Calibrating

Here is the calibration set up i am using at the moment. The resolution of the webcam works fine even for the large screen (60in). The blobs are clear and easily picked up even when the camera is positioned to pick up the reflected image of the blobs not bad considering the mirror is a standard bathroom vanity mirror from home depot.

Using the mirror on the back wall cuts the size needed for the screen in half so it fits nicely in the space. In the next "prototype i think im going to add another mirror to compact the design even further.

mini multitouch

Since Im working at Plymouth this summer i needed to build a demo multitouch unit that could fit in a suitcase. So to fit the bill i made a small 18x24 inch multitouch screen that is designed to fold into a suitcase and sit on a desktop next to a laptop.



I didn't want to hassle with getting another psu unit or bringing one to england so i picked up a couple of battery holders from radioshack and linked them together. Each holds 4 AA batteries so 2 battery holders in series kicks out 12v dc. Since the LEDs only run at about 100ma that should give me a good 10 hours of continuous use but they come with a built in switch to i can just switch it off when I'm not using.

The other great advantage to having it on the desktop is that it makes software development so much easier. I can edit the actionscript then turn on the multitouch and test the results without having to get up and move over the screen on the other side of the room.

Even with only 16 LEDs the blob sensitivity is excellent. In fact due to lack of time i didn't have time to sand down the edges of the acrylic but that did not make ay appreciable difference to the blob resolution at least at this scale. Although for the larger screen with the web cam further away i imagine it would make a noticeable difference.

Wiring Up The LED Arrays To The Power Supply

Each array of 8 LEDs was independently connected to the power supply. I used a cheap $20 power supply unit that i bought from a second hand computer supply store. It used to be part of an old compusa computer and now it powers my multitouch screen :)

In order to "turn on" the power supply so that it feeds the LEDs with a nice steady supply of 12v dc you have to rig the main "" plug. Simply cut the "" and "" wires and splice them together or short the circuit using a wire in the plug. All thats going on here is the PSU is rectifying 110v ac to 12v dc just as it normally would do when it once powered a computer motherboard but instead of the motherboard turning it on an off you can simply rig it to be "always on" when its plugged in.

Now that the PSU is hot you can (once you have safely unplugged it again) connect the leds to the "" and "" wires. Make sure you have each array is connected to the PSU in parallel.

Creating The Stand

With the first test about to begin the only thing left to do for the screen was to add the tracing paper to the back of the screen.


Then It was just a matter of fixing the frame to the stand using a couple of brackets from home depot.


The Budget for the first prototype was a little tight so i had to build the testing rig out of scrap wood i had left over from doing some renovation work on my kitchen. Three or four studs did the trick.

Framing The Screen

Here you can see the mirror clamps i used to fix the rails in place around the acrylic which has the added bonus of attaching the screen to the wooden frame.






I built a simple wooden frame form 2x1 to hold the acrylic and aluminum rails.The first prototype uses a vertical stand which is like a traditional white board.

Building The LED Array

Designing The Array
Given the dimensions of the screen (5ftx4ft) and the optimal separation distance between the leds (1.75in) I used 80 leds in total. Thats 3 sets of eight on each 4ft rail and 2 sets of eight on each 3ft rail. I grouped them into sets of eight as the power supply unit gives out 12v dc so with there being 1.5v need across each led when in series they require 12v across them to function correctly. This meant that each series of eight leds had to be soldered together and wired independently to the power supply.

of course don't forget about the resistors. Given the 100mA optimal operating current i needed a 5ohm resistor to put in each led series. They didnt have any 5Ohm resistors at radio shack so i just soldered two 10ohm resistors in parallel to create a resistance of 5ohms.

Preparing the leds
The legs of the leds needed to be bent so that the head of the led can fit comfortably inside the drilled hole and the legs can be connected together with the distance between two leds at about (1.75) inches. To do this i recommend using a pair of needle nosed pliers to grip the base of the leg then carefuly bending the leg side ways to create a 90 degree angle. This helps protect the components leg connection and also serves to create a clean/sharp bend.

Drilling the Holes in the aluminum
Looking at the dimension carefully you can see there is not quite enough leds to create a perfect grid of ir light. However if you shift the top leds slightly you can minimize any potential variation in light intensity. Mark out where each set of eight leds are to be positioned then divide the section into eight equal section.
In the case of these sf.. leds with a diameter of 5mm u used a ... drill bit which let the head fit snugly in the hole.

Soldering The Wire
leds can be easily damaged when too much reverse current/voltage is applied (as with most electronic components). Since heat makes electron flow in metal and electyron flow is equivalent to current heating a component too much when soldering can damage it. To prevent this the most effective method is to use a simple crocodile clip as a heat sink. This allows you to quickly solder the connection without accidentally over heading the component as the surface area of the clip dissipated the heat before too much of it gets to the component.

The wire i used for this was 22 gauge single core. This was appropriate given the current and voltages we are working with and the single core made that little bit easier to solder as once it was bent into shape it held its position while i soldered it. Be sure to give yourself plenty of wire to reach the power supply unit given the size of the screen and the distance to the power supply i ended up using over 90ft of wire.

Selecting The IR Leds

The infra-red leds i used where the osram SF485. I managed to get 120 for about $60. They are smaller than the ... at 5mm diameter but give a greater brightness for the same power consumption. They operate at the same voltage and current as the SF485 (1.5v at 100mA) but produce a brighter more intense beam as the radiant intensity is greater an the beam angle is smaller. The primary ir radiance frequency is at 880nm which is around the peak sensitivity for the philips .. webcam.
I purchased the leds from digykey which and they arrived the next day. At the time there was a backorder on the SF485 leds so i decided to get the SF.. instead. Slightly more expensive but i figured the extra brightness might make up for it.