img src="https://i.stack.imgur.com/kL334.jpg" alt="enter image description here"></p>

<p>Obviously use a buzzer instead of the motor and pin 9 instead of 4 :D</p>

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

<p>Buzzer +ve ==> + 9V</p>

<p>Buzzer -ve ==> Transistor collector</p>

<p>Transistor Emitter ==> battery -ve ==> Arduino GND.</p>

<p>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. </p>
