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Hi gang, not sure if this should be here in Sensors, or in the Interactive Art section.

I'm working on a an interactive light piece which is basically a large tower covered in windows:
http://www.technoetc.net/imgs/urbana_2011_gameplay_2.png

Each window will be translucent acrylic, and illuminated from behind.  To provide interaction, I'd like each window to be touch sensitive.  I'd like to use a capacitive sensing approach, but not sure if this will work until I do some experiments; I fear over such a large area capacitive sensing will be very noisy.

In the meantime, I'd love to hear any suggestions anyone might have as to how to detect users touching the windows.  I'd like to avoid having to use little buttons beside each window smiley-wink  Light or ir sensors might be an option, but I imagine will require a great deal of calibration.

The piece is still in the design stage, so I'd love to hear your suggestions/comments.

Cheers,
Andrew
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You might have trouble making translucent acrylic conduct electricity - unless you can find some with a conductive coating capacitive sensing isn't an option.  You might be able to attach a microphone to each sheet and detect the vibration when its tapped.
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what markT said, but use piezo pickups / piezoelectric sensors
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I spent a lot of money trying to get a cheap resistive touchscreen, because (at the time) the commercial ("real") ones were on the order of $900 for a 17" screen.

This might be too small for what you're trying to do, but HP is manufacturing a 21.5" lcd monitor with multi-touch technology built in for about $300. I've installed about 10 of these- half of them in a very harsh manufacturing environment where I didn't expect them to hold up, and they are performing surprisingly well. The model to look for is L2105tm.

Under XP there are HP drivers that will get you single touch. Under Windows 7 you get two points of touch. (I forget what vista support was). The monitors are designed to report up to four points, but I don't know how you get that to work. Dual-touch-screen works with this model under windows 7 (even with rotation) but XP will get confused if you connect two L2105tm's to one computer.

Hope that helps someone.

Jeremy
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Thanks for the suggestions/comments -- I'll look into the piezo idea!
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I saw a project once where they used acrylic sheets with touch-- they had a camera on the back side with the IR filter removed. Fingerprints on the outside showed up as hot spots on the inside. Just something else to consider. Wii-motes have IR cameras in them and are bluetooth enabled... I bet you could point one at each side of the building, from the inside. (from there, I don't know what you'd do with it though!)

There's also a project with Wii-motes where a guy wrote software to convert IR points from the Wii-mote camera into gestures on a screen. The initial calibration tells the software where your "screen" is in relation to what the Wii-mote can see.

Jeremy
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yeah, of course. wasnt thinking right. if you use a defuse surface, you can use use an IR camera and computer vision for detecting the touches.

for the softwareside, you could try community core vision: http://nuicode.com/projects/tbeta
if your working with processing, the jmyron (i think thats what it was called) library has worked well for me.

for the ir cams, just get yourself some regular webcams, open them up, take out the visible filter, replace it with developed unexposed film (i.e. a black dia, or something like that --> ask in your lokal photo store, they will know what you need.) (alternatly, use black trash bags, they sometimes work.) and tada. ir cam.

cheers

p.
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Hi all, an update on this touch-sensitive windows project . . .

I ended up using the CapSense library for Arduino with a row/column electrode arrangement, thus only needing 11+1 i/o pins for each 5x6 array of windows.  Thin wire was used for the electrodes, with a row/column intersection centered on the back of each window.  Despite some noise/interference problems (mostly solved w/ grounding), this solution worked pretty well.

Each window was individually illuminated by a Shiftbrite, and the whole thing assembled in a fabricated steel structure.

I've written a detailed post about the (on-going) project here:
http://technoetc.net/blog/2011/04/27/urbana-2011/

Thanks again for your input and suggestions,
ao.
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