Need advice for making an ultra-large, low-res touch panel

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For an artistic project, our hackerspace needs to make a very large, transparent touch panel (there will be no screen behind it). The required resolution is very low (on the order of 20x10 discrete positions) and the panel is very large (maybe 1.5mx1.5m). The panel will be installed in a vertical orientation, outside, in a sunny area, for several weeks if not years. Some regular maintenance is acceptable, and if things break we will be aware of this and be able to fix it without any appreciable impact.

We'd like to use glass instead of plexiglass for durability reasons, especially since we expect people to be touching it quite often. We'd also like not to have to push with loads of force in order to get the panel to respond.

I've scoured the intertubes, but haven't found anything at all. It seems that the touchscreen technology uses special coatings on glass. Considering the size and resolution, it seems to me reasonable to run some thin-guage wires between two panels of glass, but I'm not sure how that helps us.

For instance, maybe we can us such thin wire that we can bond it to the touch surface and effectively consider it a strain gauge. I'm not sure that this is sensitive enough, and temperature might wreak havoc on it.

Any ideas on how to do something like this?

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Do you mean that you'd like to use plexiglass instead of glass for durability reasons, not the other way around?
Anyway, have you looked at infrared arrays and capacitive panels? Infrared method uses a grid of transmitters and receivers across the surface. Capacitive method uses large copper pours under a thin (~mm) dielectric surface (like glass). I imagine the capacitive method would be more robust with a decent self-calibration routine (to account for changes in materials over time), but I don't personally have much experience with putting these things in the field -- just playing with dev boards (QTtouch). There are two capacitive pickup methods, too -- active and passive -- but I leave that for another discussion.

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No need to put anything on the glass at all.

Use infrared led's in a matrix around the edges. Leds on one edge and sensors on the other.
For a 1.5 meter span you might need lenses to keep the beam spread tight.

Certain plastics are transparent for infrared, so you could have a water tight plastic enclosure.

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tlucas wrote:
Do you mean that you'd like to use plexiglass instead of glass for durability reasons, not the other way around?

Tempered glass is far more durable, stronger, scratch resistant, etc.., then plexiglass. The only advantages plexiglass really has is weight, workability, and failure modes that are non-catastrophic.

Quote:
Anyway, have you looked at infrared arrays and capacitive panels? Infrared method uses a grid of transmitters and receivers across the surface. Capacitive method uses large copper pours under a thin (~mm) dielectric surface (like glass). I imagine the capacitive method would be more robust with a decent self-calibration routine (to account for changes in materials over time), but I don't personally have much experience with putting these things in the field -- just playing with dev boards (QTtouch). There are two capacitive pickup methods, too -- active and passive -- but I leave that for another discussion.

We thought a bit about infrared but rejected it because of problems with the sun. I have done a fair amount of image analysis, and sunlight wreaks havoc with cameras and image analysis. Since this could be for a year-round installation it would be incredibly hard to test for all weather conditions and sun angles. Even if we put the sensors on top and the LEDs on the bottom, you still might get weird refractions or simply overpower the sensors. And then there's the left/right direction to think of.

Thanks for the input, though. Thinking of capacitive sensors, is there anyway to DIY? I'm concerned that a having a manufacturer to make us a custom panel is going to cost many thousands of dollars. Since we don't need much precision, and have giant separations to between pixels, I think we could get away with some crude hacks.

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I'd suggest you think again about the IR solutions. Yes, IR wreaks havok on normal image sensors (which is why an IR filter is so important there). But the IR sensors mentioned above are very different animals. The IR receivers are bandpass filters, so only a narrow fraction of the spectrum gets in. Also, the signal is required to be modulated by a carrier - unmodulated signals are rejected. So they work well, are robust, and are fine in sunlight. Think of your television remote control.

The bigger issue is the spread of the transmit signal. Because you need the display to be quite large, a conventional IR transmitter would throw out a large enough "cone" of signal that would cover more than one receiver. As James pointed out, a lens might be a solution, but it's difficult to setup because you can't easily see the effect (it is IR after all). An alternative might be to use low-power IR laser diode modules. Then you know you have a tight transmit beam. Remember to modulate the signal so that it can pass through your chosen receiver module. 38 kHz is a common modulating frequency. For example here's a receiver:
http://limetronic.com/shop/index.php?main_page=index&cPath=7
You just need to modulate the power supply of your laser diode module with a 38 kHz signal to allow it to be recognised by the receiver. If somebody puts their hand in the way, no more 38 kHz IR signal at the receiver - easy detection.

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frankvh wrote:
I'd suggest you think again about the IR solutions. Yes, IR wreaks havok on normal image sensors (which is why an IR filter is so important there). But the IR sensors mentioned above are very different animals. The IR receivers are bandpass filters, so only a narrow fraction of the spectrum gets in. Also, the signal is required to be modulated by a carrier - unmodulated signals are rejected. So they work well, are robust, and are fine in sunlight. Think of your television remote control.

The bigger issue is the spread of the transmit signal. Because you need the display to be quite large, a conventional IR transmitter would throw out a large enough "cone" of signal that would cover more than one receiver. As James pointed out, a lens might be a solution, but it's difficult to setup because you can't easily see the effect (it is IR after all). An alternative might be to use low-power IR laser diode modules. Then you know you have a tight transmit beam. Remember to modulate the signal so that it can pass through your chosen receiver module. 38 kHz is a common modulating frequency. For example here's a receiver:
http://limetronic.com/shop/index.php?main_page=index&cPath=7
You just need to modulate the power supply of your laser diode module with a 38 kHz signal to allow it to be recognised by the receiver. If somebody puts their hand in the way, no more 38 kHz IR signal at the receiver - easy detection.

Interesting. I hadn't thought of carrier modulation at all. Of *course* that would work. (Kind of a face-palming moment there.)

So I guess the approach would be to have a glass panel that has a frame around it. This frame would have a slight overhang on the user side, and the overhang would have the LEDs (or perhaps just a 45 deg. mirror so the LEDs could be mounted flush with the frame) and sensors. Then when someone touches the screen, they would break the beam. It would seem like a touchscreen from their perspective, but it wouldn't actually depend on touch at all.

I like it. It would work even if the glass broke, and it wouldn't require any special preparation for the glass at all.

Edit: the light spreading problem is no problem at all. This is a uC site after all! Just use a microcontroller to pulse the output and then use it to filter the input. Choose 1kHz separations and you should easily be able to distinguish between different sources with an FFT. Maybe not with an AVR, but certainly with an STM32.

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There is a picture of a 16x16 infrared touchscreen here http://www.uiaa.org/illinois/news/blog/index.asp?id=163. I got to play with them in the late 60s, don't know how well they worked in sunlight as daytime was my sleep time :)

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AVR476 shows a capacitive touch remote control with much better resolution than what you are after, and I can make that in my kitchen sink.
It would be pretty simple to construct a capacitive panel yourself out of double-sided PCB. A 1.5m square area of the stuff (1.62mm FR-4, 1oz) would cost me $450+ in, looking at what I've paid for 8x12in sheets; you may be able to get bulk discounts. Getting it etched by a manufacturer would be pricey and unnecessary.
Here's a much more in-depth application note from Atmel on the subject: Touch Sensors Design Guide.

Another neato link, but no thick dielectric layer:
http://blog.makezine.com/archive...

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Quote:
It would be pretty simple to construct a capacitive panel yourself out of double-sided PCB.
Never seen clear PCB material :wink:
Quote:
our hackerspace needs to make a very large, transparent touch panel

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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someday I'll learn my ABC's.