Gather around children, the multitouch adventures continue. Today we will hear the story of The Acrylic Screen and 112 Little LEDs.

I decided to go for a 4:3 format for my screen and bought a 10 x 820 x 620 mm acrylic screen from Foiltek. Acrylic that size cost me around 60 euros. Couldn’t find any place cheaper here in Finland. The edges of the acrylic weren’t clear so I sanded them with sandpaper. I started with 180 grit paper and worked my way up to 1000 grit paper. Then I finished the edges with some metal polish. They should be clear enough now. Here’s my screen.

In order for the FTIR tracking to work, you need a lot of LEDs that shine the IR light inside the acrylic screen. So basically you need too make LED strips that will be placed on the edges of the acrylic. Here’s a handy calculator for helping you design your LED array.

I decided to save a lot of time, by ordering the circuit boards for the LEDs from Each of the boards will hold 2 x 7 LEDs and 2 x 15 ohm resistors I got a bunch of the boards with resistors already mounted, but not the LEDs. I ordered my LEDs from eBay. 120 for $48.

I’m using 4 of these boards on each of the longer sides of the acrylic. That’s 112 LEDs in total. This might be a little overkill, but. We’ll see once I get a power source for the strips and I can test it properly.

Next time: How to hack a webcam to see only IR light.