Hi!
Ive joined this community a couple of weeks ago. A few days ago ive constructed my own design of pressure pen and works GREAT. I see that not many people have one of this, and the ones that have one, are quite complex to build.
My design is VERY simple and i want to share it with every one.
BUT FIRST! I want to warn this kind of pen is not suitable for everyone. Because the leds is touching and slideing through your whiteboard the surface must be VERY smooth, otherwise the feeling would not be pleasent, and the led will get scratched. If you are writing on a wall or blackboard there are other suitable pens for youYou need:
1x Pilot Super Color Marker
1x IR LED (Vishay TSAL6400 or TSAL5300 for best performance)
1x Momentary switch button
2x AAA batteries
1x 12ohm resistor 1/4 watt
For the conections, instead of normal wire, i use insulated copper wire 0.2mm diameter (about 32Gauge)
The circuit cant be more simple:

Now, to assamble the pen:
First unscrew the tip of the marker and take out the cotton filling with the ink. You should keep only the tip, the tube and the cap.
Pour some alcohol inside the tube and shake it to clean the ink from inside.
Before putting the batteries and resistor inside the marker tube, insulate all the connections. The tube is aluminun and can create a short circuit between conections. I use hot glue for this.
Now, to make the presure LED first you have to drill the tip so the led can go through with no effort. Cut the legs length of the led to 3 or 4mm. Now glue the bottom of the led to the top of the button.
Make all the solder needed and then insert the led with the button inside the tip of the marker, be sure its correcltly placed and glue the button there. If everything went right the button is securely attached to the plastic tip, but the led is free to be pushed, pressing the button, and making the led to "glow". Now just screw the tip with the tube and you are done.
Below its an image so you can see the marker, button, wire and led I used.

I hope someone likes the design and make his own!

Any sugestion, question, or anything leave it here. Ill answer as soon as posible.
Bye!
PS: Here is a video of my whiteboard setup and the IR tip pressure pen in action
[youtube]http://www.youtube.com/watch?v=WEyDoEeVfc0[/youtube]