Pong Hero: Making IR Pens

After some experimentation, I settled on a nice form factor for Pong Hero, using lip balm (Chap Stick) containers. These have three 5mm IR LEDs and a rechargable AAA battery contained neatly inside them.

Background

The Wiimote contains an Infra Red camera, so it can track IR lights. IR LEDs are around 75c each but they're very directional, so in practice the Wiimote is seeing the IR light shining on the wall rather than the LEDs themselves.

This discovery allows us to get away without a switch on the IR pen: just take it more than about 5 cm from the wall and it's too dim to be seen. For a more general whiteboard, a momentary push switch near the end would be great though (fitting it on the container is hard, tips welcome!).

The other simplification is to use rechargable AAA batteries. These are 1.2 volts, rather than 1.5 volts, so we don't need a resistor. Unfortunately, they drop voltage faster than decent batteries, so this could also explain why I found the need to have three LEDs: sneaking a resistor into the circuit and using 1.5V AAAs might let you get away with a single LED.

Ingredients

1. 3 Infrared LEDs (get spares, I killed some),
2. Lip balm container (my study smells of strawberries now)
3. A rechargable AAA battery (and a charger, of course!)
4. A pen with a spring in it (free promotional click pens are good)
5. 20cm of hookup wire
6. A digital camera (for easier debugging, as it can see IR)
7. Long-nose pliers (to twist the LED legs together)
8. Snips (to cut the LED legs, shaft of container, wire, spring)
9. Soldering iron and solder
10. Glue (to secure spring to bottom of container, maybe LEDs into top)

You should be able to make two devices for under $20, assuming you have tools.

Steps

Preparing the tube

Wind the lip balm all the way out, applying to your lips if they're dry (this is really your last chance to do this, and it helps you feel you got your $3 worth!). Remove it, and then pop off the base cap of the lip balm.

The bottom part: the spring Use the snips to clip off all but the bottom few millimeters of the central shaft which went through the balm. Remove the spring from the pen and cut it in half. Strip one end of the hookup wire and wrap it around the bottom of the spring: solder it in place. Glue the spring into the truncated chap, and you should get something like this:

The top part: the holes Bore three holes through the top of the lip balm container; use one of the LEDs to check the sizing.

The LEDs

This is probably the trickiest part. I put the three LEDs into the top with the flat side of each one facing out: this puts the long legs near each other on the inside.

Twist the inner legs into one big leg. Be careful that as you're twisting them they don't pull out of the holes. Trim the twisted-together legs and solder them together. This will touch the positive contact of the battery: you can place the other half of the pen-spring around this if it makes it easier to get decent contact (and doesn't short it out!).

For the outer legs, I connected them together with two pieces of hookup wire, then snipped the legs right down to stay out of the way.

Finally, feed the wire from the bottom spring through the tube and solder it onto the outer LEDs.

Finishing and Debugging

Push (or glue) the bottom back onto the main tube, then push a battery into the tube. The flat (negative) end goes in first, and the knobby (positive) end goes at the top. You should then be able to ease the cap on, making sure that the top of the battery presses against the central twisted-together legs of the LEDs.

The easiest way to test is to look through a CCD camera: cell phones tend to work well since they have cheap cameras which don't filter out IR very well. Note that you'll only see a light from straight ahead, as the LEDs are very directional. Also, the battery should be fully charged: it gets quite dim quickly.

That's it: you have a pen! I found that taking the lid off between games to try to save battery power had the opposite effect: I suspect that momentary short-circuits as the top was pulled off drained the battery quite quickly. Some glue and insulation might help here, but I was just happy that it worked at all!