Why can't I use 3V output of arduino pro mini?
NEVER drive a motor directly from an Arduino IO pin.  You will kill the Arduino.

It's not a question of voltage, it's a question of current, and that is where the transistor comes in.  If you have a 3.3V Arduino (as it sounds like you have) then you may be able to drive the motor using the 3.3V output of the Arduino (VCC pin? Not sure with that model) and a transistor. That is the simplest arrangement.







simulate this circuit &ndash; Schematic created using CircuitLab