Hello, I'd just like to run an idea through with some people who know a lot more about this than I do.
Basically I'd like to make a capacitive multi-touch screen. It would essentially be a grid of wires spaced 4 every inch or so, hooked up to an arduino using two of those 48 q-touch chips. The charge time is then read from each row and column and interpolated to find finger positions.
I've done a very quick mock-up of 10 sensing columns using some strip-board directly connected to the digital pins of the arduino, and although that is only 10, it seems to work well. The data is being sent to processing via serial and appears stable, even without capacitors connecting them to ground.
However, do you any ideas to how well this would scale up to 48x36, using the q-touch chips?
The idea was to eventually have a touch panel mounted behind an LCD screen but I need to test that because LCD panels are obviously conductive so I doubt that will work. The other idea was to use extremely thin wire so that the LCD isn't obstructed to much.
This is going to sound nuts, and may not even work (might not even be applicable for what you are trying to do!):
Apply copper foil to glass (how? dunno. maybe find a foil that will adhere to glass - there might be something used in stained-glass work?)
Print pattern of lines on transparency (negative image - I think?).
On copper on glass, put positive photo resist (like you were making a PCB).
Put line pattern on top and expose via UV.
Etch the glass to remove non-exposed photo resist.
Remove exposed photo resist.
I think these steps are right - but consult a tutorial on PCB making using UV light; essentially, instead of using a pre-sensitized PCB, you would make your own PCB using a piece of glass and the foil.
What should be left at the end (cross fingers?) is a clear piece of glass with foil traces; if you print your lines so that they are 1 DPI wide (and use at least a 600 DPI or more printer), the lines should come out very thin; the only possible issue is that the lines may be so thin that they get etched under the photo resist or other problems occur.
You might even be able to do this with thin plastic (like transparency sheeting?) - but I am not sure how well it would stand up to the etching chemicals, etc.
This is all a very "blue sky" idea; I was just trying to think of a way you could make the lines thin enough so they aren't a distraction on the LCD, and do it in a "homebrew" way...
I think doing it in a homebrew way would mean that the wires/tracks would be thick or they would not be thick enough and would break really easily.
Yes, LCD displays usually have a metal back plate to hold it all together, if you are talking about colour displays.
The metal does not normally do anything apart from hold it together so you could remove it and hold it together a different way. I would experiment with your stripboard idea behind an LCD and see what you can come up with.
Cr0sh, I like that idea of getting a conductive-coated transparent medium. I'm trying to find some on the web some sort of conductive coated transparent film, which you could then photo-etch. I looked into that copper-track stuff but it's too thick for photo etching I think. If you could do that it would remove the need for the panel to be behind the LCD.
Mowcius, I've taken apart a couple of LCD screens (for making projectors) and the metal plate can be removed; the part I'm not sure about is whether the LCD panel itself will block the field.
Do you know how capacitive touch-screens are made in industry? How do they get the conductive material onto the glass?
I'm trying to find some on the web some sort of conductive coated transparent film
Hey, if you find something and it is fairly cheap (and easy to use), I would LOVE to know about it; I have been looking for a solution to salt water as an electrode in a copper-oxide solar cell. The best solution I came up with that was possible in a DIY fashion and wasn't insanely expensive (though still too expensive for anything big) was to homebrew a silvering solution (like for mirrors).
Unfortunately, such a solution isn't exactly safe or super easy to use for large surfaces (at least in the area I would be able to work in - if I had a larger shop...) - and disposal of the solution after use isn't easy, either (not sure if our hazardous waste program would take it or not; you can't run it down the drain, though). Ultimately, though, it is too pricey to use for larger areas.
I have this weird and probably unworkable "dream" to make screen-printed copper oxide solar cells using copper-oxide bearing anti-barnacle ship paint, an aluminum back plate, and a transparent but conductive front plate (or grid). If I ever find enough time to experiment with it (and I can find someone to give me a pint or less of the paint - it isn't cheap, and I couldn't use it up quickly anyhow), I would probably find it didn't work anyhow.
Funny you mention that because I was looking at the very same thing! The mirror silvering page I found talked about using copper sulphate solution and a zinc suspension to copper-plate something but I'm sure somewhere on the vastness of the web you can buy something pre-made. Also I found this research paper about ink-jet printing a circuit with a metal-salt ink onto OHP film then applying a reducing agent to create the metal layer. That would be extremely cool, but it is still in it's early stages.
I'm going to experiment with having the wire grid behind a the TFT screen itself and i'll let you know the results. If this works it opens the possibility of making 17" inch multi-touch screens for < £50! But it won't obviously.
I am currently trying to do very much the same thing. I attempted to build the grid of conductors and monitor it with an arduino(no capacitors connecting it to ground either), but my results were basically noise. Do you have pictures or a slightly more detailed description?
-Michael-
The clear conductor that is used for capacitive multi-touch LCDs, in things like the iPhone, is called Indium Tin Oxide (ITO). Its expensive and in fairly short supply on this planet.
-Michael-
Um well, I never got past soldering some strip board to some wires going to the arduino to be honest. It seemed to work ok though. Maybe you need to change the resistance to make it less sensitive?
