[SOLVED] Need help with plate sensing a load

[qbone]

Hey guys. I am looking for a way to have a plate where I can put a glas or beer can and the measure wether something is on it.
At the moment I have made this with 4 push buttons in each corner of the plate, but this seems like a messy way of doing it.
Does any of you guys have an idea?
Bear in mind that it has to be able to sense the weight of full beer atleast

Now, I have look at FSR's but they are rather small, and if I need a decent size I fear it will amount to alot of money.

 

[jiripolivka]

Hey guys. I am looking for a way to have a plate where I can put a glas or beer can and the measure wether something is on it.
At the moment I have made this with 4 push buttons in each corner of the plate, but this seems like a messy way of doing it.
Does any of you guys have an idea?
Bear in mind that it has to be able to sense the weight of full beer atleast

Now, I have look at FSR's but they are rather small, and if I need a decent size I fear it will amount to alot of money.

Many shoe stores and similar workshops recently introduced floor mats connected via UPC to a computer, which sense pressure when you step on it. Fine details of pressure and plane coordinates are output for analysis.
Such mat with only limited sensing accuracy you can build from conductive foams, or, use a thin layer of foam to separate a grid of fine contacts. You must make holes in the foam layer to allow contacts to get together. I have once tested the black foam "boards" used to package semiconductor components, as a similar sensor. Some offer a resistivity of 100 Ohms to 10 kOhms/square, so you can again make a metal sheet on one foam side, and a grid of contacts on the other side, and make a suitable sensor for your beer glass...

 

[qbone]

Oh, nice, I did not know they came with different resitivity, but it is certanly something I have considered with foam, I was just wondering if there was some fancy way to sense something being put on a plate, like vibrations or something, cause it only need to sense when a beer is put down and when its liften again, a simple 1/0 signal

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Oh, I actually just stumbled upon this tutorial or what you wanna call it: http://www.nerdkits.com/videos/weighscale/
Where they are using the strain gauge from a bathroom weight.
I saw Sparkfun has the strain gauge for only 10$ https://www.sparkfun.com/products/10245 so I think I am gonna try playing around with that. This seems like a rather sturdy method aswell.
Unless ofc. someone comes with a brilliant idea

 

[jiripolivka]

You can possibly invent more ways to sense a glass on a plate. If your task is so simple, try a piezo sensor (from an old digital watch or a beeper) glued under a thin board which would bend under glass weight. The output from such piezo disk can be several mV DC, so you will need an opamp to e.g. light a LED, with some 100x or more gain.

Strain gauges can be used if you find a cheap one - again, their output is one mV or so, then you again need an opamp.

 

[qbone]

Hey jiripolivka
Sorry for not replying sooner.
Thanks for the suggestion, I will look into piezo discs. We just got a sample kit for some of those at my work

 

[betwixt]

I'm not sure bending a piezo disc will produce a usable voltage but you can use two discs, one fed with an ultasonic signal and the other picking it up. There would be a change in the level when the mass of the plate changed so it should be able to detect anything of reasonable size placed on it.

Brian.