Transistor switch "Hello World"

According to the Netduino Spec, the Netduino has a maximum current output of 8mA, which is not enough to power very much at all, so we need a way to control circuits with more power in them via our Netduino without that power going through it.  Transistors are the answer to this, searching the web for "Transistor" gives you any number of explanations of now they work.  I'm not going to go into the details but, in very basic terms, the resistance of a transistor can be changes from high to low by varying a third current.

There are two types of transistor, NPN and PNP.  Again, searching will describe these to you better than I can, however, for this I'm using an NPN (I discovered after allot of wasted time that a PNP doesn't work here!).

We are going to make a circuit which will flash an LED on and off, the LED is powered by a 9V battery and the current from this battery is controlled from a digital pin on the Netduino.

The circuit diagram is this:

Transistor Circuit

The transistor is Q1, the three pins are called the collector, emitter and base.  In our diagram the collector is the top pin (connected to 9V), the base the middle pin (connected to the Netduino pin) and the emitter, which is the bottom pin.

The resistor R2 reduces the power so we don't blow up the LED, SW1 is the Netduino pin, switching between ground and 5V.

The operation is very simple, when SW1 is connected to ground, the resistance of Q1 is high enough for almost no current to flow, so the LED of off.  When SW1 is connected to 5V, the resistance in Q1 plummets and allows enough current to flow to power the LED.

NB: A PNP transistor has a different symbol and doesn't work in this circuit, the resistance doesn't get high enough to turn off the LED.

Also, the resistor R1 is in place for two reasons, firstly, it only takes a small current lower the transistor's resistance as much as it will go.  This is called saturating the resistor.  Secondly, when the transistor changed from low to high or back a pulse of the full voltage and current might be briefly exposed to the Netduino and may damage it, the high resistor stops this pulse from being at a dangerous level.