SparkFun Load Sensor Combinator (Ver. 1.0)

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.

  • [Schematic](https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/SparkFun Load Sensor Combinator v10.pdf)
  • [Eagle Files](https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/SparkFun Load Sensor Combinator v10.zip)
  • Hookup Guide
  • Getting Started with Load Cells
  • GitHub

SparkFun Load Sensor Combinator (Ver. 1.0) Product Help and Resources

Getting Started with Load Cells

June 11, 2015

A tutorial defining what a load cell is and how to use one.

OpenScale Applications and Hookup Guide

July 22, 2016

OpenScale allows you to have a permanent scale for industrial and biological applications. Learn how to use the OpenScale board to read and configure load cells.

Load Cell Amplifier HX711 Breakout Hookup Guide

July 22, 2016

A hookup guide for the HX711 load cell amplifier breakout board

Is it time to go to the beer store?

Harold Pulcher (Microsoft MVP)

Seems I can never figure out how much beer is left in the keg. Time to let the IoT have a crack at this.

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Comments

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  • Someone can help me? I am using this combinator with 4 Tal220 Load Cell and a HX711 ADC Amp, but when i run the code with nothing on the scale, and the read value is constantly increasing ( starts 0.00 lb, up to 0.01, etc), but i'm not understanding why this is happening. Is supposed the center tap be S+(Green in my load cell) or S- (White), or doesn't make any difference?

  • When wiring the sensors, are the non-center tap wires specific to B or R? If so, what is the correct wiring. The hookup guide really only mentions the center-tap wire specifically.

    • The hookup guide has a typo, and the center-taps from your load sensors should be connected to "R" on the combinator (not "W" as specified in the guide). "B" and "W" don't matter as long as they're the same for each load sensor, ie. "B" is all constant resistance and "W" is all variable resistance or vice versa.

      Hopefully that saves someone else from having to reverse engineer the circuit after soldering it incorrectly the first time.

      • You are right. The trick in hooking the 8 variable resistances of 4 half-bridge load cells (like the ones in bathroom scales, and the ones Sparkfun sells) up as a 4-resistance wheatstone bridge is to identify the center taps, wire the non-center-taps into a big ring with matching colors, and then excite with two opposite center taps and sense on the other two center taps. The wheatstone bridge then has two tension (or compression) resistances on each leg, the two tension legs (four tension resistances) are on opposite diagonals, and the two compression legs (four compression resistances) are on the other opposite diagonal. The taps of the wheatstone bridge are the center-taps of the 3-wire load cells.

  • Does the combinator board require four load cells in total? Can it work with just two?

    Also: for the Sparkfun straight bar load cells - what happens to the green wire? Does it just float?

    • With two half-bridge, three-wire load cells, you don't need the combinator. Identify the center taps and hook them up as S+/S-, and wire the remaining wires to their opposite colors (e.g. white to black, and black to white) and hook them up to E+/E-. The color-flipping puts the tension resistances on opposite diagonals of the bridge, so adding tension to them unbalances the bridge constructively. If you wire it up with matching colors, you get it backwards, and two half-bridges with the similar loads will balance each other out.

      The four-wire load cell is two separate gauges, you can make a four-wire cell into a three wire half-bridge by hooking one wire from each pair together to make a common terminal.

  • just in case people get as confused as I was, in the cheapo bathroom scale i bought, the load cell wires were red, white, and blue. in this case, blue = black, red = white, white = red. should save a couple hours.

  • Hey there, is there a diagram that you can point me to that shows a schematic of how the four, three wire load cells tie together to form a Wheatstone bridge?

  • Oh Sparkfun, why is this only 3 cells?!? The Wii Fit Balance Board has 4 cells :(

    I know that it only needs 3, but it would have been such a nice form-fitting retrofit project!

    edit: I get it now. Was looking at it the wrong way! Load cells have 3 wires each, not 4...

    • This product looks so simple but when we started talking about it around the office lots and lots of people got confused. Thanks for posting and coming back to post your find. This board is really powerful once you understand that it's designed to work pretty much exclusively with pre-made scales, that it combines four load sensors (not load cells), and that your scale is often pretty far away from your Arduino (I need to combine these four things into one 4-pin wire). The Load Sensor Combinator comes in really handy once you've tried to build a scale or two.

Customer Reviews

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Reasonable results, but watch out for calibration

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.