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

Comments

author
SprinklesP made it! (author)2015-03-05

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.

author
App Programmierer made it! (author)2012-05-08

wow how did you come up with that?

author
GoldCannon made it! (author)2008-11-11

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?

author
guunhan made it! (author)guunhan2012-01-20

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

author
Hoaxe made it! (author)Hoaxe2008-11-18

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.

author
SargentMcFluff made it! (author)2011-07-15

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

author
arjay1491 made it! (author)arjay14912011-09-23

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

author
srvliveson made it! (author)2009-03-16

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

author
cliffyd made it! (author)cliffyd2011-08-13

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

author
Opalinus made it! (author)2008-08-26

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.

author
cliffyd made it! (author)cliffyd2011-08-13

ahh pull up a bar stool, LOL

author
zack247 made it! (author)2011-04-28

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!

author
mikebook made it! (author)2010-12-13

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?

author
ermapundi made it! (author)2010-05-12

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

author
tanmanknex made it! (author)2010-04-05

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.

author
bowmaster made it! (author)2010-04-04

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

author
VTguy made it! (author)2010-04-04

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

author
Pumpkin$ made it! (author)2008-09-25

genius.... pure genious

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Christopher Jette made it! (author)Christopher Jette2008-09-27

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

author
VTguy made it! (author)VTguy2010-04-04

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

author
greenie7888 made it! (author)2010-02-14

I was wondering if your box is made of wood and what the holes on the back are (if they are air vents should i consider putting some fans in, is over heating a problem, do i have to use the same projector?) , do you have it plugged into a computer or does it have a dedicated computer.

author
kristoff1313 made it! (author)2009-12-20

I dig your project man. 
 This project and NuiGroup inspired me to make somehting similar to this. I am changing a couple things from your design as a basis.
1. Screen will be glass and sit 30-45 degrees for a more ergonomic approach for extended use. Like a drafting desk.
2. I think I will go with a 1/4 inch tempered glass (clear no tint and without manufacturer bug engraved)
 Seems everyone is migrating to lasers over the surface for touch(less) interface. Would you considering doing an updated project with IR lasers?

author
RapidMustang made it! (author)2009-09-24

Do I have to use 850nm leds, or can I use any IR led?

author
JG96 made it! (author)2009-07-07

Could you make it in to a (for example) 13 inch screen, and embed it into a desk (so its facing upwards)?

author
CaptHowdy1173 made it! (author)2009-07-02

"DI don't require acrylic" ... is there a disadvantage to acrylic? I designed mine using acrylic and was going to coat the bottom side of it with a material that's about 20% opaque (using silicone glue). If anyone's having trouble understanding this there are TONS of materials elsewhere on this project.

author
maxpower49 made it! (author)2009-06-27

what is the special lense and the link doesn't work

author
m.kitchen made it! (author)2009-05-31

Is it possible to put the projection screen on the bottom layer? This way you're touching the acrylic and not the rosco.

author
biotecnix made it! (author)2009-05-14

endlightin plexi glass you will find works allot better if your using 850nm leds

author
Pyshotic Wards made it! (author)2009-04-30

sexyyyyyy!!!!!

author
housewright30 made it! (author)2009-02-17

about how thick did you make the rubber layer? i tryed this without anything on the acrylic and it was hard to use i put a thin layer of rubber on then tryed it again with only the rubber and it didn't show anything. i understand how this is suposed to work but how do you get the RI light to go into the rubber so that when you tuoch it, it is reflected back down to the web cam?

author
jantunes91 made it! (author)2009-02-12

Hi. I'm doing a project similar to this for school and I can't seem to understand the purpose of the Special Lens and the IR Filter. I've seen other tutorials where these items are not required to modify the webcam. I you can light me up on this, I'll be very grateful. Thanks in advance, jantunes

author
ItsTheHobbs made it! (author)2009-01-03

Can't you use a regular $15 webcam from like, Staples?

author
Archive555 made it! (author)Archive5552009-01-08

You could, if you wanted to use a setup which is called a Diffused Illumination setup. This relies on (usually) outside sources of ambient light shining on a diffused surface with the camera in an enclosed space below. The camera picks up dark patches (fingers) on the surface and software on your computer is able to translate that into trackable objects. This system however has a lower accuracy, and is more prone to 'False Blobs' (tracked objects that aren't actually there). Hope that helps. You can find a ton of info over at the NuiGroup forums (nuigroup.com/forums/). The setup he is using is an FTIR setup (which uses infrared light 'rebounded' off of objects on the surface back towards and IR camera (and interpreted as trackable objects), and offers far greater accuracy and isn't prone to 'false blobs'. -Archive

author
09wklaehn made it! (author)2008-12-18

Christopher, I am at teh point of adding the screen in, thanks to your great plans I have been able to build eveerything else, but I am a bit confused on the placement of the rosco aabove the screen. Should I put the spacer along the 1/2 inch edge without silicon? and tehn put tapw o the sides and stretch the rosco? Thanks for any help, William!

author
09wklaehn made it! (author)09wklaehn2008-12-22

So I have the LEDs in, and everything else, but you said in your post to never tape directly onto the Acrylic. I don't see how you can't? how did you attach and stretch the rescue without taping on the back of the acrylic. Sorry to bother you again, but thanks for all your help. Have a merry Christmas!!

author
Christopher Jette made it! (author)Christopher Jette2008-12-18

Yeah that's exactly it! after you tape your protective foil over this! Chris

author
MFeinstein made it! (author)2008-12-18

Hi Christopher, I am from brazil and I really like your work, actually i want to make my own :P I was wondering, the image quality is like an LCD tv, like those in the market? second think. you have sad that the ambient IR ligth may cause interference in the system , so i have the ideia to make the IR LEDs blink in some frequency, and an software can filter those blinking bloobs, dont? and did you thougth about making an "processor board" to filter the bloobs and makes the PC have less work (with microprocessors like AVR or PIC)? thanks for the atention Michel.

author
Christopher Jette made it! (author)Christopher Jette2008-12-19

Hi Ad Matay, The quality of image only depend of the projector and the type of screen you choose (ex. rosco grey). Mine is 1024x768 with 2000:1 contrast ratio but you could choose any you want. The pulsed light would be a great way to improve the reliability of the system but I think it would not solve the problem unless you take a pic with light on and another with light off to compare. This way you loose half of your tracking speed so i don't look to this. For the processor board, I've been looking for it like the board in optical mouse. But for me its a much more complex thing to do than the actual screen. I'm looking into different way to improve my system but for now I'm close to be able to make it work in any light environment even with big changes.

author
MFeinstein made it! (author)MFeinstein2008-12-20

Christopher, Tanks for your reply, please continue with your great work and keep us posted! I will soon test some things (it will take some time to buy it on brazil :P) thanks again! Michel.

author
cham2020 made it! (author)2008-12-15

man, i am amazed by your work. just a question though. inside the frame, there are two mirrors to reflect the projection image from the projector to be visible on the screen. so what's the size of both mirrors? thanks !

author
Christopher Jette made it! (author)Christopher Jette2008-12-15

You will have to do some experimentation but the smaller is about 1 square feet(1x1) and the second 16 sq. feet ( 4x4)by memory... Glue first so it fills the scrathes and you dont have to polish it. My paint touch the leds and there is no problem for me with my paint. Hope it helps! Have a nice day!

author
junior2387 made it! (author)2008-12-05

ok i have a couple questions. 1. u applied the glue first into the wholes, then you slid the LEDs in? and second if i were to use the paint, any paint in black would work? and do i want the paint to NOT touch the LEDs legs, or is doesnt matter that much? or would u thing it is better to paint the edges and then make the wholes? let me know please. thank you.

author
Hoaxe made it! (author)Hoaxe2008-12-11

paint is not conductive so it would not matter. i hope this answers your concerne

author
junior2387 made it! (author)junior23872008-12-12

it does thank you

author
junior2387 made it! (author)2008-12-10

does it matter of the paint touches the LEDs

author
junior2387 made it! (author)2008-12-05

what is the silicon rubber for, and what do i use for protective foil and where do i find it? and i run electric tape on top of the black paint all along the edges? is to reinforce ir leaking in?

author
09wklaehn made it! (author)09wklaehn2008-12-06

silicon rubber allows your finger press to slightly indent the Ir layer, which then cuts intothe ir path, producing the blob. The protective foie is just there to protect the projectr screen, you can find it from his link, or just buy window wrap from any hardware store. Te black electric tape, and black paint is indeed used to help ftop IR leakage.

author
junior2387 made it! (author)junior23872008-12-08

ok i understand that, but one thing iam still a little bit iffy on is, when i put the paint on... do i also paint the edge of the class where the LEDs are on.. where i drilled and inserted the LEDs.. does that edge get paint also...

author
thehackerdude made it! (author)2008-10-05

Wow, nice to see my idea actually would have worked. I had a similar idea to this about 4 years ago. My only problem was getting a rear projection tv, never had the money. My idea was to use dual cameras and a bit of software to calibrate the two to cut off the overlap of the two cameras thus providing higher resolution and higher accuracy/response. I'm soo glad someone was smart enough to articulate the same idea. Thank you for making dream come true. Now I only wish I had a rear projection TV.

author
09wklaehn made it! (author)09wklaehn2008-12-06

? touchlib, the main open source multitouch software was only released 2 years ago.... 4 years?

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