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Making an LED touch pen and UV-responsive writing surface

Step 4: Construct Darlington Array touch switch

Picture of Construct Darlington Array touch switch
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I freeformed the transistors to fit within the pen body. I have a series of closeups of that process but here's the basic thought:

take the two transistors bottom up so their collectors, bases, and emitters line up. Slide one down so you can line them up horizontally and bend it's collector over to make contact. You'll need to bend it with needle-nose pliers into a sort of step shape. This is where you'll connect one end of the LED so leave some space when you clip it.

Take the emitter of the "bottom" transistor (I ended up putting my transistors in with their leads facing the back of the pen) and bend it carefully to the base (middle pin) of the second transistor. Solder these together and trim the excess - that connection point is done.

Now, thread through the pen cap three lengths of wire. One (positive) will need to be long enough to reach the LED, which will end up mounted at the tip below the switch, the other two will be connected to the switch set. My pen cap had convenient lanyard loops I enlarged slightly for this procedure - you might need to do some drilling or some such. Keep in mind that if your pen has an inner curve like mine, you'll need longish wires for the LED to fit far below the switch.

Take the LED and solder the resistor to the positive (non-flatted) lead. measure it to stretch a little longer than the length of the pen to the cap, it'll twist some as you install if you have a screw cap like I did. Trim it to fit as snugly as you can. Heat shrink the connection all the way to the LED, then slide another piece of heat shrink over and leave it. Solder the positive lead to this, then slide the unshrunk heat shrink over and shrink it down.

Note: at first the resistor on my LED was wrong (I had it prewired for 12V) - then my color coding on the LED was wrong, I had to take it out and flip it around at the end, so just check your own LED as you go and don't pay too much attention to the particulars of the pictures. It doesn't really matter which side you put the resistor on - just make sure the positive from the battery goes into the curved side of the LED and the flatted side as follows:

Bend the remaining collector lead (connected to both transisotrs) down and towards the LED's other lead. Do a little wire origami to try to get all the pieces to line up along the pen's center; the joints will flex somewhat as you install, but try to get it as good as you can. Heat shrink the base of the LED's lead, leaving just a little to solder onto, then slide another piece of heat shrink on top, like before. Trim, then solder, then heatshrink.

Almost there - measure the whole length against the pen for the ground wire from the cap, which will be connected to the remaining emitter. Slide some heatshrink far up the wire, solder it to the emitter, then trim the emitter and heatshrink the connection. For the last wire, solder a 100K resistor to the remaining unconnected base. This will form the detector part of the touch circuit outside the pen. I used the blue wire for this and the yellow for the positive wire.

When you're done install it in the pen.
 
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