Using BJT transistor as switch using arduino digital pin
Your circuit should look something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

A high output from the Arduino will turn on Q2 which will pull Q1's base low, turning it on as well.  Q1 should be selected to handle the current required by your device. You should also have a resistor between Q1's base and emitter to be sure it turns off when Q2 turns off.