Intro: Pressure Sensitive Tip IR Pen
I am a school teacher and use a wiimote whiteboard on a daily basis in my classroom. I have always made my own IR pens using a simple lever switch and old highlighters but was always intrigued by the idea of a pressure tip pen that would more closely mimic actually writing. There are a few commercial options but all are at least $30-$40 + shipping and I wasn't willing to pay.
This ible shows my v.3 pressure tip pen that I absolutely love now and can't believe that I used the clicker pens before. All the parts can be purchased easily.
Step 1: Parts List
Highlighter with removable tip. - If you were creative you could probably get most highlighters to work but I found it easiest with the kind where the whole tip of the pen is removable.
Tact Switch - Come in a pack at Radioshack for around $4.
Epoxy - Simple 2 part, fast drying epoxy
Glue Gun - Low temp is sufficient
IR LED - I use TSAL6400 940nm LEDs. You can source these from Digikey, ebay, or lots of other websites
Wire - thin is good
Battery - AA or AAA
Optional Battery Holder
Step 2: Gather Everything
Break apart your pen and dispose of the ink and tip. You can also rinse out any residue from the pen and make sure everything is dry before continuing. If you can find a marker with a removable tip it will be easier.
Step 3: Tact Switch
As of the writing of this article, you can buy these tact switches from Radioshack either at their brick and mortar stores or online. Here is a link to the exact item with some specifications.
It is important that the switch is the high type because that is where you will attach the LED.
Step 4: Prepare the Switch
This is definitely the most painstaking part.
Start by using a dremel or other rotary tool to cut 2 small notches on either side of the plastic button part of the tact switch. I have tried to take a picture of this step so that you can see. Basically the legs of the LED will fit snug and help to hold everything in place.
Cut down the 2 legs of the LED remember which one is (+)anode and (-)cathode. You can also tell by the orientation of the internal parts of the LED if you forget. Or use your camera to view whether or not the IR LED lights when you provide a small current.
Solder the cathode to one post of the switch and then another negative wire coming from the adjacent switched leg. A multimeter might be handy in making sure you have the correct legs as these particular switches have 2 sets of legs.
Solder another wire to the anode. Make sure that your two long wires are long enough to pass through the body of the marker.
Use a tiny amount of epoxy to secure the led to the tact switch making sure not to get any glue on the switch so that it still moves freely.
Step 5: Chop Down the Pen Tip
Cut down as far as you can without taking off so much material that your LED falls through. It might be best to take a little at a time until you see exactly how much material you have.
Step 6: Widen the Pen Nozzle
Use 2 different drill bits to carefully widen your pen tip. The smaller of the two will pass all the way through while the larger drill bit will be sunk right up to the edge but not pass through. LEDs have a small ridge that will hold the LED from falling all the way through the tip.
The two sizes of drill bits that I used are:
However, the bigger of the two isn't as important as the small one which seems to fit perfectly for a 5mm LED. I used a normal cordless drill for this step while holding in my hand but it might be easier in a drill press.
Step 7: Create Access Holes
At this point you could probably do a lot of things to secure your switch into the pen body but I drilled a few holes and pushed in a little bit of hot glue. I was careful not to push in too much glue because I just wanted it to hold in place so that the button could still move.
After the glue sets up I filled in some epoxy behind just to make sure it never pushes out.
Step 8: Adding a Battery
You can use either a AA or AAA battery to power the LED. It is going to be underpowered so there is no need for a current limiting resister but you could always throw a 1-ohm resistor on if you are worried about that. I have used many of these LEDs for years and never had a single one fail. I have also never had to replace a battery although I would anticipate that the battery would need to be changed every year or so under really heavy use. Since the light only activates when you switch it and no current is used when the switch is closed it should last forever. Even using cheap batteries I have seriously never replaced a battery and I've made 15 or 20 pens for myself and colleagues.
Since I don't plan on replacing my batteries too often I just solder my leads to the battery. Please do this at your own risk and if you're not that experienced or fast at soldering I wouldn't recommend heating up your batteries too much. Alternatively, most pens should fit a AAA single battery holder and AAA battery.