CrossRoads:
Sharp looking table.Sounds like you need something like one of these:
http://www.pololu.com/product/2464
http://www.pololu.com/product/2458
Pololu - QTR-L-1A Reflectance Sensor (2-Pack)
The top one is quite expensive if I need 300 of them and the second two say they operate at 6 mm distances (and so presumably wouldn't work through frosted glass at all well). I've been looking for IR sensors and how to connect them for some time now and although the connection methods you guys have suggested seem really promising unfortunately I've been unable to find suitable IR sensors. I've almost finished making a 12 by 24 RGB LED array with 100's of frames per second animations. Each pixel has cost me £0.31 ($0.52) which is pretty good. It seems like adding IR sensing to each pixel would cost around $5 per pixel which is a lot more (10x). I can't really justify that when I could just increase the RGB LED resolution by ten times instead of making the table interactive, for example. This is obviously quite regrettable because I've been trying to figure out a solution for quite some time. If any one has any ideas I'd be very grateful!
Grumpy_Mike:
Sorry late on this one but the sampling rate is way way over the top.
I think you only need to scan them at four times a second, you won't notice any faster response.
Always glad for more input! Yes, 4/sec is probably fast enough for most things I would do. The only time I could see needing more is for swiping gestures or perhaps something fast-paced like LED air hockey or ping pong! It's a compromise I would probably be happy with though. Thanks for pointing that out.
The backup plan was to use IR sensors with an ON or OFF digital output only like these. I could probably find them a little cheaper especially if I'm buying so many. I still think it might be too expensive but out of interest - how would I connect these? Is multiplexing the best idea then, because they don't need an ADC - I would just have 288 1-bit inputs. What's the best approach in this case? I'm genuinely interested even if I don't use them in the end.
I really appreciate all the responses and help and I have actually learnt quite a bit and will no doubt use many of these ideas in other ways even if I don't do analogue ADC/MUXing in this project.
Lastly, given that IR sensing isn't as good of a solution as I had hoped, does anyone know an alternative touching sensing method? These are the criteria: multi-touch, preferably able to detect different touch levels, cheap, easy to add to my already half-finished table, and has to work on a thin table with the RGB LED matrix I have already designed. For that reason - projector/camera options are out. I've found capacitive touch films you can add to glass but they cost $1000's. I can't see a way of using resistive touch either. That leaves SAW (surface acoustic wave) but I'm not sure that's easy or if it's multi-touch. Any ideas welcome so long as it's works out at around $0.50 per pixel (300 pixels = $150). Finally, it would only require 1 or 2 inch touch resolution!