Multi touch interface (part 2): Prototype scheme

At last, I received the first components to build the FTIR interface an do the first tests.

I have 100 LED (I bought 100, but I will use only 50, but just in case). I choosed the Oshram SFH485, because them have the best results with FTIR configurations: A wavelengh of 880nm and an emission angle of 40º. With superior waveleghts it’s harder to filter the light for FTIR, and the 40º angle it’s perfect for my plexiglass.

 In the other hand, I received at last the acrylic (I was waiting too long for it). It is 8mm thickness and is 76×56cm (panoramic view). I could use thiner one, but due it dimensions, I prefered to don’t risk and ensure it will not bend with the pressure strokes. In any case, the LEDs aer 5mm thick, so an acrylic of 5-6mm will be fine.

I ordered a couple of aluminium profiles in L shape, 90cm each. I will use them as “rails” to put the acrylic in and to mount the LED arrays. I choosed the L shape for the prototype, but of course when I reach an stable prototype I will change them for a good aluminium frame. 

The first thing to do at this point was to decide the LED scheme for the project. I will use 25 LED, with 2cm between them on each side of the acrylic, to illuminate the entire panel.

The LEDs operate at 100mA, with a forward voltage of 1.5V  I will use a source of 12V, so I will do a 10×5 array, with 47ohm, 1W resistor on each row. Perhaps I need a less power resistor to get the LEDs brighter, but I will try first with that, to don’t risk burning the LEDs (they are not cheap).

Note: The resistor formule (following the Ohm’s Law) is that:

R = (Source Voltage - Forward Voltage) / LED intensity

 And then we choose the nearest standard value wich is greater.

I still have to decide how to make the silicone complaint surface, I don’t have neither projector neither diffuser yet, but I have enough to start testing and generate the first blobs.

This week I will start this, so I will post some pics and explain the obstacles I could encounter in the process.