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livinded

what kind of sensor

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I'm currently building a new beer pong table and planning on making a scoreboard for it that will automatically set the score based on the presence of a cup in the designated locations (where the sensors would be). I want to use plexi glass for the top to keep drinks and water from soaking into the actual table and was planning on cutting holes in the plexi glass for depressions for where the cups will go. I'm trying to figure out the best way to handle determining the whether or not a cup is there. I was thinking about using weight or pressure sensors but I don't know how well they will work if located under plexi glass or some other material to protect them since the table will get wet and the weight really isn't that much. The other idea I had was using a photoresistor, but since the area can be either extremely light (during the day) or fairly dark (at night) I don't know if it will be sensitive enough to use at night time to distinguish it being dark and a cup on top of it. I'm planning on using an atmel avr for handling all the I/O and controlling it but looking for advice on what type of sensor to use.

Edited by livinded
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I think the easiest way would be to put a magnet on the bottom of the cups and use a hall effect sensor or reed switch. The plexiglass would have to be pretty thin, or the magnets pretty strong though. And it wouldn't work with any old cup.

Photoresistors will work, but it'll be tricky. You'll need to "set" the system at the ambient light of the room. If the light level drops more than X or to Y between any two samples, there is a cup there. To prevent any shadows from triggering it, you'll need to set a pretty hefty drop in light. Also, the cups will have to be opaque.

I think you'll have a lot of trouble with piezo elements used as pressure sensors. You'll have to distribute the weight from the cup evenly over the sensor, and you'll have the same problem as with the photoresistors.

Needless to say, you need to order up a bunch of these.

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I think I'll try prototyping with the photo resistors because it's going to be the cheapest and easiest solution if I can get it working. The magnets just aren't going to happen because we use disposable opaque cups and it's going to be a pain to be constantly putting magnets on them.

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That's commonly referred to as a photo-interrupter, and probably won't work here. You need something to block the IR from the LED to the phototransistor (labeled as "baffle" in the diagram). What could do that in this situation?

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That could work, depending on how reflective the cups and plexiglass are to IR. Though, it's essentially the same as the photoresistor only more complicated and expensive. Remember he's going to need at least 12 of these (depending on how they play beer pong). Those are $2.50 each, you could probably get all 12 photoresistors for $2.50.

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Don't know how well this would work.. but here's my two cents...

What if you mounted little speakers, or condenser microphones, whatever you can get enough of the cheapest, in the board?

Alot of plastic cups have that raised bottom, or lip. Could that be used as a resonance chamber? When the ball lands in the cup it's gotta be kinda 'loud' down there. Wire the speakers/condenser-mics up as inputs and watch for when one's signal is significantly larger then the others.

It has problems... esp in a loud party environment. I do like the IR idea, but would that require clear cups?

-If I had a ping pong ball I'd do some research.. Someone with the tools, please lay down on your side with a cup sitting on your ear and have someone drop a ball in the cup. ;)

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That's a terrible idea. The kind of analysis needed to distinguish a cup being over the microphone and room noise would be prohibitive. Though what you're describing is something like an ultrasonic range finder. That would work, but it would be too expensive and way too complicated for this application.

No, the cups would have to be opaque or otherwise reflect IR light. The point is that the IR LED and phototransistor are both beneath the cup, and that the cup reflects a certain amount of IR light back to the phototransistor if there is a cup there. But using this also has the added complexity of IR in the environment, either from the other emitters (IR likes to bounce around a lot), lights or sunlight.

The photoresistor will probably be the simplest of the solutions. It's dead simple to hook up to a microcontroller, cheap and should be able to detect the cups reliably as long as they're opaque. Even the translucent "foggy" cups should work, but not the transparent cups. The colored opaque cups would work best.

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We always use opaque cups (red or blue). I'm thinking photo resistors will be the easiest, cheapest, and possibly best solution for the materials and cost available assuming they are sensitive enough to distinguish between nighttime darkness and a cup being over it.

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I guess I played a different version of the game. I just checked the wiki, and it seems there are many variants to the rules.

OP - For clarity, please define your table setup.

I was thinking the cups were empty. When a ball landed into a cup, it compresses the air space between the bottom of the cup and the top of the table, causing the voice coil to move in the speaker. It goes to a simple bank of analog sample and hold circuits (diode and a cap) long enough until the uC polls for voltage with it's ADC's (a couple of ms) . When one is significantly higher then all others a score is reported -- I thought the inverse square law would make it easy to rule out room noise...

... my bad.

edit..

But if the cups have beer in them, and they are emptied upon being observed with a ball in them.. Why not just use a pushbutton switch and check to see if the cup is lifted?? a cup of beer surely is heavy enough to hold down a push button.

Edited by PurpleJesus
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What about having a metal circle on the bottom of the cup that makes a connection with a broken circle on the table. It would being connected when the cup is on the table and disconnected when the cup is removed. You would also have the added advantage of not having to cut any large cup-shaped holes, just mounting (gluing) a circle (or any other shape really) to the bottom of the cup. It's kind of like a giant game controller.

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That requires modifying the cups, which he already said he didn't want to do. Also, you have to account for spillage which you know is going to happen. Is beer conductive? Is it going to short out these metal rings? Is beer corrosive? Will it dry up and accumulate? Will the metal rings take the punishment this table is going to receive?

Ideally, the top of the table should be flat plexiglass. In the bottom of the plexiglass, cut channels for wiring and indentations for the sensors. You can use paint or something to mark circles where the cups should go.

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I have no idea how to answer your question, but I wanted to chime and say that your idea is incredible. Good luck!

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Hey, have you thought of using something like a limit switch or a pressure switch? You could "hack" together on rather easily with just a normal button and a small piece of metal(or something) and a spring that keeps the metal(or whatever) off the button, but the weight of cup would be enough to push the button.

This solution would probly give less trouble than using IR, since you need to worry about false reading and IR pollution. Fluorsent bulbs tend to reak havok with some IR sensors.

...just my 2 cents!

best of luck livinded! sounds like a really fun idea!

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I'd use a photoresistor as well -- a cup covering it should block enough light to cause a noticeable change in resistance. Alternately, you could use a separate disc in the base of the hole on which the cup would rest, with a small plunger extending from its center. This plunger could activate a small switch, either directly or via lever, if the force of a partial cup of beer wasn't enough. Mechanical methods are probably less desirable though -- with our beer pong table, at least, there's usually enough spilled beer that it'd seep through any open gap in the surface.

For the IR solution, you could transmit a recognizable blink pattern from your infrared LED and have your control unit pick up on it. That way, ambient IR and stuff from TV remotes, etc, wouldn't mess with the sensor. You could even wire all of the IR LEDs to blink at a certain predetermined rate, even using just a 555 and a power transistor, and have your controller check to see that there's a certain blink frequency. This adds complexity, but in software rather than hardware.

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I need to wait until I get my paycheck, but I plan on starting to prototype out a design using photo resistors next month and will post results and possibly pictures. If everything works out I'll price it out and if I can do it affordably I will probably build it and make sure to take pics of the end result.

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