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14 <h1>Pong Hero: Making IR Pens</h1>
19 After some experimentation, I settled on a nice form factor for
20 Pong Hero, using lip balm (Chap Stick) containers. These have
21 three 5mm IR LEDs and a rechargable AAA battery contained neatly
26 <img src="complete.jpg" align=right>
33 The Wiimote contains an Infra Red camera, so it can track IR
34 lights. IR LEDs are around 75c each but they're very directional,
35 so in practice the Wiimote is seeing the <em>IR light shining on
36 the wall</em> rather than the LEDs themselves.
40 This discovery allows us to get away without a switch on the IR
41 pen: just take it more than about 5 cm from the wall and it's too
42 dim to be seen. For a more general whiteboard, a momentary push
43 switch near the end would be great though (fitting it on the
44 container is hard, tips welcome!).
48 The other simplification is to use <em>rechargable</em> AAA
49 batteries. These are 1.2 volts, rather than 1.5 volts, so we
50 don't need a resistor. Unfortunately, they drop voltage faster
51 than decent batteries, so this could also explain why I found the
52 need to have three LEDs: sneaking a resistor into the circuit and
53 using 1.5V AAAs might let you get away with a single LED.
58 <li> 3 Infrared LEDs (get spares, I killed some),
59 <li> Lip balm container (my study smells of strawberries now)
60 <li> A rechargable AAA battery (and a charger, of course!)
61 <li> A pen with a spring in it (free promotional click pens are good)
62 <li> 20cm of hookup wire
63 <li> A digital camera (for easier debugging, as it can see IR)
64 <li> Long-nose pliers (to twist the LED legs together)
65 <li> Snips (to cut the LED legs, shaft of container, wire, spring)
66 <li> Soldering iron and solder
67 <li> Glue (to secure spring to bottom of container, maybe LEDs into top)
70 <p> You should be able to make two devices for under $20, assuming
75 <h3> Preparing the tube </h3>
77 Wind the lip balm all the way out, applying to your lips if
78 they're dry (this is really your last chance to do this, and it
79 helps you feel you got your $3 worth!). Remove it, and then pop
80 off the base cap of the lip balm.
87 <h3> The bottom part: the spring </h3>
89 Use the snips to clip off all but the bottom few millimeters of
90 the central shaft which went through the balm. Remove the spring
91 from the pen and cut it in half. Strip one end of the hookup wire
92 and wrap it around the bottom of the spring: solder it in place.
96 Glue the spring into the truncated chap, and you should get
101 <img src="bottom.jpg">
109 <h3>The top part: the holes</h3>
111 Bore three holes through the top of the lip balm container; use
112 one of the LEDs to check the sizing.
116 <img src="top-holes.jpg">
128 This is probably the trickiest part. I put the three LEDs into
129 the top with the flat side of each one facing out: this puts the
130 long legs near each other on the inside.
134 <img src="top-with-leds.jpg">
143 Twist the inner legs into one big leg. Be careful that as you're
144 twisting them they don't pull out of the holes. Trim the
145 twisted-together legs and solder them together. This will touch
146 the positive contact of the battery: you can place the other half
147 of the pen-spring around this if it makes it easier to get decent
148 contact (and doesn't short it out!).
152 <img src="LED-diag.png" title="LED diagram: long leg is positive, flat side is negative">
161 For the outer legs, I connected them together with two pieces of
162 hookup wire, then snipped the legs right down to stay out of the
176 Finally, feed the wire from the bottom spring <em>through the
177 tube</em> and solder it onto the outer LEDs.
183 <img src="full-pen.jpg">
190 <h3>Finishing and Debugging</h3>
196 Push (or glue) the bottom back onto the main tube, then push a
197 battery into the tube. The flat (negative) end goes in first, and
198 the knobby (positive) end goes at the top. You should then be
199 able to ease the cap on, making sure that the top of the battery
200 presses against the central twisted-together legs of the LEDs.
204 <img src="with-battery.jpg">
213 The easiest way to test is to look through a CCD camera: cell
214 phones tend to work well since they have cheap cameras which don't
215 filter out IR very well. Note that you'll only see a light from
216 straight ahead, as the LEDs are very directional. Also, the
217 battery should be fully charged: it gets quite dim quickly.
221 <img src="top-lit.jpg">
227 That's it: you have a pen! I found that taking the lid off
228 between games to try to save battery power had the opposite
229 effect: I suspect that momentary short-circuits as the top was
230 pulled off drained the battery quite quickly. Some glue and
231 insulation might help here, but I was just happy that it worked at
235 Rusty Russell: rusty@rustcorp.com.au.