I've seen this force plate from Vernier and it requires another USB interface device. They have an SDK that will they allow us to communicate with the force plate.
Is there something that I can create on my own that also measures weight? I think that I've seen a strain gauge thread on this forum.
There's this load sensor from Sparkfun (mentioned in thread) but it maxes out at 125lbs.
I essentially want to create a plate for someone to step onto and be able to measure their weight.
Like this? http://www.walmart.com/catalog/product.do?product_id=5740778
You could buy one and interface to its strain gage. Or even its measurement electronics.
If you are making a computer interface to a scale, try to stick to the SMA Serial Communication standard: http://www.scalemanufacturers.org/smastandards.htm Then your scale will be instantly compatible with all software.
You can use a Nintendo Wii Balance Board. It can be used to calculate weight, as this is what the Wii Fit games do.
You can connect to the balance board using Bluetooth HID.
You could use levers to reduce the load on the sensor. Take apart a typical mechanical/analog bathroom scale and you'll see that your entire weight is being carried by a small spring, with the help of some levers. I have a picture at home that I can upload if you need more explanation.
I'd design a platform that gives you some horizontal stability before looking for a sensor, because the platform may have some mechanical multiplier in it. If this is a one-off project, I'd re-use an existing mechanical scale and replace the spring with a different sensor, or a digital one and piggyback on the sensor or LCD readout.
Based on the discussion on Sparkfun (as found by following the link you provided) I understand that the general idea is to use more than one of these sensors to support your measuring platform, so that they share the load. For example, you could put one of these sensors at each of the four corners of a plate and you would be able to measure up to 200 kg/500 lbs.
There is some discussion on this topic at this question: How to wire up a 3-wire load cell/strain guage and an amplifier?
In my answer I discuss a project I built using a gutted bathroom scale and a Stellaris LM3S6965 evaluation kit to give the scale a web server that could be used to monitor the weight remotely. Since you don't really describe your project goals I don't know if that will be useful or not, but hacking a bathroom scale is certainly an option and it will likely be a lot cheaper and easier than building something with pieces you buy from the various hobby vendors, especially since their stuff always seems to have a very limited range while a bathroom scale is already designed to weigh people. From your description I could conclude that you should just buy a bathroom scale and weigh people and write the results down on a piece of paper but I assume you have something in mind that requires customize hardware and software or you wouldn't be asking.
There isn't a sparsity of sensors you could use; clearly mass-balance scales fill out a myriad of well documented critical design, leaving the choice of sensor up to your ingenuity and your subjects' (should they decide to form a human pyramid or play Twister as they shuffle over your foyer.) If you pick a fiber laser spectrometer to sense weight though, the data should be worth the trouble...is it worth knowing who came in wearing Farragamo v. Jimmy Choos?
