I have an urgent problem, as my project submission is due in two weeks, and I am unable to resolve this issue. I am conducting a bending test within a press and have chosen a quarter-bridge strain gauge configuration for measurement. My setup consists of:
I am trying to capture the voltage variations from the strain gauge, amplify them, and convert them to digital values using the HX711. For testing purposes, I am not using an actual strain gauge but instead three resistors in series, which total 350Ω, just like the strain gauge would. I have carefully balanced the bridge, and the differential voltage is close to 0V, with only minor deviations (~1mV).
However, my HX711 outputs fluctuating values, which make the measurements highly unreliable. I have read multiple times that the HX711 typically has deviations in the double-digit range, but in my case, the fluctuations are in the five-digit range, making the measurements extremely inaccurate.
What I have tried so far
Shielding & EMI Reduction:
I have connected the YLW output to ground as it is supposed to reduce electromagnetic interference (EMI), which has somewhat minimized fluctuations, but they are still significantly high.
Testing Different HX711 Modules:
I have tried several cheap green HX711 modules from China – they were terrible.
The red SparkFun HX711 modules are noticeably better, but they still produce unstable readings.
Verifying Bridge Voltage:
The bridge voltage is stable, and the differential output is near 0V with ~1mV drift, so the instability must be happening inside the HX711 or due to external noise.
Request for Help
I am running out of time and getting desperate, as I need to submit my project soon. Does anyone have any idea how to fix this issue?
As a token of appreciation, I am willing to send €10 via PayPal to anyone who provides a working solution that completely resolves the problem.
I have attached a graph/image of the fluctuating values for reference and more. Any help would be greatly appreciated!
Your wiring between the HX711 and the resistors and strain gauge is a bit of a mess. It will be prone to electrical noise.
Also the signal will change with temperature as the resistance of the resistors will change differently to each other and the strain gauge.
For best results, you need to use a classic 4-wire strain gauge, which is actually four strain gauges connected together in bridge formation. One gauge works in compression, one in extension, and the other two provide temperature compensation. Keep the four wires twisted together as much as possible, to minimise the “loop area”.
Also, why do you have one end of the potentiometer grounded?
I see that you are using it to trim the resistance in one quarter of the bridge. But connecting one end to ground in your circuit is the wrong thing to do. You can only do that if the resistor + potentiometer is at the “bottom” of the bridge, providing the connection to ground.
Try moving the bottom end of that black ground wire, so it connects to the blue wire instead.
You want to eliminate potential noise.
As PaulZC mentioned.
Not only is the wiring prone to noise but your whole breadboard layout is too.
Get a pack of 10 small proto pcb’s solder it up (Short lead lengths).
Don’t be afraid of junking a few of the PCB’s and resistors etc.
They are cheap and your time is not (2 wks).
Also try powering from a battery.
One of the cleanest sources of power and useful when trying to eliminate potential noise.
Handy tip.
You can often use machine pins on your proto pcb to mount the potentiometer or other components you don’t want to damage by reusing them on a new design.
Hi thank you very much for answering. When I connect the bottom of the black wire to the blue one where do I have to put the yellow one then? Do I connect both of them to the blue or shall I remove the black one completely?
Still way too big, I’d chop one of those PCB’s into four.
I’d investigate this more “This version of the SparkFun Load Cell Amplifier features a few changes that you specifically asked for! We have separated the analog and digital supply”
The wire leads are still far too long. They make excellent antennas for picking up electrical noise from fluorescent lights, household wiring, nearby computers, etc.
