Back Projection 56 Inch Multitouch Television.

149,218

381

128

Posted in TechnologySensors

Introduction: Back Projection 56 Inch Multitouch Television.

After reading this Instructable, I have built a nice multitouch display during my spare time as a hobby. In this article, I will share plans and explain how to construct a 56 inch multitouch screen that looks like a TV. It's totaly homemade, it's not made from an old tv but you could do so. This article is for people who already have basis in this kind of thing. It's not an easy setup and it's not suitable to be commercialized but for personal use it's fine for testing new softs.You can look my website at: www.Softmt.com

 

Step 1: Basic Principle

Basic Principle

The principle is to modify a webcam to allow it to capture only IR light and point it to your screen. Then the software analyzes the images and detects press where there is a blob. A blob is the light projected back of your screen when you press onto it (Scattered Light on the schema).

Multitouch technology grows at a lighting speed during those years. There are many different methods for sensing the touch on the screen. As far as I know there's electronically and optically based setup. Iphone and Ipod touch are multitouch devices and use electronic circuit for sensing touches.

Optical systems are the simplest to build for home users. Those systems require a low light environment and may not be as stable as electronical systems. There's two big principle in multitouch optical system, the first Is FTIR (Frustrated total internal reflection) like I use and DI (Diffuse illumination) like Microsoft surface uses.

Frustrated Total Internal Reflection

When light encounters an interface to a medium with a lower index of refraction (e.g. glass to air), the light becomes refracted.
The extent of refraction depends on the angle of incidence, and beyond a certain critical angle, it undergoes total internal reflection (TIR).
Fiber Optic cable is a common technology that employs TIR.
However, another material at the interface can frustrate this TIR, causing the light to escape the waveguide there instead. Ref : http://www.cs.nyu.edu/~jhan/ftirtouch/

Under "ordinary conditions" it is true that the creation of an evanescent wave does not affect the conservation of energy, i.e. the evanescent wave transmits zero net energy. However, if a third medium with a higher refractive index than the second medium is placed within less than several wavelengths distance from the interface between the first medium and the second medium, the evanescent wave will be different from the one under "ordinary conditions" and it will pass energy across the second into the third medium. (See evanescent wave coupling.)
A transparent, low refractive index material is sandwiched between two prisms of another material. This allows the beam to "tunnel" through from one prism to the next in a process very similar to quantum tunneling while at the same time altering the direction of the incoming ray. Ref: Wikipedia

Diffuse illumination
In DI system, The IR light isn't inside the acrylic but projected onto it by the top or the bottom. The acrylic should be evenly projected onto the screen to be reliable.
+ Simplest construction
+ DI don't require a compliant surface (Rubber)
+DI don't require acrylic, it can be made out of any transparent material
+ Allow tracking of Fiducial
-Blobs are not as clear and sharp compared to ftir blobs

http://youtu.be/Wi8DqXYYehE

Step 2: Material Needed

Material needed

This is the component I use now in my system. You can use different gear and get same or better results.

-Computer core2 recommended
-Projector 550$ on ebay
-Webcam 55$ on tigerdirect
-Special lens 25$
-IR Filter 25$
-36 x 48 inch Plexiglas 1/2 inch 150$
-36 x 48 Rosco Gray Rear-Projection Screen 30$
-168 Osram SFH485 880nm IR LEDs 115$
-Wire and resistors 20$
-Silicone 6$
-Toluene 20$
-Protective foil Different from mine--- Seems similar 30$
-Mirrors
-Tapes
-Glue
-Black paint
-Mdf, hdf or plywood sheets
-Front speaker fabric 40$
-Many different tools
-screws
-wood finishing product
-paint filter
-paint can
-Sound system 115$

Step 3: Preparing Acrylic

Silicone rubber

**WARNING**
DO NOT Use the pouring method as this has been found to result is Crazing (Cracking) of the Acrylic due to the Toluene.For best results use the spray method in thin layers. I suggest you to run some tests on spare acrylic before you do your final screen.

The silicone rubber layer helps coupling between the Rosco projection screen and the acrylic. It allows the user to have brighter blob for less pressure onto the screen. Without it users should have to press really hard to get decent blob.

You can watch a video that illustrate the differences at:



'''''Dowload my explicative PDF for rubber'''''

Step 4: Ir Section

Ir leds
In this setup I put one led every inch on four side of the acrylic. 36x2+48x2= 168 led. Big quantity of leds helps to have really bright blobs. For my experience 1 led every inch is enough to get nice result. I got the best performance by drilling 5mm holes on side of the acrylic and then glued them with testors glue. Then I wired the leds and paint the acrylic edges using really dark black paint. I don't use any frame on side of acrylic, black paint block all IR and it's a lot simpler to avoid leaking of IR.

Step 5: Fixing Other Layers

Your acrylic is now ready to be covered with other layers. Before you stretch the Rosco on the acrylic you have to put a 1mm spacer on edges of the screen to avoid the Rosco being directly on the silicone rubber. I personally use arborite which is 1/32 inch thick. This small air gap helps to avoid the Rosco from sticking to the acrylic.

The easiest solution I find out to this problem its tape, I used the best tape I found. For the simplest result you start by stretching the center in both directions horizontally and vertically and then you process to the outside in rotation. Go by small steps to avoid wave in foil. First you have to stretch the Rosco than your protective foil. I use to cut both exactly the same dimensions of my screen so it simple to tape it on edges and at the backside.

As protective foil I personally use Insulating film which are really thin and retract when you blow warm air onto it. I took the thicker of the 3 size. You have to stretch it using the same method as the Rosco and then you blow warm air from an air dryer to eliminate waves. My results with this product were awesome compare to those obtain with my previous vinyl which was too thick and hard to hold with tape. I suggest you to put some layers of tape around the edges of the acrylic to help other tape to hold.

NEVER STICK TAPE DIRECTLY ONTO THE ACRYLIC OR COVER IT WITH BLACK PAINT!

Step 6: The Box Plans

Size of the box
Height: 73 inches
width: 50 inches
deepness: 24 inches
Size of the screen
33.75 x 45 inches
56 inches diagonal (4:3 ratio)

If making this box is a problem you will find a lot information on this website:

http://www.woodworking.com/dcforum/dcboard.pl

Step 7: Position of Components

Position of components

Step 8: Software and Link

Step 9: Videos

Share

    Recommendations

    • Spotless Contest

      Spotless Contest
    • Space Challenge

      Space Challenge
    • Science of Cooking

      Science of Cooking
    user

    We have a be nice policy.
    Please be positive and constructive.

    Tips

    Questions

    128 Comments

    I actually have a similar project that I'm barely starting in on. It's an old crt rear projection TV that I found on the side of a street not too far from where I live and decided it'd be a very cool thing to take apart and I did want to miss this opportunity. So, I brought it back home and checked the screen out, the colors were far outta whack and it needed blue coolant but I ended up tearing it down carefully and an idea sparked, I'm planning on using my dad's old 37" lcd Sharp Aquos which needs a tuner board and putting the lcd TV in this cabinet, which has a larger screen capacity (might need to make some shims and braces, better a few inches too large than too small). Once that's done, I'm planning on throwing in a gaming computer into the cabinet and build it from the ground up, plus put in a good audio system, fan system, hard drives, and a liquid cooling system for the CPU to help manage the computer components. Basically, I'm making a large All-in-one computer.

    is there such thing as a projector lens that can project a fully proportional rectangular image when mounted at the side of the screen, not in the center? If I were to build a multi touch desk and need at least some space for my legs under the table, i would need to put the projector somewhere at the opposite side with a special lens, but i still want the image to reach to the edges of the table. Would this be possible?

    2 replies

    I believe the setup your looking for is a telecentric lens.

    what you need is an "optical engineer" or an "optical designer", that sort of thing, to help you figure out how to build this lens of yours. I have never seen anything that could do that, but hit me up if you do. there's probably an e-mail in my profile or something. you could also try pushing the projector back and tilting forward and then just use a regular Isosceles trapezoid regulating feature seen on most projectors to fix the picture. a last try is to project the image from the top. the picture would appear on your hands when on it but other than that, i don't see why not.

    I understand that you can use Microsoft paint and all... but, can you get on the internet while using the software?

    1 reply

    of course you can, its a computer after all
    its just a computer with a projector and webcam installed

    Could I theoretically use a projection TV to create this? I have a sony 43 inch.

    1 reply

    ya me too, have RCA 52" work exactly the same, theoretically!

    Very nice project. You show how easy it is to use Multitouch even for a huge surface. The only Problem is - I think - you must stand. It isn't very comfortable, but I don't think you want to work with that for hours, as with a "normal" computer. But at least very cool.

    1 reply

    ahh pull up a bar stool, LOL

    i have a question. hopefully you can answer?
    would this work if i used a LCD screen?
    is there any way to make something like this with a lcd screen?

    please post back and let me know!

    I understand how your finger triggers the IR to bounce back, by why doesn't the silicone/rubber do the same thing? Maybe I'm really just missing a big concept here?

    Could I just add IR LED's and an IR webcam to this TV? TV link  or would I need some layers of some material

    You know, I think this could be used as a table also, with the slanted part where you sit under.  It might not be quite as effective, but it might work.

    Amazing. This would be the perfect thing for my geek pad. I WILL make one when I have the money.

    Ever see the Homemade Multi-touch on the Hak5 show?

    No its Jeff Hann who is the genius beside this technology...

    Jeff Hann may be a genius but he was rude to me, I talked to him on the phone to get an interview, he told me he would.... never answered my call again