HX711 with 4 equal resistors in a wheatstone bridge config

Wawa:
Connecting the bridge to E+ E- A+ A- is the only way.

Still no picture of the setup.
Leo…

Here, I took a picture of the setup attached below. I still get the readings stuck at 13108 when using this config, are you able to tell if there is an error in the setup?

Thanks for your great help.

MarkT:
If they aren’t 0.01% resistors on a temperature-stable heatsink yes they will drift, resistor values are not
constant due to temperature changes caused by self-heating.

Actual strain guages are thermally bonded to each other and well heat-sinked, temperature drift is much
less (still an issue though).

Remember you are performing measurements with this kind of setup to ppm levels of precision…

This is true, I will switch to strain gauges once I understand why I get those messy readings with resistors. I just can’t find an explaination online to why I get those readings stuck at 13108 when I do not connect my E+ to 5v on the uno and E- to GND.

Here is code again, in case:

#include <Arduino.h>
#include <SPI.h>
#include <Wire.h>

#include <HX711.h>

HX711 scale;

#define LOADCELL_DOUT_PIN 5
#define LOADCELL_SCK_PIN 6

#define CALIB_FACTOR -200

void setup()
{
  // put your setup code here, to run once:
  Serial.begin(9600);
  scale.begin(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN);
  scale.tare();
  scale.set_scale(CALIB_FACTOR);
}

void loop()
{
  // put your main code here, to run repeatedly:

  int scaleReading = scale.get_value(10);
  Serial.print("read: ");
  Serial.println(scaleReading);
}

While waiting for your image...

E+ should be 4.25volt, E- should be 0volt.

A+ and A- should both be 2.125volt, and there should not be more than 10mV between them
(with DMM pins on A+ and A-).
Leo..

On the green HX711 module GND is not connected to Arduino GND directly but via an input protection diode and so E- is not 0V (measured to Arduino GND) but about 0.6V. I don't remember what is E+.

Never seen that, and I can't imagine it to be right, looking at the HX711 datasheet.
That would seriously reduce the excitation voltage.

Do you have a link to the diagram of that module, or clear pictures of both sides of the board.
Leo..

https://www.eevblog.com/forum/projects/how-can-a-chip-(hx711)-work-without-a-ground-pin-connected/

So sorry! The image on next post (it failed to upload on this one)

Wawa:
While waiting for your image...

E+ should be 4.25volt, E- should be 0volt.

A+ and A- should both be 2.125volt, and there should not be more than 10mV between them
(with DMM pins on A+ and A-).
Leo..

This is interesting, I have 2,7V on all 4 pins (A+/- and E+/-) with this current setup. Do you think of a reason why? I have checked continuity between the 4 pins and none of them is.

ss

Wiring seems ok, apart from too much solder on the pins of the module.

Remove all the wires. and move the module to another position on the breadboard (BB do have bad contacts).

Then only connect VCC/GND to 5volt/GND of the Arduino, and measure E+ with black lead of the DMM on the metal shield of the USB socked (GND).

It should be 4.25volt..... ±0.1volt.

If not, check/re-solder, or try a new HX711 board.
Leo..

Wawa:
Wiring seems ok, apart from too much solder on the pins of the module.

Remove all the wires. and move the module to another position on the breadboard (BB do have bad contacts).

Then only connect VCC/GND to 5volt/GND of the Arduino, and measure E+ with black lead of the DMM on the metal shield of the USB socked (GND).

It should be 4.25volt..... ±0.1volt.

If not, check/re-solder, or try a new HX711 board.
Leo..

Thanks, I measured 0V from E+ to GND, and made sure the 5V was reaching VCC of the HX711 module. I am wondering if it might be a setting that deactivates the E+/- pins sent to the chip (maybe it would be saved on its eeprom?) I am not the first person who used this specific module and I am unaware of any changes that could have been made firmware wise. I cant find anything on the datasheet though..

You could be right.
The chip has a power-down mode, activated with the clock pin.
Not sure if that will also affect the E+ excitation supply.
Haven't got a board handy at the moment, so I have to pass that problem on to another forum member.
Leo..

One of my spares, with the clock pin held low (tied directly to ground):

Vcc=4.97v from a wall wart
E+=4.96v
E-=0.81v

All are relative to the GND pin.

E+ to E- = 4.15v

If the clock pin is floating, the behavior may be undefined.

Something fishy going on.
According to the datasheet, E- must be tied to ground, and E+ comes from a PNP transistor that is being regulated down by the HX711 to ~4.25volt (if you work out the feedback resistor divider ratio).

Like to see clear pictures of both sides of these boards.
Leo..

I just realised I had extras, and were able to measure a 4.25V on E+ on a new one (DT and SCK were floating). I dont know what happens if I try to ground them, I could check

Don't ground an output pin (DT)!

pics

E+

E-

front

back

traces with backlight
Annotation 2020-07-08 084159.jpg

This appears to be one of those boards in which E- is not grounded (as mentioned by @Smajdalf). And, there's no ground plane (for noise reduction)...

I also have a Sparkfun HX711 but haven't soldered pins to it. Its E- is tied directly to the ground plane.

This probably ought to be split off as a separate topic, since it now appears to be unrelated to the OP.

Annotation 2020-07-08 084159.jpg

Thank you for the images.

E- is normally grounded through the board.
The green board doesn't have E- connected to ground through the board.
If you don't, then the E+ regulator also isn't working, because it's 8k2 feedback resistor (8210) is floating.

It seems that with these boards you also must connect E- to Arduino ground.
Leo..

Wawa:
If you don't, then the E+ regulator also isn't working, because it's 8k2 feedback resistor (8210) is floating.

It seems that with these boards you also must connect E- to Arduino ground.

With E- grounded, E+ is 4.39v

This appears to be another example of "you get what you pay for" and caveat emptor.

The 8k2:20k resistor ratio on this module, with the 1.25volt reference of the HX711, works out to
(8200 + 20000) / 8200 * 1.25volt = ~4.3volt

0.09volt off is ok for a 5volt Arduino, but 4.39 volt is a bit close for a Nano on USB power (~4.6volt supply).

You must of course know what you're doing if you order undocumented cheap parts.
Leo..

Hi,

I have what seems the same board and looks same, seems wrong as per everything I have read on the net.

I have also other boards with different layout that I am willing to analyse per DaveEvans study of Noise Free resolution test. Here my post: https://forum.arduino.cc/index.php?topic=365107.msg4676152#msg4676152

Thanks for all your work.

My best regards,

Wawa:
Thank you for the images.

E- is normally grounded through the board.
The green board doesn’t have E- connected to ground through the board.
If you don’t, then the E+ regulator also isn’t working, because it’s 8k2 feedback resistor (8210) is floating.

It seems that with these boards you also must connect E- to Arduino ground.
Leo…