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The SparkFun Load Sensor Combinator is a bare PCB that combines four load sensors into a standard 4-wire wheatstone configuration. If you open up an electronic bathroom scale you’ll find a large rats nest of wires. The Load Sensor Combinator was created to combine the 12 wires found in a bathroom scale into the standard 4-wire wheatstone bridge configuration.
You can either use four individual load sensors or simply purchase an off-the-shelf bathroom scale and hacking the combinator into it rather than trying to design a base to properly mount four load sensors.
This board works great with our Load Cell Amplifier breakout board; the five pins on the edge of the combinator line up directly to the five pins on the amplifier.
If your amplifier and supporting electronics are more than a few inches away from the scale an RJ45 footprint is provided. The four wheatstone pins (E+/E-/S+/S-) as well as the shield pin are connected to twisted pairs within a standard cheap Ethernet cable. This allows the amplifier board to be placed many feet away from the scale itself.
The combinator board also includes a footprint for the DS18B20 one wire temperature sensor. This allows the user to gather the temperature of the scale in case there is a large variance between the scale and the amplifier. These three pins are accessed through the RJ45 connection as well allowing remote temperature readings to be gathered over one twisted pair Ethernet cable.
This skill defines how difficult the soldering is on a particular product. It might be a couple simple solder joints, or require special reflow tools.
Skill Level: Rookie - The number of pins increases, and you will have to determine polarity of components and some of the components might be a bit trickier or close together. You might need solder wick or flux.
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I made a dog-bed weight scale from 4 load sensors, this board, a load cell amplifier, and an Arduino. It worked the first time I turned it on, and gave reasonable results.
Once I started calibrating, I noticed one thing: because each load sensor requires a slightly different calibration number, and because calibration happens at the Load Cell Amplifier, the calibration won't be as accurate as it could be. Instead of a linear relationship to weight, the Amplifier output is a messy polynomial.
That said, without doing anything fancy, I'm weighing a 40 pound weight with +/- 1/3 lb error (18Kg +/- 0.151Kg). That accuracy will probably work fine for many uses.
I'm exploring using each load sensor in a wheatstone bridge connected to its own Load Cell Amplifier, so I can a) get more accurate results, especially as the dog leans on one side of the scale, and b) calculate the dog's center of gravity (position) on the scale so I can see her flopping about in the night.