Introduction: Interactive Multitouch Display

Picture of Interactive Multitouch Display

Between the Apple iPhone and Microsoft's interactive table, multi-touch displays are all the rage. This instructable will show you how to turn your lcd projector into an interactive multi-touch display table using a few cheap components readily available from the hardware store.

Here is a video of my display in action:

Step 1: Theory of Operation

Picture of Theory of Operation

This multitouch display screen design is based on the description in Jeff Han's paper,

Han, J. Y. 2005. Low-Cost Multi-Touch Sensing through Frustrated Total Internal Reflection. In Proceedings of the 18th Annual ACM Symposium on User Interface Software and Technology

The figure below comes from his web site.

An acrylic panel is edge lit with infrared leds. When your finger comes in contact with the acrylic, it scatters infrared light out the back where it is visible via infrared camera. As long as nothing is touching the acrylic, very little of the light escapes, instead just reflecting around inside. Image processing takes care of detecting tips of fingers and relaying their location to application software. Since the camera "reads" the whole display in parallel, it is easy to detect multiple fingertips at once, even those belonging to multiple users. All this sensing goes on in the infrared spectrum, leaving us free to utilize the visible spectrum to display interactive software.

Since most hobbyists can't afford multiple projectors (i don't even own one, just borrowed it from dr.eel), my design uses a ceiling mount that swivels so that the projector can be used either in standard mode (say for watching movies) or can be aimed downwards, bouncing off a reflector and onto the multitouch display screen.

The screen itself can be constructed from hardware store materials and hand tools. Excluding the projector and modified webcam (commodity items these days), the only thing complicated is the software. Halfway through this project, I was happy to discover that there is thriving DIY community which has already undertaken the task of writing the image processing code and several cool open source demos which can be found here:


Step 2: Screen Frame

Picture of Screen Frame

The primary component to be constructed is the screen itself. This is a piece of acrylic, with a frame which holds the diffuser and IR leds in position. I opted for a sandwich style construction out of 1x2s and aluminum channel. My acrylic was 30x36x0.25 inches so I made two frames to match, mitering the edges and assembling with screws and construction adhesive. The sandwich design is simple and leaves plenty of room for wiring the leds.

Step 3: Led Rails

Picture of Led Rails

The leds are mounted in extruded aluminum c-channel, pressed into holes. The channel serves to hold the leds in place as well as providing a baffle to direct the light into the edge of the acrylic. Additional spacer blocks cut from acrylic keep the panel from sliding around and pushing the leds out of their holes.

Step 4: Led Mounting and Wiring

Picture of Led Mounting and Wiring

I used 88 infrared leds I ordered surplus online. Their maximum output was 10mW @ 940 nm. Each led needed 100mA at 1.45V so i wired sets of 8 in series along with a 5 ohm resistor. I wired these sets in paralell across the 12V rail of an old computer power supply I had lying around. Since the total current draw is over an amp, you can't get by with a wall wart.

Pour yourself a glass of whiskey and fire up the soldering will take a while to wire up 88 leds.

Step 5: Prepare Acrylic

Picture of Prepare Acrylic

It is valuable to polish the edge of the acrylic to maximize the light that enters from the diodes. Some folks advocate drilling holes in the edge so the leds seat down in but they seem to work ok just butted up against the polished edge. Start with 200 grit sandpaper and work up to 600 or realms beyond. Sanding always takes longer than you think.

Step 6: Screen Sandwich Assembly

Picture of Screen Sandwich Assembly

Once the frame, rails and acrylic are prepared you are ready for the final assembly. Clamp the layers together for easy drilling and bolt them in place. Once the sandwich is assembled, it is necessary to caulk along the edge of the aluminum in order to block any light that might spill out thru the crack. We want it all nicely ducting along the inside of the acrylic instead.

In order for the projector to display on the surface it is necessary to also introduce a diffuser (not shown here). A sheet of thin drafting paper was included in the sandwich supported by a second thin layer of plexiglass.

Step 7: Modify Webcam

Picture of Modify Webcam

In order to image the infrared light, we need an infrared camera. Fortunately CCDs in cheap consumer cameras are quite sensitive to IR, so sensitive in fact that it is necessary to filter out the ir in order to get good pictures. I got lucky and had an old intel web cam lying aroud in which the ir filter was a cinch to remove. Replace the ir filter with a visible light filter (so the camera doesn't see the projected display). I used the exposed end of a negative. See e.g., this instructable for more details.

Step 8: Projector Mount

Picture of Projector Mount

A key aspect of my design is the projector mount. It allows dual use, whereby the projector can be used with a standard projection screen, or swiveled down for use with the mutitouch display. I cobbled something together using scrap metal from the closet. A central support and cross piece with holes drilled to match the mounting holes on the bottom of the projector are attached thru a bolt to the wall. The bolt provides a natural place for the projector to hinge from.

Step 9: Framing Support

Picture of Framing Support

Last but not least, build some framing to support the screen. I started from a coffe table frame as a base and a couple wood planks to get the screen up in the air. The exact geometry is dependent on the optics of your projector. The one I borrowed wouldn't focus at close distances so it was necessary to have the light follow a longer path, bouncing off the mirror and back up onto the display. The angle of the display surface matches that of the projector (angle of incidence = angle of reflectance) in order to minimize keystone distortion.

Step 10: Software and Demos

Picture of Software and Demos

The key thing to making this all work is really the software that converts the detected blobs of light where your fingers are into awesomeness...of course a good soundtrack also makes a huge difference. I used touchlib which basically just worked out of the box. There is a calibration utility which saves results to a configuration file that is shared among the other apps. I also downloaded some Flash apps, including the light-box shown in the video. These interface with touchlib via OSC, a neat protocol for relaying timestamped events which should provide a good starting point for building your own apps.

Step 11: Enjoy

Picture of Enjoy

Excluding the projector, the total material cost is on the order of $100-200 depending on what you already have lying around. Construction time was around 15 hours, mostly soldering and sanding. There is a very active DIY/academic community which has started exploring this space; lots of reward and interesting things to be done for little work with very simple materials

nui group
multitouch blog



epicnoobpwn (author)2016-04-12

How's the resolution on this (for use with a 40" 4k Samsung TV). will the camera work from the front?

sunnycanuck (author)2013-06-26

so it is now a few years later, do you still use this system? Any changes?

tcarr2 (author)2012-01-23

can you use this for surfing the internet?

redinc (author)2009-09-14

Is there a maximum number of fingers that can be used on a multi touch screen?

squiggy2 (author)redinc2010-01-21

no there is no limit to the amount of fingers you can use with a FTIR screen. the limits are

1. how many fingers you have - the average is 10

2. how many your computer can handle - the smoke demo especially uses a lot of processing power and the more fingers the computer needs to track and create effects for, the harder it will be.

vlezcano (author)squiggy22010-06-27

Sorry, but the amount of finger cant be average as 10, because that means, more or less, that for everyone that have a missing finger, there must be one with 11 fingers, and if someone have missed 3 fingers, you will have someone with 13 fingers. the stabdar number of fingers is 10, but is not the average

copiesofcopies (author)vlezcano2010-09-15

It's true, 10 is the mode, but it was still a pretty funny response.

squiggy2 (author)copiesofcopies2010-09-15

k fine my bad.
**'how many fingers you have - your average joe will have 10'
cheers copiesofcopies :)

Apollo2543 (author)squiggy22010-09-16

Well... Since a much larger number of people tend to have 10 fingers that would bring the average up. No need for someone with 11 fingers. If you have 20 people with 10 fingers and 1 person with 9, the average is 9.95 fingers. Close enough to 10 in my opinion.

90mp11 (author)Apollo25432011-06-19

well, technically modally everyone has 10 digits, but only 8 fingers. Two Thumbs are quite common I hear...

jadronx (author)90mp112012-01-12

i thought everyone had 20 digits.....just sayin

DehLeprechaun (author)2010-06-01

would a set up like this work?

Nauscar (author)DehLeprechaun2010-07-16

yes but you need a short-range projector. . which are more expensive.

DehLeprechaun (author)Nauscar2010-07-17

what if i made a serese of magnifying glasses to shorten the projection?

My thought was using some form of modified LCD/OHP setup... that way, you run most of it gutting an old laptop, and just using the lenses to project the screen directly...

I'd love to see this as a modified gaming table

M0HIZ (author)JeffShortland2012-01-04

I had roughly that idea, just jazzed up a bit. My idea was to use an old table or build one from wood, then give it a touchscreen and LCD. I would then put computer hardware into the base of the table, I would try to make it like the touchscreen table out of Quantum of Solace:

M0HIZ (author)M0HIZ2012-01-05

One question, though: is there general rule of thumb for how many LEDs should be used per inch or perimeter or something, I mean, how do you decide how many to ue? Thanks!

I was thinking as a workshop table, but using the gutted laptop is a good idea and then creating a custom interface or something, that would be epic!!!!

You'd have to elaborate on what you meant by workshop table - all I can think of is like an actual work bench, covered with sawdust and small tools.. hahaha

I was thinking for running paper and pencil type games - being able to use an interactive table map that was solid enough to play on. There are a lot of programs out there that can be used (not in this intended way of course) to handle map generation and cloud of war effects.. but it would be awesome to do it with a table

Interactive project layouts, autocad renders, that kind of stuff, not like a wood shop table or anything.

that sounds brilliant.

I wonder how additional surface pressures/noise would impact that? like paper or white board markers, or rulers for example..

or, in my case, dice on the table surface...

Though my understanding Is minimal of how this works I believe that the only problem would be drawing on the surface with dry erase markers because you would be pushing on the "screen" maybe a program that creates a button in the corner that can turn off it's sensitivity on all but te button. I'm not sure how papers and things lying on the surface would react but it would depend on the weight of the object and the sensitivity of the would have to be experimented with, unfortunantly I have neither the time or money to do so, with school starting it makes it impossible for me to tinker or in some cases think straight :-(

aagrlp640 (author)2012-01-04

Before nothing thanks for this web page, its awesome, i have a question does it have to be of a specific size or it doesnt matter? for example of 32" or even 60", obviusly with a good projector and more leds on the sides

epicnoobpwn (author)2011-11-24

Could I use a large tv (rear projection/flatscreen) instead of the projector. I would have the transparent acrylic touch panel on top of the screen.

By the way, where did you put the ir camera? I never saw you mention where to mount it.

rwhite18 (author)2011-11-14

you could make a cheap projector yourself by using an old lcd screen and an overhead projector. made one my self and works as good as a sony.
repurpose the LCD as you only need the screen itself. not the lighting that came with it. the overhead projector supplies the lighting and can be adjusted for short range use. you would have to look it up on the web for details of the build.

eluna3 (author)2011-11-12

How much are the thicknesses of the 2 acrylic?

Tarzan88 (author)2011-11-01

¿que tan separados están los LED's? ¿son 2 acrílicos verdad?

lshoesmith (author)2011-10-31

Hi there.

Im just wondering how this method of input acts in 'normal' computing e.g what can be achieved with a mouse pointer.
Basically, does it play well with programs that are not designed for touch or multi touch like word or Opera internet browser?

Thanks muchly :)

acieslak-jones (author)2011-09-14

hi :) i am thinking of making this. i was just wondering how thick you reccomend the acrylic to be? thanks

zack247 (author)2011-04-28

hey, i was wondering, could this work with a LCD screen?
i have a 15" one and i want to repurpose it, is there any way i can convert this to work with a LCD?

the only problem i can think of is the webcam being able to see the infrared light, especially since the LCD would need lighting too. (obviously not the original backlighting system, of course)

any help would be great, thanks!

dancmarsh (author)2011-03-05

I know someone asked before, but there was no answer, so i wonder if anyone has tried it.

Has anyone tried mounting the projector under the table yet and setting the screen to rear projection?

I'd like to know if it works, would make for a neater and more portable install.

Aphylian (author)dancmarsh2011-04-14

it works but the focus length tends to be longer than the distance between projector and the screen, you then also have the reciever in the way of the projector leaving a dark patch on your screen.
you can have the projector in the top corner near your screen and reflect it off a mirror at the bottom of your case. that works too

psalanoa (author)2011-02-24

how do you press the holes into theextruded aluminum c-channel?

Tangoforce (author)2011-02-03

Nice music man, what is it?

Oh great project too.

10fellowsd (author)2011-01-31

not the acer x110 its the best

softlux (author)2011-01-26

Sound’s good. Just one question for you. Can your multitouch recognize an object such as a glass resting on the surface? Thank you!

abadfart (author)2010-12-29

couldn't you just set the projector to rear project and put it under the desk

JamCat (author)2010-01-07

  I used to work in a plastics shop, and I guarantee the best options for nice clear edges are flame-polishing or clothwheel-wax buffing (but as buffing is pretty straightforward, and similar to other media I won't go into any tips)...
Flame polishing is really the best, and potentially fastest, but you should really practice on a similar thickness scrap first because you risk burning if you move the flame too slowly. I recommend using an oscillating power sander with 200 grit, then 600 grit til the edges are silky smooth. Then use a MAPP or hotter (for best results, we even used to use oxy-propane smallllll tip) torch to *QUICKLY* run the furthest tip of the feather of the flame around the edges at a backwards angle (so the cone is pointed opposite the direction you are moving) - after all dust and masking is cleared from the edges. You'll see the hazy sanded area turn clear and reflective very quickly if you are looking at the right angle.
If you miss a spot, or if some deeper sanding scratches remain, you can retouch with quick swipes keeping the flame even further back; if you burn it (and its VERY easy to do if you don't keep it moving) just resand the edge with 600 grit, and use some soap and water with a cotton (or preferably microfiber) rag to remove any scuz from the faces of the acrylic sheet. NEVER USE PAPER TOWELS ON ACRYLIC - they have wood pulp and will scratch it, eventually turning it cloudy...
This will give you nice crystal clear (acrylic is of course clearer than glass...) edges that will transmit wayyy more of your IR light!
Good luck!

killersquirel11 (author)JamCat2010-11-16

Would a soldering iron be able to do the trick? According to wikipedia acrylic melts at ~160c, and soldering irons tend to be able to break 200.

Hossein (author)JamCat2010-10-17

Dear JamCat
i know what i wanted to ask is out of line but believe me i wouldn't ask if this was not my last chance .
I really really need to have the PDF version of this multi-touch screen here coz i took it as my major project and i have to build it in a week time and to be honest i don't have the money to be a pro member and since i'm a student i don't have any visa card as well.
I was wondering to ask if possible can you downloaded it for me and send it to my mail i would mean a lot to me .
THANK you so much

turkey tek (author)JamCat2010-01-07

wow jamcat, many thanks for the useful advice and insights!

yosaulox (author)2010-11-08

man what software you used¡?

zeeebus (author)2010-03-28

Awesome Job!  This may be an idiotic question, but....I want to use my table mostly as a whiteboard for writing and brainstorming--hopefully converting into typed text too, if possible.  Would the side palm of my hand resting on the screen while I write interfere with the writing recognition?

Hossein (author)zeeebus2010-10-17

Dear Zeeebus
i know what i wanted to ask is out of line but believe me i wouldn't ask if this was not my last chance .
I really really need to have the PDF version of this multi-touch screen here coz i took it as my major project and i have to build it in a week time and to be honest i don't have the money to be a pro member and since i'm a student i don't have any visa card as well.
I was wondering to ask if possible can you downloaded it for me and send it to my mail i would mean a lot to me .
THANK you so much

squiggy2 (author)zeeebus2010-09-15

your hand resting on the side of the screed would create IR light that the wii remote would see, and yes this would mess up writing recognition software, but you could put a clause in the software that ignores any blobs bigger than a certain size - for a palm is much bigger than a finger, or any blobs that are stationary (to a certain degree) or something like that, which would fix things up and make it possible.
let me know how you go! (or are going)

BIGHAIRYDUDE (author)zeeebus2010-05-26

i think so

Neoferatus (author)2010-10-10

Interesting!! Thanks alot!!

UniBot (author)2010-03-30

Well if you use the 3v output from your standard ATX power supply, you can hook up in parallel N modules of 2 LED's in series (MODULES in parallel, LEDS in series), being theese in a total power consumption of 2.8v... 3v will do, since (I think) that's within the LED's electric tolerance. Then you hook up (as i said) N modules, with this I mean until you complete the average total amperage of a normal ATX power supply (commonly 19amps), with that you can plug as many LED's as you want (as long as theyre within your power supply capabilities).

shinjiyuubix (author)UniBot2010-07-10

Without any resistors? Can you draw here a schematic? Thx

UniBot (author)shinjiyuubix2010-09-26

Im sorry for the delay, but heres a quick schematic. The thick lines are the 3v power output from the ATX power supply, (positive and negative respectively). the rectangles are (supposedly) LEDs hooked up in series of two LEDs drawing a total voltage of 3 volts (YAY! thats exactly the same voltage than the ATX! xD), thats what we'll call a "module". If we assume that each LED draws 50 Ma (milliamperes), then each "module" will draw that amount from the power supply, if you hook up more modules to the power output of the ATX power suply in PARALLEL you can keep up until you fill the 19,000 Ma (19Amps [depending on your power supply capacity]) of total capacity... that's a HELL LOTTA LEDS MAN!

PS: im sorry for the lame paint-made schematic.
PPS:I know, I made a short story long. Who cares? xD

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Bio: thanksgiving! ...and bringing technology to this traditional celebration of excess.
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