A transistor switch allows you to use a smaller flow of electrons to control a much larger flow of electrons. In this case, we are using the Netduino’s 3.3V output to control turning on and off a 9v supply.
It took me a while to figure out how this should be wired up, but after a lot of looking around I found some arduino examples. This one in particular was very helpful
My take away is that you should have your circuit set up like this. Effectively your transistor is blocking the current from going to ground until some input to the base is given from the Netduino.
Some notes on the switch: There is a formula for figuring out the value of the resistor going to base is, however it is outside the scope of this tutorial. I’ve just used 1k and it works fine. Also be sure to connect both the ground from your power, and the ground from your Netduino. For the longest time I didn’t have my Netduino grounded like in the images above and could not figure out why it was not working!