I want to track the mass of a hanging window farm project. It seems like I should be able to get some sort of load cell device to interface with my arduino. What is the simplest way to do this?
If you want ease of implementation, value for money, and good accuracy - then a cheap commercial 2 kg scales interfaced by whatever means is possible is liable to seve you well.
You can buy load cells and instrumentation amplifiers and roll your own, but the Chinese already make just what you want by the millions at a price which is liable to be far less than you can buy the parts for new.
The best (and in some cases also the cheapest) kitchen scales use a "real" load cell with 4 strain gauges in a bridge plus hopefully a 5th temperature compensating strain gauge (places on the load cell where stress/strain is caused only by temperature changes. The photo below shows a typical load cell used in low cost scales.
These can be converted to "hanging" operation either by removing the load cell from the scales body and custom mounting it (not very hard)
or by placing the scales on a beam and using a "question mark" shaped stiff wire to convey the force (weight) out around the scale body and onto the pan. I use a similar arrangement with the scales described below to suspend objects for weighing in an out of water for 'Archimedes' density measurements with good success.
Performance oc cheap kitchen scales varies from not-overly-marvellous through to unbelievably superb.I bought some end of line 2 kg kitchen scales here a few years ago for about $10 each, that were so good that I went back and bought the rest that were available - about 10 sets.
Mine looked very like these. Similar appearance no guarantee.
These are now about 'used up' and hopefully some equally good and cheap ones will turn up shortly.
Factors which relate are accuracy, linearity, repeatability and temperature independence. The first 3 are close travelling companions but not identical.
Obtain several hundred one and two cent coins. These will probably weigh close to 1.000 gram and 2.000 gram each. If not that then some other fixed mass. This is partially so banks can check amounts by weighing. They are amongst the best value for money calibration wights you can but. Make a few larger test weights by using eg coins to calibrate them. Say 100 200 400 800 gram would be easy. These can be made out of almost anything stable. Even eg plastic screwtop jars with water in - as long as they are airtight.
My scales would track linearly for any number of coins aded or removed and whether removed or added one by one or N at a time. Superb. Some scales are not so good.
Some scales are poorly temperature compensated. Mine can have a hairdrier waved over them on high until they are toasty hot (50C plus) with little of no display deviation. Superb.
Once you have found ones which are accurate enough for you, you nee to interface them to the Arduino. Most will have an anlog voltage(amplified strain gauge signal) which is converted to digital by the display controller. Signal level should be in the volt or few range - easy for an Arduino ADC to measure.
If you can't get at the analog signal you may be able to access the controller at the digital stage. Long ago people have gone so far as decoding 7 segmont multiplexed display signals for processor interface - but if it's that hard then finding ones with analog signal access would beeasier.
Some may use a V to F (Voltage to frequent) converter. These are easy to read by either pulse counting in finite time or by seeing how long it takes to make N clock cycles.
Since it's a hanging system you're going to want to use something like a stretch sensor:
A load cell wouldn't work to well in this particular case. You can calibrate the stretch sensor using a weight calibration kit to map the sensor values.
I wrote a comment "how about using accelerometer?" I was curious, googled and found out many smart people before me found the answer. You can use a simple spring and an accelerometer.
This message board discussion has the data.
What they don't say though, you need to calibrate your system since the spring constant will probably change over time. So, if you can have a known weight and do a calibration mode once in a while, this could be very low cost measurement system.