Building a Multi-Touch Sensitive Table

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David Smith

In conjunction with David Holman at RWTH in Aachen, Germany, we've constructed a multi-touch sensitive table that actually works! The table makes use of Frustrated Total Internal Reflection (FTIR), as described in a short paper at UIST 2005 from Jefferson Han at NYU.

This serves as a construction guide for building tables such as ours. The only rule for using this guide for your own work is that after you've built your table, please come back and contribute with any comments or improvements.

I'm not at all an electrical engineer, so I gloss over some of the details of the circuit. If you have more detailed information to add, please feel free :)


Contents

Materials

  • 1 Large piece of Clear Acryllic - Mine is approximately a square meter, about 7.5 mm thick, which was too thin. I would suggest at least 1 cm thick.
  • IR LEDs - I used 8 75mA 30-degree LEDs I got from digi-key. That was more than enough. David Holman says he's using less than that.
  • Resistors - Resistance depends on how many LEDs you use and their operating voltage, and current. To propperly calculate the required resistor use the following formular.

File:Formular 1.png

Vin is your input voltage (12V) Vd is the LEDs operating voltage, which can be found in the LEDs datasheet (Vd is typicaly between 1,5 and 2V). Qd is the quantity of LEDs you are using, and finaly Id is the operating current which also can be found in the datasheet (in our example 75mA). If you punch all the numbers in corerclty, and remember the parantheses you should have a usable result. Since there is probally no resistor of the precise value you calculated, use the closest one you can find, it can be 10% off or even more with no hassel. If you use many LEDs the total operating voltage of the leds can exceed the supply, in this case use a bigger power supply.

  • Wire - Pretty much any wire will do.
  • 12 V AC adapter - Pretty much any 12 V AC adapter will do.
  • Reflective tape - You need tape that is very shiny on the sticky side. I found some really good stuff in the car trim section at Canadian Tire. It's tape designed to look like chrome, but is made of plastic and is shiny on both sides. Apparently you put this on your car to mock up a chrome accent.
  • Rear projection screen - I used the stuff mentioned in the paper. It's made by Rosco and is just a sheet of gray vinyl. You can get it directly from the Rosco distributor in Toronto.
  • Rubber border - You'll want a border for your surface to cover the electronics. I chose to use some rubber trim from Home Depot. I'm not sure what it's really used for, but I found it near the linoleum. It's essentially a black strip of rubber about 4 inches wide that's been folded in half longways. It comes on a roll.
  • Epoxy - a couple tubes at least to glue everything together
  • IR camera - you need a webcam or a video camera that's been modified to only use IR. For this buy an IR filter (Kodak Wratten 87 filter) and manage to position it on the captor itself. A piece of unexposed color film negative should work as an IR filter as well.

Tools

  • drill - doesn't need to be a big one, just enough to handle acryllic
  • soldering iron - just soldering wire and LEDs, so nothing major here
  • pliers - used as a heat sink. I ended up using a Leatherman
  • scissors - for cutting the rubber border
  • wire stripper - I just used the pliers

Construction

Step 1: Prepare the acryllic

First, cut the acryllic to the desired size / shape. Try to have as nice an edge as possible, and make use of the polished edges that come on the acryllic as best you can.

Next, drill holes into the edge of the acryllic for the LEDs. The LEDs will be embedded inside the acryllic, so you want the hole to be just big enough for the LED to fit in. I had 8 LEDs, so I drilled 8 holes, 2 on each side of the acryllic.

Picture to follow

Step 2: Insert the LEDs

Insert an LED into each of the holes. And bend the leads to be parallel with the edge of the acryllic. The important thing is to make sure the leads are all in the same direction. So for each hole, make the long lead go right and the short one go to the left. Place a small piece of tape over the back of the LED to hold it in place.

Picture to follow

Step 3: Tape the edges

Put the reflective tape around the edge of the acryllic, making sure not to cover the leads.

Picture to follow

Step 4: Prepare the circuit

Measure and cut the wire to connect the LEDs together into a single loop. Leave one corner open for the AC adapter. Insert the resistor into the loop between the AC adapter and the first LED.

Picture to follow

Step 5: Solder the circuit

Solder together all the connections in the loop. When you're done, tape the wires against the edge of the acryllic.

Picture to follow

Step 6: Attach the screen

Cut the vinyl to the same shape as the acrylic, but leave approixmately 1 inch extra all the way around. Lay the acryllic in the center of the vinyl. Fold the vinyl up over the circuit and epoxy in place on the other side.

Picture to follow

Step 7: Attach the border

Cut the rubber border material to length on all 4 sides of the acryllic, leaving each piece approximately 1 inch too long. Using a pair of scissors, mitre the ends 45 degrees to make the corners. Epoxy in place.

Picture to follow

Software

The system software is open-source, and will be hosted in Subversion on this server. Once we get the account management sorted out, a link will be made available here.

Links

Retrieved from "http://equis.cs.queensu.ca/wiki/index.php/Building_a_Multi-Touch_Sensitive_Table"