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I'm trying to hook up the load cells from a Wii fit balance board to an esp8266 using the HX711 ADC. Ideally, a way using only one HX711 but, a hookup guide for two HX711 with two load cells apiece would be helpful as well.

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  • \$\begingroup\$ any updates so far? \$\endgroup\$ – goblingift Jan 8 at 13:01
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In short it can be directly connected to the ADC. If you want to remove errors involved in low frequency DC signals you will need a more complex analog front end. Things such excitation methods should be consider, ratiometric measurements, differential sourcing of your load cell to improve CMRR, source loading of your sensor, common and differential mode filtering, noise, 1/f. Just top of my head. It realy depends on what accuracy are you trying to achieve. If accuracy id not your concern, than just ignore all of the above mentioned. Oh snap forgot the sensitivity of your load cell is also an important factor that must be consider. Together with you scale resolution it determines the resolution required of your ADC.

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  • \$\begingroup\$ To clarify are you saying that I can just hook them all up in parallel to the HX711 and it will read the combined weight of the load cell just not with the best accuracy? \$\endgroup\$ – Bradley Nelson Apr 10 '18 at 15:12
  • \$\begingroup\$ When it comes to the desired accuracy a lot more variables come to play, like thermal noise, flicker noise, offset, drift. All of these influence your reading. But yes you can connect the load cell directly to the ADC. The input of ADC have high impedance and most come with PGA to match the dynamic range of the load to the ADC dynamic range. \$\endgroup\$ – Gianfranco Walrus Schwengle Apr 10 '18 at 15:27
  • \$\begingroup\$ He asked how you can connect all four full bridge load cells to a single HX711. You did not address the question. \$\endgroup\$ – Stephen305 Dec 19 '19 at 20:07
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Four 3-wire load cells can be combined into one 4-wire bridge using the following schematics: enter image description here

Here R#C and R#V represent constant and variable resistance inside each cell. W/B/R is one of the common color combinations for load cell wires, for +/-/C respectively. Your sensors can have different wires, of course. If that is the case, use multimeter to find the common wire. Sparkfun has convenient load sensor combinator board and very informative hookup guide for these.

Unfortunately, Wii Fit board uses 4-wire load cells that cannot be combined as above. You have two options here, either replace the cells and connect them as shown, or connect all 4 in parallel and hope that the load will not be too much for the voltage regulator.

Some other, more complex, options are to connect to the balance board via Bluetooth or hack the internal PCB to tap into I2C line. See here for related discussion.

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