Thursday, March 20, 2008

Taking Apart My LCD Monitor

It was one of the scariest moments of my life, but it had to be done. It takes some guts to take apart a 19" LCD screen that cost $120, but if you keep the multi-touch table in mind, it is worth it. I didn't want to take apart my screen only to find out it doesn't work, so I did a lot of research. A great place to check if your screen will work or not is LumenLab. Make sure your model doesn't have FCC issues, which means there are non-movable components that block the LCD panel. My model was the Acer AL1916W, which worked perfectly.

What you are trying to do here is strip down the monitor until you are left with the LCD panel and the electronics. The LCD panel is just a thin plate of (glass, plastic?) with some ribbon wires attached to the top. The pixels on the LCD panel can turn different colors but are mostly transparent, especially to IR light. So lets get started!
My monitor:

After taking all the screws out I could, it still wasn't coming apart. I used a screwdriver and slowly pried it apart, breaking the little plastic snaps all around it. This was the point of no return and came surprisingly early for this project.

Here is the back of the monitor with the plastic case taken off:
Here is the front of the monitor. It is actually on right now. The little green dangly thing at the bottom is the controller for adjusting brightness etc. If you look closely you can also see the little gold strips at the top. These are the ribbon wires that are connecting the LCD panel to everything else.

Here is the LCD monitor with the panel lifted away from the backlight.
I disconnected everything I could from the actual LCD panel (except for the controller), including the power supply.

After you take everything apart, you will be left with a lot of extra parts. The sheets of foggy papers are diffusers. For a nice looking picture, you will need to put these between your LCD screen and the backlight to make the light even. I also kept the lights from the backlight, to see if I could use them myself.

Tips: Make sure you don't wear socks or do your disassembly on carpet. In short, keep everything static-free. I didn't, and now I have a few lines of discolored pixels.
Make sure everything is unplugged as you take everything apart. I only plugged it in for a second to reassure myself that nothing was broken yet. Watch out for capacitors too, as they may still be carrying a charge.
When you plug in your monitor to see if it still works, and it doesn't. Don't panic until you try another outlet. The one you are using is dead.

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Planning My Multitouch Setup

As it turns out, not all multi-touch surfaces are created equally. There were many decisions that had to be made before getting started. Here is what I decided on.

Touch Sensing
For homebrew multi-touch systems, there are basically two different ways to detect your touches. I will briefly go over each. The first is direct illumination, or DI. Basically, DI uses an infrared source to illuminate the touch surface and an infrared camera to see what is on the touch surface. This approach allows you to see anything on the touch surface, whether it be fingers, a paintbrush, or fiducials.

The other method uses Frustrated Total Internal Reflection, or FTIR. The theory is that when infrared light is shined inside the edge of a material such as glass or acrylic, it will be reflect internally to the other end of the material, never escaping. However, when a finger touches this material, it "frustrates" the internal reflection, and causes the infrared light to escape through the surface of the material. An infrared camera then tracks this on the opposite side of the material. Unfortunately, this method does not allow much else than fingers from being tracked.
The method I chose was FTIR. To me, it looked more stable than DI, not needing as much calibration and not being as susceptible to outside interference.

One of the coolest parts of multi-touch is the illusion it creates of manipulating virtual objects as if they were on the table in front of you. This requires the touch surface, and the display to be on top of each other. Much easier said than done. One method is to use a projector to shine the image on your touch surface. Projectors aren't cheap though, and I didn't have any spares lying around.

I decided to go with a more affordable method, using an LCD monitor. I know what you are thinking. "But you have to track the infrared blobs through the display, and LCD monitors are very much opaque." That is exactly what I thought. As it turns out, the actual LCD matrix in an LCD monitor is clear. It is just a piece of glass(?) that can color itself. Then, behind this is a backlight which allows you to see the image. By ripping apart an LCD monitor (gulp) you can take out the LCD matrix and mount it on your multi-touch table. You just have to figure out your own way to backlight it.

Multi-touch Surface
The final major design consideration is how you are going to display your multi-touch surface. Because I will only be using a 19" display, a wall display wouldn't be very impressive. Also, this being a science fair project it can't be overly large or I won't be able to fit it. My goal is to enclose it all in a box, about 2" on each side.

So that is my planned multi-touch table. I will post more progress, and some photos soon.

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