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Cheap, Small LED or LCD Touch Sensitive Screens?
emf2268 asks: "I'm looking to either purchase or build (I'll do the circuitry myself if I have to) several dozen, small screens for an arcade game that uses a touch interface. Each screen, which should be around 6-10 inches, needn't be extremely advanced in the display department, since 16 colors will do just fine. An LED or LCD would do the job. But each screen also needs to be touch sensitive...it only needs to know if it's been touched, not where it was touched. How, can I build this as cheaply as possible?"
the cheapest way to do this is to probably have a small (plexi)glass pane over the cheapest display you can find and have the glass pane be resting on a bump switch(sensor) that has enough tension to hold the glass up but will register when it is touched.
What I have done in the past is to buy a bunch of old Windows CE.NET devices off of Ebay and use their touchscreens. For some (like the Aquapad) you can even get Linux drivers - search for Midori Linux and Aquapad.
Its certainly cheaper than trying to source new screens.
Punch Punch Revolution?
If all you need is to detect a press anywhere on the screen without regard for where it was touched, just get small LCD display guts and float them on spring loaded contact switches and wire that switch up to some input.
"The great thing about multitasking is that several things can go wrong at once." -me
Unless your game is Sim Paint-Drying, I think you'd be better to have an actual switch to press rather than have the player press the screen. Excited humans don't know their own strength, and I've seen industrial coin-op joystick that were yanked out by the roots.
One line blog. I hear that they're called Twitters now.
If I were trying to do this on the real cheap, I'd use conductive bags. Cut a square of the material, place it backwards (inside out) over a plexi backer, with the LEDs behind it. A non-conductive) bezel over the front to keep all in place. Connect the conductive inner layer of the bag material to a high impedance circuit with a spring contact and sense the "body antenna effect" (as on the old microwave ovens),
It's much cheaper than metallized glass. But maybe you want to go that way.
Back in the day, we used to see articles for homebrew touch screens that were basically picture frames with a few infrared LEDs on one side, and some phototransistors on the other with a bit of circuitry. Your finger would break a beam and get detected.
For a tiny production run, this might be an acceptable method. (For a *real* product, you'll want something better.)
Another thought: If you can get a conductive transparent plastic sheet, then you could make a sandwich where one sheet is against the screen, and the other is floating a small distance away. Then you can just detect the conductivity change when someone pokes the top sheet enough to make it touch the bottom sheet.
"Time flies like an arrow, fruit flies like a banana" --Karl or Groucho, I forget...
Cheap, small, and good. Pick any two.
It turns out you can use LED arrays as both displays and sensors. People have successfully used them for touch sensor controllers. See this blog for some experiments. The original concept came out of the NYU media lab.
Resistive touch panels (not including the screen) are incredibly simple and cheap... They can register position, but if you don't care, you can still use them to see if they've been pressed. Get some palm-pilot touch panels, peel off the overlay on the back for the graffiti section, and you have a nice ~4 inch touch sensor. Buy custom sizes from a place like 3m or digikey, or get old palm pilot touch panels (no screen, just the touch part) for $4 each from http://www.halted.com/ Look up how they work... they basically act as a voltage divider when pressed... a simple comparator circuit is all that you need to register a press, and it's much more elegant than resting the screen on a push switch... -Taylor
Worldwide Military budgets: $2100 billion. Worldwide Space Exploration budgets: $38 billion. Really, world? Really?
just get an optimus keyboard, they have lots of OLEDs and it's touch sensitive since it's a keyboard. http://www.artlebedev.com/portfolio/optimus/ oh, you may have to wait a little while for the release of this product, the prototype will be ready any day now...can't wait to use it to play DN Forever.
earthlcd has been a savior on many a project
http://store.earthlcd.com/
if you really just need on/off - do the spring loaded, or piezo, or even IR (www.acroname.com - sharp ir sensors)
if anyone has any other leads on cheap overstock LCDs... post away!!!
(1) capacitive switches -- inexpensive, digital (on/off)
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m e3/strain.html
This reverses the idea of protecting a high-impedance circuit from stray capacitance introduced by a limb or finger -- turn your accidental 'people detector' into just that:
http://www.discovercircuits.com/C/capacitance-sw.
(2) frustrated internal reflection -- not necessarily cheap (needs a camera), or easy (needs video analysis) but can handle multi-touch and large screens
The idea is to shine light in from the side of a class or plastic screen, and have a camera look at the backscatter introduced by finger contact, which scatters the sidways light rather than allow it to reflect at the bounary. The camera turns the touch events into a video stream which is then analyzed to compute touch events.
http://mrl.nyu.edu/~jhan/ftirsense/index.html
(3) strain gages -- not necessarily cheap ($5 - $8 for each strain gage) but can provide a very sensitive analog signal with wide dynamic range.
Put some strain gages on several centimeters of half-inch square steel tube and you can easily measure touch events, as well as strong pushes.
http://www.omega.com/literature/transactions/volu
Here is a simple, inexpensive (if you make it yourself) amplifier for strain gages that I've tried, and can vouch that it works well:
http://www.staramp.com/
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I found this neato LED interface the other day from hackaday
Some more links to projects like this can be found on the story on hackaday.
in girum imus nocte et consumimur igni
LEDs are photodiodes too! http://mrl.nyu.edu/~jhan/ledtouch/index.html
Why several dozen for an arcade game? Will this be used as a prototype menu interface, or are kids going to be thumping on this thing with rubber mallets at the museum of science?
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