HX711 and Strain Gauge HW debugging

OK, I understand it's necessary to post an image about that :smiley: (I've done with KiCad if you tell me a better tool I really appreciate that)

@DaveEvans
With a 3.3v microcontroller, you should use an HX711 (like SparkFun's...link below) that can use different VCC (5v) and VDD (3.3v).

I'm using ESP32 LilyGo-T-Call-SIM800 that it has on pin 16 a 5v output. It should be fine, Is it right?

@DaveEvans
The voltage between E- and ground should be zero, with and without the strain gauges. Since you measured a bit less than 5v, you may have a defective HX711.

I've measured more times and it's not always 5v (without the strain gauges) sometimes it's 0v. So I guess HX711 could be defective.

@DaveEvans
The voltage between E+ and ground should be about 4.3v, with and without the gauges, not "a bit less than 5v." You may have a defective unit.

I measured again and it's 4.94v without the gauges.

@DaveEvans
With the strain gauges properly wired, A+ and A- should be about half of 4.3v under no load.

I measured 0v.

Wawa
Some (green) HX711 boards have have no ground plane,

Yes, I have a green HX711.

@DrDiettrich
Do you want to form a Wheatstone bridge from 4 strain gauges?

Yes, I want to form a Wheatsone bridge with 4 strain gauges. Unfortunately, I can't read the value even if I am supposed to have linked it all correctly.

@DrDiettrich
That would never give absolute weight.

What does it mean? I don't understand with "absolute weight".

In the end, I guess my HX711 could be defective and I have not a wiring issue. Are you agree with me? If that is correct I'll buy sparkfun controller as suggested by @DaveEvans

. Regarding wiring: a "picture" is worth a 1000 words. Draw a schematic. And carefully read this website: Wheatstone Bridge Circuit | Strain Gauge | HBM

Thanks for sharing :+1:

Using a strain gauge like this

What is the correct configuration for my use case?

A simple strain gauge has 2 connections and is combined with 3 resistors to a Wheatstone bridge. Your strain gauges with 3 wires seem to be something different - a data sheet is required.

If you combine 4 strain gauges into a single Wheatstone bridge then 2 gauges changing their resistance in sync can leave the bridge voltage unaffected.

You should build a Wheatstone bridge from each strain gauge and use 4 channels (2 HX711) to measure 4 distinct values.

Hi,
Your strain gauges are already half of a wheatstone bridges.

This might help, I know it is a destructible project, but it uses the gauges you are.

This may also be of assistance;
https://circuitjournal.com/50kg-load-cells-with-HX711

Google;

arduino using multiple strain gauges

Tom... :smiley: :+1: :coffee: :australia:

No, as explained before, the HX711 will output 5volt logic with a common (Chinese) HX711 board, which could damage the I/O of your ESP32 board.

E+ should be 4.3volt with the bridge connected, and E- should be 0.0volt.
A- and A+ should be about half of E+.
As said, it could be a board without ground plane (E- floating).
You should post a link to where you have bought it, or a clear picture of both sides of the board.
Leo..

A note on nomenclature, since using the correct search terms will give better results:

This is a strain gauge:

Screenshot 2021-11-09 210529

This is one type of load cell (which has one or more strain gauges in it):

Screenshot 2021-11-09 210958

Neither type of device give weight (or "absolute weight", whatever that is) directly, but they can be used to determine weight, either by calculation (using principles of mechanics and the strain gauge properties) or by calibration.

:grinning:

Now I understand what you say...

Dave's post resolves my doubts...

Maybe easier to use a "Load Sensor Combinator" for four 3-wire load cells.
Leo..

Hi,

I've just checked the projects posted by @TomGeorge :+1:

  • Arduino Bathroom Scale With 50 Kg Load Cells and HX711 Amplifier
  • 50KG Load CElls with HX711

And thery are exactly the same project that I've done :yum: (The only difference I use ESP32 instead Arduino UNO) - they are using output 5v to supply HX711 and they are connecting the DATA input to Arduino UNO as me.

I little bit confuse :roll_eyes: when @Wawa said

No, as explained before, the HX711 will output 5volt logic with a common (Chinese) HX711 board, which could damage the I/O of your ESP32 board

I don't believe there is a difference between me and them.

  • Could you confirm if that is I supposed it's right? Or there is a difference?

You should post a link to where you have bought it, or a clear picture of both sides of the board.

@Wawa I've bought my load cell here on Amazon

Based on the voltage measured on HX711 controller I think it's clear there's something wrong on it.

  • Are you agree with?
  • What should I do as next step? Should I buy a new HX711 by SparkFun? Should I buy Load Combinator as reported by @Wawa?

PS: I'll be more precise after DaveEvans's explanation...thanks a lot :grinning: :ok_hand: :+1:

  • Yes, there is a difference between your project and those other projects.
  • Yes, you should buy the SparkFun HX711.
  • Yes, you should buy the SparkFun combinator, too.

Because...

As mentioned or implied multiple times already, your ESP32 operates at 3.3 volts and the UNO used in the other projects operates at 5 volts. If you use the standard HX711, it will send 5 volt signals to your ESP32 and probably damage it. The SparkFun HX711 can (and in your case MUST) be configured to send 3.3 volt signals to the ESP32. The combinator is not required...just makes the wiring easy.

If you power the HX711 with 5volt, then the chip will obviously output 5volt signals.
Not very good to shove that 5volt logic into the inputs of a 3.3volt processor that might not be 5volt tolerant.

You must however power the analogue part of the HX711 with 5volt, because it's 4.3volt voltage regulator for the load cell needs it.

The designers of the chip have anticipated this problem, so they have split up the supplies for the analogue and digital parts of the chip. So you can power the analogue (load cell) part of the chip with 5volt, and the digital part (that connects to the ESP32) with 3.3volt.
Chinese designers didn't bother to bring out those two supplies, and just joined them onto one power pin (cheap).

Sparkfun did the right thing, and made the two separate chip supplies available to the user.
So if you are using that Sparkfun board with a 5volt processor, then power AVDD and DVDD both with 5volt.
But if you're using it with a 3.3volt processor, then power AVDD with 5volt and DVDD with 3.3volt.

If you can solder, then you could modify your green board to the Sparkfun standard.
The circuit diagram is on their website.

Hold your green board against a strong light. If you can see the light through it, then it's the board with E- not connected to ground. You must correct this (ground E-) for the board to work.
Only then will E+ be 4.3volt and E- 0volt.

A combinator is just an easier way to connect the four 3-wire load cells.
You can do that also yourself. The diagram is also on the sparkfun site.
Leo..

OK! :ok_hand:

Now it's everything clear!

Thank you so much for your patience. :pray:

Amazing :smiley:

Unfortunately, It's a little bit complex for me to modify my HX711 as sparkfun circuit diagram. Now I'm going to buy HX711 Sparkfun load cell to finish my job.

Anyway I have a curiosity:

Could it make a sense put a voltage divider between green HX711 and ESP32 input in order to have a 3.3v?

There is two-way communication between the HX711 and the ESP.
You could lower the 5volt data line from the HX711 to the ESP with a 1k:2k voltage divider.
But it's not easy to increase the 3.3volt clock from the ESP to the HX711 to 5volt.
Leo..

Does it make a sense put a pullup on SCK?

The pullup resistor between SCK and 5v connected to Hx711 VCC

PS: Probably it's better to do something like this. What do you think about it?

Waste of time.
If you want to tinker, then modify the board to Sparkfun's standard.
It's not as hard as you think, assuming you have basic soldering skills.
Leo..

I don't have solid soldering skills but I don't fear it :grinning:

Following your advices I've already bought sparkfun load cell amplifier and until it will not arrive I'm without work :weary:

@Wawa Could you help me sharing something to adapt my hx711, please? I've googled and I found this article but you are not agree with that..

Not a good way.
Lowering excitation voltage is degrading the performance of the setup.

If you don't have any soldering skills, then wait for the Sparkfun board.
Because modifying that green board includes de-soldering and lifting pin1 of the HX711.
Leo..

Also, if the photo in the OP's Amazon link is accurate (and the OP's voltage measurements are accurate), the OP's green board may be another one of those with floating E- (as is the board in the honeybee hive monitoring project).

OP's board from Amazon:

Honeybee board:

Board known to have floating E-:

This is what I would do to modify that board.

  1. de-solder and lift pin1 (near the dimple) of the chip from the board.
  2. scrape/expose the copper of the long trace (VCC) near the dimple, and connect to pin1.
  3. cut away the short trace between C8 and the long VCC trace.
    Now the VCC pin of the board is the 5volt (AVDD) connection.
    You should add a 100n decoupling cap from VCC (pin1) to ground.
  4. connect a wire to pin16 of the chip (C8). This is the 3.3volt (DVDD) connection.
  5. solder a wire between E- and GND. This fixes the design fault of that board.
    Leo..