Sensitive Proximity Sensors

I just got my first arduino about a week ago, and have some basic code running.

End result- I want 20+ proximity sensors that are sensitive enough to detect through a sheet of glass the presence of a plastic cup filled with a bit of liquid (Think beer pong table)

I've put together a makeshift capacitive sensor with a 4x4 piece of cardboard with aluminum foil on it. I've set up some code to read in the values it takes to charge/discharge, aka the capacitance (lol I THINK that's what it's called...). I've then hooked it up to an LED to vary brightness based on the closeness of an object (or the change in capacitance).

I've got all this working pretty well, and even included some code to work out smoothing it so it discards random spikes and drops in capacitance. The system works perfectly with something like a hand- turning the LED on when a hand is about 6 inches away from it and evenly increasing brightness to max by the time you are actually touching it. I've even put in editable constants for how much smoothing one wants, and average resting value and average close value, etc... (I'll upload it here when it's completely refined :P) *NOTE: This code is derived from code originally by Paul Badger(2007) that I found online. It has, however been drastically changed to fit my needs

But, that's beside the point- A cup filled with a little bit of water does NOT have the same effect on the sensor as a hand does.

Questions: A) How can I either make the sensor more sensitive so it can accurately detect the presence of a cup?/ How can I make the cup more "capacitive" (?) so the sensor can easily detect it?

B) Is there another sensor I could consider, knowing I have a rather tight budget? (However, information on the best possible sensor regardless of price might still be somewhat useful information...)

C) Is there any way I could refine the sensor EVEN FURTHER to detect the presence of a ping-pong ball in the cup as opposed to no ball?

Restrictions: A) Price B) Would like to (if possible) not require any modifications to cups/balls, as we would like them to be easily replaceable and not have to worry about their destruction C) Pressure sensors/buttons/switches/etc... are out of the question. They would require the compression of the sensor, which would require a moving part, which would require some kind of hole in the table, which would allow for spilled beer to get in between the cracks, which could mess up the electronics. (I would like to have the top be one solid sheet of glass) If I am wrong in this description, any help on that would also be appreciated :P

I think that's it! Please help me figure this out/point me in the right direction ;)

Please? :-[

What you have is a passive capacitive sensor. While it might work with big blobs of water (bodies) it is not going to work with little lumps like your cup. Assuming that you want to stick with the capacitive effect for sensing then you need an active sensor. Basically this is an oscillator who's frequency is dependent on your capacitive sensor, any LC or RC oscillator will do for this. Then you look and see if the frequency has changed. You can get commercial sensors of this sort but they are not cheap:- http://uk.farnell.com/jsp/search/browse.jsp?N=500006+1004302+5449022&Ntk=gensearch_001&Ntt=proximity+sensor&Ntx=mode+matchallpartial&No=0&getResults=true&appliedparametrics=true&locale=en_UK&catalogId=&prevNValues=500006+1004302&filtersHidden=false&appliedHidden=false&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D500006%2B1004302%26Ntk%3Dgensearch_001%26Ntt%3Dproximity%2Bsensor%26Ntx%3Dmode%2Bmatchallpartial%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26catalogId%3D%26prevNValues%3D500006%2B1004302

to detect the presence of a ping-pong ball in the cup as opposed to no ball

You must be joking aren't you.

Yeah, those sensors are definitely out of my price range… :frowning:

But about the ability to detect blobs of water- that’s all I need it to do! I don’t need it to be able to detect an empty cup, there is either going to be a cup partially filled with water there to be sensed, or no cup at all. The problem is that in tests with cups filled with water, it still only changes the value I’m detecting by one or two, with a little bit of fluctuation… so not that reliable.
Is there any way I can just up the sensitivity of the sensor?

Lol and no, I wasn’t joking. I understand it might not be realistic, but hey, it can’t hurt to ask, right? ;D

What about Hall Effect sensors? This would require having a magnet in the base of each cup but it would work. Also parts are $.50 to a couple bucks US.

Is there any way I can just up the sensitivity of the sensor?

I told you how to do it, use an active sensor. Try reading about capacitive sensors and get to learn how they work.

The problem with hall sensors is that then generally need to be closer than the thickness of the glass, but with a super strong magnet you might just swing it.

generally need to be closer than the thickness of the glass

Did not know that. Thought they would work at least an inch away.

But...

http://www.amazon.com/Applied-Magnets-Strongest-Neodymium-Magnet/dp/B0012AWV9A

;D

With those a 1mm thick magnet should have a range of about 7.6mm assuming you need a magnetic flux density of 5mT to trip the switch.