Please refer to the following image:
I hope the quality is not too bad. Anyways, as can be seen, the circuit has its own external power supply from a wall wart (AC to DC transformer). The arduino is being used only to close the circuit by means of providing the signal current to the base.
My question is: is the only thing that needs to be done is to connect the Arduino output to transistor's base? Is there a need to somehow make use of the
GND pin in the Arduino (for example, by connecting the -Ve of wallwart along with the emitter to the GND)?
While I am at it, I have another question: how to find the +Ve and -Ve on the female DC jack?
As can be seen, there are 3 pins sticking out of the DC jack, (it is centre positive I think. I bought the jack because it makes it easy to connect the wall wart to the motor, i.e., without having to cut open the adapter's male jack).