Hi there, I wish to use Arduino to measure the weight of a non-moving object as it increases from ~18 pounds to ~20. The Arduino is programed to trip a relay off when the object reaches ~20 pounds. I used Force Sensitive Resistors, but the Force / Resistance curve is very flat after 5 or 10 pounds: other variables (noise) is greater than the weight induced resistance change. (used 4 FSR's in series, tried parallel, wheatstone bridge circuit, etc, no help.) Question - I'm looking for an interface that would be more linear in the 20 pound range. Any ideas, please? Thanks in advance!
hack up a bathroom scale ;D
You should probably look into a load cell of the appropriate range. A bathroom scale is one source of them, but you can also get them from Aercon Systems for $30 (http://aeroconsystems.com/cart/catalog/Load_Cells-12-1.html).
Interfacing is a little tricky, because they're wired as wheatstone bridges internally and the output is proportional to both applied load and applied voltage, and is on the order of millivolts. My company sells a $50 amplifier board (http://www.logos-electro.com/1x3-ana-ina125-1/) that takes care of all of that and puts out a nice analog voltage.
If it could be suspended, I'd hang it on a suitable spring and then rig up an optical sensor (IR LED & photo-transistor) at the end of the spring. When it increases in weight, the spring stretches and cuts off the optical path and Bobs your uncle......
Or if your goal is to measure package sizes while they fill up, make a classic balance scale with a proper counterweight. When the beam crosses a certain point, you have reached your 20pounds. Some old style packaging machines for powders work that way. The beam closes a contact which closes the funnel and send the bag off to the next station.
Have you tried using a current to voltage converter as an input preamp? The FSR handles up to 1mA / cm2 in quite a linear manner. I obtained FSRs which were 40mm x 40mm square (16cm2) giving a max loading of over 100kg before saturating.
The current to voltage converter is best done in an op-amp.