Intro: Making an LED Touch Pen and UV-responsive Writing Surface
Create a touch-switch sensitive LED light pen! This instructable uses parts most hobbyists probably have lying around the house to make a pen that emits light from an LED. I used an ultraviolet LED to "ghost-write" with; you can coat a white surface with glow-in-the-dark paint and the pen leaves a temporary glowing line when you write with it.
The effect achieved by the Rustoleum GLOW paint isn't the best; if you're going to attempt this instructable, it'd be worth it to investigate the super bright glow in the dark coatings available now from various online suppliers.
UPDATE: I've got a sample with two coats of Glow Inc's brightest done, it looks great! The writing persists for minutes and is bright even in subdued natural light... The picture below shows me writing on my business-card sized sample piece. This paint is much more expensive than the latex based formulation I was using earlier, and will take a few more coats to get nice and even, but already it looks great!
Step 1: Materials
You'll need the following materials:
3 cell battery holder (I used a 4xAA battery holder and soldered to the 3rd battery's terminal to emulate)
A pen you don't mind destroying. It should be fairly wide and be easy to disassemble, including removing the ink well and nib.
1 UV LED (You can use any color, though, for light writing with a camera - adjust your resistor accordingly)
1 LED load resistor - I used a 150 ohm resistor, which should be good for any LED. Technically, I could've used a resistor as low as 80 ohm for the UV LED I was using (use ohm's law to find out the brightest resistor value possible)
2 NPN transistors (any rating)
1 touch switch resistor - 100K (this is also somewhat forgiving)
2+ conductor wire (I'm using 3-conductor shielded but it's way overkill for this)
1 notebook or other modifiable paintable source (I taped white inkjet paper inside a leather binder)
Glow in the dark paint - I used Rustoleum GLOW which is adequate to demonstrate the effect, but you'll achieve much better results with a different formulation
UPDATE: I've ordered glow in the dark paint online from Glow Inc. I'm currently producing test pieces but it looks like it'll take a few coats - I ordered 2x 1/2 fl oz of their brightest paint... the glow in the dark effect is pretty awesome, and the container comes in a nail polish bottle with applicator. I tested it with the pen and was able to leave long-glowing spots with the fluid in the bottle. I'll update with pictures when I've completed testing.
In addition you'll need the following tools:
Hot Glue Gun
Small Paint Roller w/ Tray
Step 2: Construct and Paint Notebook
I cut to size two sheets of inkjet paper and taped them to the inside of my binder.
In the pictures you can see before, (immediately) before, and after shots of the first coat. It took 5 coats of GLOW to get the so-so results seen on the first page. See my notes in the intro about getting better quality glow in the dark paint. I waited a minimum of 30 minutes and a maximum of 24 hours between coats - when I painted it only 30 minutes after a coat, the roller tended to push the wet paint around a little, so I decided to give it a little longer.
You can see the paper tends to swell up and wrinkle once wetted with the paint - apparently poster board and other kinds of treated paper will hold up better without absorbing paint, and will increase the quality of the glow, if you use it.
Step 3: Schematic
Here's my sloppy hand-drawn circuit diagram.
It's pretty simple: Power flows in through a resistor and the LED to the collectors on two transistors.
They're wired in a darlington array. You can actually get those as SMD ICs
The "output" transistor, whose emitter sinks to ground, has it's based tied directly to the emitter of the "sensing" transistor. Because this is a low-impedence path, a very slight current is required to push the output transistor's base to fully on.
The sensing portion of the circuit includes a 100K protection / callibration resistor. Too small of a value and it will oscillate from the capacitance in the sensor section, too high and it won't push the output transistor into saturation.
The other leg of the touch circuit is simply tied to the positive rail.
Step 4: Construct Darlington Array Touch Switch
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.
Step 5: Mod Your Pen for the Touch Sensor
Take a paperclip, straighten it out, and build long rails to go along the outside of your pen. Solder the ends to the positive wire and the detector wire. You should also go ahead and solder the ground wire and the positive wires to the power cable at the same time.
I just soldered the other end of the cable, one end to the outside of the battery holder, the other to the same-marked lead in the empty last battery box (if it's marked +, it's connected to the next battery's - and vice-versa)
when you're done, take it all out, connect the batteries, and test the touch circuit before you start gluing stuff!
When you're satisfied that it works, install it back into the pen body and hot glue the connectors in place. Also apply a generous dab of hot glue anywhere the wires might be flexed outside the pen body to reduce tension on the delicate solder joins.
Note: if you had a large enough pen, you could use 3 button cells actually inside the pen body and eliminate the external box entirely. If you do this, the ground wire doesn't even need to be threaded outside, and you can sort of mount your makeshift battery holder above the transistors.