Step 1: A Little Insight....
The Darlington transistor, or more commonly, the Darlington pair, consists of two separate bipolar transistors sharing a common collector, and when combined, offer an exponentially higher gain than both transistors separate.
The idea was created by engineer Sidney Darlington of Bell Labs in 1953. He also patented the idea of having 2 or 3 transistors on a single chip,sharing a collector pin.
In a way, the pair could be seen as operating in 2 ways. The 1st, and most commonly accepted, is where both transistors act like amplifiers, the 2nd stage further strengthening the output of the 1st stage. I have a slightly different view of it, where the 1st stage acts as an amplifier, beefing up the input, so that it saturates the base of the 2nd stage, turning it on, like a switch.
With all that being said.... LETS BUILD SOMETHING!!!!!!!!!
Step 2: What You'll Need
In this rendering of the circuit,there are only 3 actual components, or 4 if you decide to run 9 volts instead of 5.
All parts can be had from Radio-shack, or any hobby shop for around 3-5 bucks, assuming you already have a breadboard.
2 NPN transistors (i used MPS2222's , but any NPNs should work well)
Short jumpers, preferably some small solid core wire you could cut to fit
LED capable of handling 5 volts, 5 mm whites work well
560 ohm resistor (only if using 9 volts, value not critical)
5 or 9 volt supply, your choice, i just had a USB supply handy.
A pair of old nail clippers works well for the cutting and stripping of wires as well.
Step 3: Placeing the Transistors
Step 4: Adding the V+ and Ground Jumpers
Step 5: Insert the Collector-collector & Emitter-base Jumper
This step involves jumpering the emitter of the right transistor to the base of the left transistor, and jumpering both collectors together.
Step 6: Installing the LED and Touch Wire
The next step is installing the LED and touch wire. KEEP IN MIND the led is polarized, meaning it will only light when it has -voltage to - and +voltage to +. I will attach a drawing of how to tell the difference,just in case you don't know.
As you will see,the touch sensitive wire attaches to the base of the right transistor.