i would like to build a coffee table with a slab of white perspex on top and an array of RGB LED's underneath to shine through. The LED's will respond to items being placed on the coffee table or a person touching it.
However, how can this be done? What methods are there out there for making a touch sensitive surface? I don't need high resolution and don't want external devices (cameras', etc.) as i wan teh surface itself to detect touch.
Does anyone have any suggestions about how this could be done please? Like I say, resolution and accuracy is not important as it won't be runing an LCD screen or anything like that.
EvilMadScientists already has an analog (non-Arduino, non-digital) board that is roughly like this. It comes in 12"x12" boards or kits that can bus together, and you can put frosty or clear covers over them. However, the existing sensors are light/motion sensitive, not touch sensitive. I expect this could be changed. The grid of lights and sensors is pretty sparse to keep costs down, but you could adapt the design to be more digital and more dense as well.
If your not worried about resolution, Run some thin, wire across and along the table (maybe an inch or so apart) and sequentially measure the capacitance of each one. When someone touches the table the capacitance of the nearest wires will go up. Use this as a signal to switch on an LED. You'll probably been to work out some circuitry to expand the number or pins from the board. But other than that the coding is fairly simple.
The IR LED solution would not work with white acrylic or perspex I don't think. It would need a transparent tabletop.
I don't want to run wire across as this would be visible, no matter how fine it was and it wouldn't be possible to affix it to the tabletop without it being clearly visible or without it creating a bumb on the surface or attracting dirt. I want a nice, clean smooth white surface.
Any other ideas? This must have been done before I would guess.
I too think the capacitance change sensing method might have the most successful per buck costs. Might take a bunch of analog components to map out a grid of sensor plates and generate a variable voltage, but with say six sensor areas the six analog input pins could be used to read the area sensors and take actions to light or blink led arrays assigned to that area. Cool project idea.
Perspex gets scratced very easyli. I can see from your posts that estetichs and "good looking" is important. So i think you should consideer alternatives to perspex.
The capacitive sensor does not have to be on top of the perspex, it can be below the top layer. While they do work best with direct contact, there is a measureable effect just on close proximity. I've got a working version myself (only 1 sensor unfortunatly). So long as there's no metal above it and the gaps no more than a couple of mm it should still work. Will only trigger for 'wet' or conductive objects though. A cup won't set it off, a cat will.
It does get scratched reasonably easy but the scratches can also be buffed out easily (plus perspex is very cheap nowadays if it needed replacing). I guess a thin layer of glass over the top of the perspex would solve that problem.
The problem with capacitive sensors comes when you want to make many of them work in close proximity.
The biggest matrix capacitive sensor I have seen is for only 48 switches and using several of them is going to be tricky.
I have seen devices using the Frustrated Total Internal Reflection technique but this involves a camera looking through the glass from below which rules that out. However, I was wondering if an array of IR LED's and IR sensors along the edges of the glass would have tjhe same effect?
If your going to go with light beams, I'd go with laser diodes, they'll give you a lot better range and density. Though cost could quickly become a factor unless your could find a cheap supply.
an array of IR LED's and IR sensors along the edges of the glass
IR light does not travel through glass so it would have to be plastic. However the IR will disperse too quickly to prevent any great accuracy. So I would try lasers. However if you want to look at my project with an array of IR LEDs it is here:- http://www.thebox.myzen.co.uk/Hardware/The_Sound_Square.html
It's not a table but it is an array of IR LEDs and sensors.