Multitouch interface: Prototype schematics


After a long wait, at last I have received my materials to start working on the project and make the firs tests with the FTIR phenomenon.

I bought 100 LEDs (100, but I only need 50 for this). I have chosen Oshram SFH485, because are optimal for an FTIR configuration: It’s wavelength is 880nm, and their emission angle 40º. Wavelengths avobe that are harder to filter for our purpose, and the angle is optimal for my plexiglass thickness.


In the other hand, the acrylic (I had to wait a lot to obtain it). It’s thickness is 8mm and it’s measurese 76×54 cm. I could use a thinner one, but it’s so big, so I prefer this thickness to avoid bending when applying pressure over it. LED’s are 5mm, so a 5 – 6mm acrylic should be enough.

I got a couple of 90cm L-Shaped aluminium profiles too. That will act as a “rail” where to support the acrylic and assembly the LEDs. I have chosen L-shape because is comfortable to make a prototype, but obviously U-shape will be the chosen when the prorotype is finished.

The first thing I did was to think my LED scheme. I will use 25 LEDs with 2cm sepparation between them, on each long side of the rectable panel, filling the entire screen.

LEDs work at 100 mA with 1,5V voltage falloff. I’ll be using 12V power, so I’ll make 10 parallel series with 5 LEDs each, using 47ohm resistors, 1W each serie.Perhaps I will need less power resitors to allow LEDs to be brighter, but first I’ll try with this to avoid burning LEDs, because aren’t cheap.

Esquema LEDs

Note: Formula for resistors (following Ohm's law) is this:

R = (Power voltage – diode voltage falloff) / LED current

and then we choose the resistor with the next standard value near our R.

I just need to think how to make the complaint surface with silicone, and I don’t have a projector neither diffusor yet, but still I can start doing tests and start generating my first blobs.

This week I’ll begin, so soon I’ll be posting pictures explaining the issues that can rise during my experiments.