Run 5V/2.3W water pump on Arduino Pro Mini 5V
Hi Florian you can take power from USB fro both Arduino and the water pump.This is the schematic you have asked for your project. An arduino Pin can give maximum 20mA current that's why we need a limit resister to limit the current below 20mA and also this current should be such a way that it can drive the switching transistor. So the challenging thing is choosing the correct resister
I have taken the value 1K 
So the current at Arduino Digital pin is Current I = 5V/1k = 5mA which is below of 20mA and sufficient to drive the Transistor. you can use BC548 transistor also.







simulate this circuit &ndash; Schematic created using CircuitLab