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.
Electrical – 4 full bridge load-cells with the HX711
esp8266load cellwii
Related Solutions
In order for the bridge to produce an output between S- and S+ the bridge must become "unbalanced" when weight is applied. So if all 4 load cells produce equally increasing or decreasing resistances when weight is applied the bridge will remain balanced and the output will not change.
If the load cells are arranged such that the left and right halves of the bridge move in opposite directions, then the difference between S- and S+ will increase when weight is applied.
It's likely that on each corner of the scales the cells are half-bridges not quarter bridges. The only 3-wire quarter bridges I've seen are for when trying to cancel long-wire volt drops.
You wire the four half-bridge load cells like this: -
But you have to take care you get the wiring polarity correct or you can end up with weight cancellation and no signal. You just have to experiment a little but that's the basic wiring.
For the first diagram you showed, each half-bridge is excited in parallel with the other half-bridges and two half bridges are paralleled for each input. It does the same as the above but is less sensitive (or put the other way has a larger full-load capacity).
As for amplifying the signal I'd use an instrumentation amp (inamp) like this: -
It's basically 3 op-amps connected as below: -
There are plenty of options and optimization or best choice of inamp relies on knowing the power supply you have available for your project.
Best Answer
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.