Instructables
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

Picture of 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

 
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wow how did you come up with that?
GoldCannon5 years ago
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?
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?
of course you can, its a computer after all
its just a computer with a projector and webcam installed
srvliveson5 years ago
Could I theoretically use a projection TV to create this? I have a sony 43 inch.
ya me too, have RCA 52" work exactly the same, theoretically!
Opalinus6 years ago
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.
ahh pull up a bar stool, LOL
zack2473 years ago
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!
mikebook3 years ago
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?
ermapundi4 years ago
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
tanmanknex4 years ago
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.
bowmaster4 years ago
Amazing. This would be the perfect thing for my geek pad. I WILL make one when I have the money.
VTguy4 years ago
Ever see the Homemade Multi-touch on the Hak5 show?
Pumpkin$6 years ago
genius.... pure genious
Christopher Jette (author)  Pumpkin$6 years ago
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
greenie78884 years ago
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.
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?
Do I have to use 850nm leds, or can I use any IR led?
JG965 years ago
Could you make it in to a (for example) 13 inch screen, and embed it into a desk (so its facing upwards)?
"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.
maxpower495 years ago
what is the special lense and the link doesn't work
m.kitchen5 years ago
Is it possible to put the projection screen on the bottom layer? This way you're touching the acrylic and not the rosco.
biotecnix5 years ago
endlightin plexi glass you will find works allot better if your using 850nm leds
sexyyyyyy!!!!!
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?
jantunes915 years ago
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
ItsTheHobbs5 years ago
Can't you use a regular $15 webcam from like, Staples?
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
09wklaehn5 years ago
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!
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!!
Christopher Jette (author)  09wklaehn5 years ago
Yeah that's exactly it! after you tape your protective foil over this! Chris
MFeinstein5 years ago
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.
Christopher Jette (author)  MFeinstein5 years ago
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.
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.
cham20205 years ago
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 !
Christopher Jette (author)  cham20205 years ago
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!
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