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+ <head>
+ <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+ <title>Pong Hero</title>
+ <link href="style.css" rel="stylesheet" type="text/css" />
+ </head>
+ <body>
+<center>
+ <object type="image/svg+xml" data="../logo.svg" NAME="pong-hero-logo.svg" width="646" height="162"><img src="../logo.png" title="Nintendo Wiimote PLUS Linux bluetooth PLUS Infra-red Torches EQUALS Pong Hero!">
+ </object>
+</center>
+<hr>
+ <h1>Pong Hero: Making IR Pens</h1>
+
+ <table>
+ <tr>
+ <td> <p>
+ 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.
+ </p>
+ </td>
+ <td>
+ <img src="complete.jpg" align=right>
+ </td>
+ </tr>
+ </table>
+
+ <h2>Background</h2>
+ <p>
+ 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 <em>IR light shining on
+ the wall</em> rather than the LEDs themselves.
+ </p>
+
+ <p>
+ 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!).
+ </p>
+
+ <p>
+ The other simplification is to use <em>rechargable</em> 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.
+ </p>
+
+ <h2>Ingredients</h2>
+ <ol>
+ <li> 3 Infrared LEDs (get spares, I killed some),
+ <li> Lip balm container (my study smells of strawberries now)
+ <li> A rechargable AAA battery (and a charger, of course!)
+ <li> A pen with a spring in it (free promotional click pens are good)
+ <li> 20cm of hookup wire
+ <li> A digital camera (for easier debugging, as it can see IR)
+ <li> Long-nose pliers (to twist the LED legs together)
+ <li> Snips (to cut the LED legs, shaft of container, wire, spring)
+ <li> Soldering iron and solder
+ <li> Glue (to secure spring to bottom of container, maybe LEDs into top)
+ </ol>
+
+ <p> You should be able to make two devices for under $20, assuming
+ you have tools. </p>
+
+ <h2>Steps</h2>
+
+ <h3> Preparing the tube </h3>
+ <p>
+ 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.
+ </p>
+
+ <hr>
+ <table>
+ <tr>
+ <td>
+ <h3> The bottom part: the spring </h3>
+ <p>
+ 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.
+ </p>
+
+ <p>
+ Glue the spring into the truncated chap, and you should get
+ something like this:
+ </p>
+ </td>
+ <td>
+ <img src="bottom.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <table>
+ <tr>
+ <td>
+ <h3>The top part: the holes</h3>
+ <p>
+ Bore three holes through the top of the lip balm container; use
+ one of the LEDs to check the sizing.
+ </p>
+ </td>
+ <td>
+ <img src="top-holes.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <hr>
+
+ <h3>The LEDs</h3>
+ <table>
+ <tr>
+ <td>
+ <p>
+ 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.
+ </p>
+ </td>
+ <td>
+ <img src="top-with-leds.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <table>
+ <tr>
+ <td>
+ <p>
+ 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!).
+ </p>
+ </td>
+ <td>
+ <img src="LED-diag.png" title="LED diagram: long leg is positive, flat side is negative">
+ </td>
+ </tr>
+ </table>
+
+ <table>
+ <tr>
+ <td>
+ <p>
+ 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.
+ </p>
+ </td>
+ <td>
+ <img src="leds.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <table>
+ <tr>
+ <td>
+ <p>
+ Finally, feed the wire from the bottom spring <em>through the
+ tube</em> and solder it onto the outer LEDs.
+ </p>
+ </td>
+ </tr>
+ <tr>
+ <td>
+ <img src="full-pen.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <hr>
+
+ <h3>Finishing and Debugging</h3>
+
+ <table>
+ <tr>
+ <td>
+ <p>
+ 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.
+ </p>
+ </td>
+ <td>
+ <img src="with-battery.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <table>
+ <tr>
+ <td>
+ <p>
+ 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.
+ </p>
+ </td>
+ <td>
+ <img src="top-lit.jpg">
+ </td>
+ </tr>
+ </table>
+
+ <p>
+ 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!
+ </p>
+ <hr>
+ Rusty Russell: rusty@rustcorp.com.au.
+</body>
+</html>
projects / ponghero.git / blobdiff