All applications are working properly. The tabletop knows perfectly where touches are made and sends a signal back to the correct place. Tomorrow, the tabletop will be tested for the first time in practice. The pre-schoolkids will get in touch with this new factor in their daily classroom-environment. I’ve made various existing applications ready for use, they can discover themselves. Meanwhile, the tabletop is colored in black and the white frame on top is finished. There’s also a new piece in front of the tabletop that creates an extra firming and ensures protection of the mirror.

Ready for the big day tomorrow …

The box is ready and everything can be tested together at a fixed location. Unfortunately, the drop of my beamer is not large enough to cover the whole acrylic plate to provide a full screen image. No disaster because this is a prototype, I will fix this later. I adjust the camera on the image that has been dropped by the beamer. In Touchlib I make sure he sees only the dropped image. I rule the filters and see my ‘blob’ as very bright. The calibration does its job very well and the first touches are a fact.

It is important to have a solid box that’s toddler-proof. It should be a hard enclosure that sets and keeps all the hardware in a safe way, on the desired location. Inexpensive still firmly pressed wood is an interesting solution. The box consists of 4 separate parts which can quickly put together and take apart. On this way it becomes mobile and compact during transport. On the surface there is a hole where the acrylic plate and ledframe will come. On the other side of the surface, there is room for a laptop to set up for observations that can be done. A handy and useful work surface. Under this surface is a second floor where the mirror with a beamer takes place. The camera is set on the beamer where it captures the screen trough the mirror.

The next step is the same setup but with a beamer which projects the signal through a mirror on the acrylic. Among the acrylic I’ve pasted a piece of chalkpaper. Especially important now is to know whether the light of the beamer is going to be held by the IR-filter in front of the camera and whether the infrared light will still be visible through the chalkpaper. It works perfect and I can start building my box that holds all the elements and protects all the hardware.

The moment where I can put everything together is reached. For now I use 2 trestles that must support my acrylic. The frames with LEDs are mounted on the sides (provisionally with adhesive tape) and the camera is underneath the plate. With Touchlib I play with the options (mainly through the use of filters), making a clear as possible picture of my touches on the plate.

In this first test I notice that direct sunlight or other ambient light of the sun is a bad factor because it’s the largest source of infrared light. From now on there will only be tested in a dark room with minimal ambient light. The camera gives a clear picture of the touches (after the outbreak of the IR filter in front of the lens!).
Below a picture without the filter on my camera.

For the wiring of my leds I use a wire of 0.5 mm with isolation. I place in every hole a led and hang 8 leds at a time to each other by connecting every positive pole with the negative of the next one. After every 8 leds I hang a 1-ohm-resistor to this row and a wire that I connect to a ‘breadbord. This is a small connection plate which connects the incoming wires very simply. This step, I repeat 5 times on each side and in total 10 times to create a fully connected frame with leds. I test the fresh wired circuit with positive results.

Before I start wiring the leds I first finish the frame where the leds will take place in. Because I am using the most powerful IR LED who can handle a relatively large luminous, I decide to put on every long side 40 IR LEDs. Which means a total of 80 leds. So I drilled on every half a centimeter a hole with a diameter of 5mm (the actual diameter of my leds). Because my plastic frame is deeper than the thickness of my acrylic, I clear a few millimeters of this strip. After this, the hole can be drilled right in the middle on the edges of the plate, and gives a perfect position to be able to reflect in the glass. Thereafter, the frame has to be sawn at an angle of 45 ° sawn to be connectable to each other to create a complete framework.

Despite my electronics background I still keep in touch with my local electronic shop and the worldwide web so my leds certainly will not be overloaded with too much power. I used the ledwizard. This is a small online application that perfectly tells you how to wire your leds and how much resistors from the kind you need. As power source, I use an old PC power supply with an AT connector. This normally gives a direct output to the motherboard of the computer and gives a DC voltage of 12volt. I should put my leds in 10 different rows of 8 LEDs in a parallel circuit. Each row should contain a resistor of 1 ohms so my leds cannot be overloaded. After testing the I concluded that an infrared led has a total different characteristic than a normal led. The anode and cathode of an IR have led have reverse characteristics compared with an ordinary led. Therefore, I connect my source in reversed order. Before I connect everything together I first test 1 row with 8 LEDs and 1 resistor . The leds are shining. Now I am sure that the 7 other rows will also work and I can get started with the wiring of 80 leds.

Now I am pretty sure that my filter is working correctly, I can get started with my acrylic plate. To save costs, I have only let them saw my plate and I have not chosen for the polishing-option. It is important that the edges of this acrylic plate are very bright. Because infrared light has got to go through it and be able to reflect through the plate. I started sanding with the largest grain and so on until the smallest grain. I used sandpaper of 5 gridtypes: 360-400-600-1000-1200. Water during the sanding creates an even finer edge and can do wonders. After the finest grain (1200), I have finished everything with silvercleaner and then with polish to a have a very smooth clear edge. Normally I use only the long sides of the plate for putting in the leds, but I have also grained the short sides. Here also will be reflection, because I will use a white frame.