Building Process

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(This is a quick setup of the building process, I will be editing it soon, to make it have more sense)

This is for the FTIR approach (see Multitouch Technologies )


Step 1: Acrylic

Polishing Acrylic Edges

Drilling:

Drilling holes for the LEDs can give you a cleaner-looking, more compact surface. The jury is still out on whether it improves performance. Unfortunately, it is difficult to avoid frosting the holes as you drill.

To get a clean hole, you need a sharp bit with the [url=http://www.hawkfish.org/snailman/acrdrill.htm] edge ground off. After you've drilled the holes, you have two options to get a clean view into the acrylic:

-MCBond - an acrylic solvent that will dissolves acrylic. The acrylic will reform around your LED, with a perfectly shaped, clear path for the light to travel in. MCBond will not dissolve the epoxy resin most LEDs are made of, but if you don't mind having removable LEDs, this is an excellent choice.

-Loctite Plastic Epoxy - This will give you a cleaner edge than doing nothing, but it will still leave a hint of frosting. It is the only substance found so far that will both hold the LEDs in place and provide a relatively clean, bubbleless bond.


      • Cementing LEDs to acrylic, or reforming acrylic to the LEDs, has the effect of disabling the lens of the LED. ***

LEDs are typically cast in epoxy. Because the index of refraction for epoxy is much higher than air, these castings typically function as lens as well as housings. Such differences of index of refraction are exactly how lens work. Thus, altering any index of refraction is to alter lensing properties. Removing the air between the LED and acrylic effectively removes the differences of index of refraction and thus effectively removes the lens of the LED housing and its viewing angle will be MUCH higher.

If you really really want to directly couple LEDs to the waveguide, and thus disable the lens of your LED to increase the radiation angle, you might want to consider cementing them with plain old clear RTV silicone glue (much less work and risk compared to drilling).

If you place LEDs in a polished hole and keep an air gap, the hole itself becomes a lens (whose shape is determined by your drill bit) and radiation pattern is thus altered for better or worse.




Step 2: Frame

   * prepare the frame by sawing or cutting the material (plastic/pvc/aluminium or wood)

make sure you have enough room for LED's but not too close to the edge either

   * mount everything (for example is you have 4 pieces of wood, use screws)

Step 3: Circuit Note: Reading this article first on how to wire LEDs may help you to understand this section more clearly. I highly recommend wiring in parallel. It is more reliable and gives you more freedom when wiring.

As the author mentions below, limit the number of LEDs per parallel circuit to a maximum of 10 (general number). I have found over 10 tends to heat up. If you are willing to take the time to build a test circuit you can monitor the heat of the resistor before installing them, which may help you avoid problems in the future. -O'Malley

Image:DSC08116.jpg You don’t need a series resistor for each IR led. You can put several LED’s in parallel. Putting 100’s of them in parallel is not a good solution either, since the current becomes very high for the series resistor (hot).

I used 2 strings of both 18 leds (=36 in total) Every string has 3 strings of 6 leds (=18 total) For every 6 leds i used 1 resistor, so for 18 leds i used 3 resistors, each 68 ohm

You'll probably work with different numbers since you might use less or more LEDs or less or more powerful ones.

Do yourself a favor, and build clusters of LEDs in series, like http://www.sport4minus.de/UdK/led_cluster.gif to calculate the needed resistor, go here: http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

If you're not sure how to set up your circuit, use http://ledcalculator.net or http://led.linear1.org/led.wiz to calculate resistors and layout. Please mind that the resistors may warm up a little bit, so it's better to leave some space around them for cooling.

You can as well build parallel LED clusters, which could be even better, in case one LED is destroyed, the others will still work, which is not given when you put them in series

   * Mount your LED's to the edge of the acrylic to hold it in place. 

Notes:

People used UV glue as well to mount the LEDs to the acrylic. But that way the setup is petty fragile and harder to tear down and to test with it. If a LED is broken it's harder to change.

Wider angle led's are better.

Measure and cut the wire to connect the LEDs together into a single loop.

Leave one corner open for the AC adapter.

   * Attach leds/resistors/wires 
   * attach wires to molex connector of old computer supply or to a 12 volt adapter
   * Solder together all the connections in the loop.

A very fast and simple way of connecting the LEDs and holding them in place is by using a perfboard. This is a very common and quite cheap prototyping circuit board that you can find in every shops of electronic components. You can cut the perfboard in stripes that will fit in your frame for example with a dremel: Image:Perfboard-cutting.jpg and then solder the LEDs on the copper side of the board, this will allow you to place the board stripes directly in the alluminium frame if you're using alluminium or just to hold the LEDs in place and make a clean cabling: Image:Perfboard-led.jpg

Step 4: Hook up and test Power up your LED array, if all is ok you should see nothing with your eyes but the LEDs will be clearly emitting light when you look at them with the IR cam: Image:LED-first-test.jpg