3

schematic

simulate this circuit – Schematic created using CircuitLab

Wiring is based on the scheme presented below - enter image description here

(Reference: https://electronics.stackexchange.com/a/174580)

I am using Arduino Mega 2560, HX711 and four load cells to build a weighing scale but having a hard time calibrate the unit. I have tried different libraries and calibration schemes but none of them seem to help. I have found that the raw values corresponding to any given weight (including zero load) are not consistent. Firstly, the readings do not settle even after 10 minutes, and even if I take an average, the next time I put the same weight, I get an entirely different reading making the last average useless. I am seeing errors of the order of +/- 30 lbs when I put a known weight.

I first thought that maybe my load cells are the root cause but when hooked them up directly to Arduino and read the voltage across the bridge, I read stable constant values. Verified the voltage readings with a Voltmeter. (Yes, their resolution is not as good as HX711 but at least they are stable and consistent). I get the voltage reading close to my hand calculations.

Next, I tried testing the HX711 in different ways. In one test, I created a full-bridge using resistors other than load cells, forced the bridge out of balance but cannot read anything other than 16777215. Next, I wanted to see if the board is reacting to any voltage at all, so, I removed the bridge and did not connect anything to E+ and E- pins but supplied known voltages from a DC power supply (from 0 to 4.5V) to the A+ and A- (GND) pins. Now, it is reading only 8388607, no matter what voltage I supply. If I supply +5V or +3.3V from Arduino to the A+ and A- (GND) pins instead of DC Power supply, it then only reads 16777215.

I don't know how to interpret this behavior. How can I resolve the issue and bring the error down? Thanks for your help.

I have included the schematic below. All four Load Cells (LCs) are 3-wired (White, Black & Red). Resistance between W & B wires is around 1980 ohms while that between R & W or R & B is around 990 ohms. SO I connected the four LCs in the manner shown in the schematic. Wire colors have been labelled.

Code:

Code is pretty straightforward as I am simply reading raw values from the HX711 using the following HX711 library - github.com/queuetue/Q2-HX711-Arduino-Library.git 

#include <Q2HX711.h>

const byte hx711_data_pin = 4;
const byte hx711_clock_pin = 8;

Q2HX711 hx711(hx711_data_pin, hx711_clock_pin);

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  Serial.println(hx711.read());
  delay(500);
}
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3
  • Can you draw as a schematic or wiring diagram how everything is connected? And please provide your code in your question. Maybe there is something wrong with one of them
    – chrisl
    Commented Jan 9, 2020 at 8:15
  • I have included both of them in the post. I think the variation in the values that I am seeing is because of residual strain and hysteresis but not sure of that. I have been noting the scale readings for past four days. I have not put any load at all and these are the readings I have obtained - Day 1: 84442XX Day 2: 84447XX Day 3: 84442XX Day 4: 84447XX If zero lbs weight correspond to 84442XX, the same empty scale starts reading a different weight the other day. This is the problem I am trying to solve. I expect variation in readings since HX711 is 24-bit but what I have is too high.
    – PSharma
    Commented Jan 11, 2020 at 19:54
  • If you getting readings at the limit of the HX711, (2^24=16777216) the load cells might not be close enough to identical, and you might have to bias them closer to zero. Do you have a high resolution voltage meter that you can test the voltage difference between the signal+ versus signal-? If it's more than 10-20mV, you could add a trim pot to bias a leg of the wheatstone bridge higher or lower. // Zero could move around significantly from day to day dependent on temperature and creep. Most use-cases of cheap load cells/scales have a means of zeroing them out immediately before use.
    – Dave X
    Commented Oct 3, 2021 at 19:07

1 Answer 1

Reset to default
1

enter image description here

Here is is a picture of how I have seen the sensors connected. Is this how your is?

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14
  • When you connected the cells directly to the board, did you do it one at a time or all at once? Is the board combining the weight values or is it done with the sensors/library?
    – Jeff A
    Commented Jan 7, 2020 at 0:34
  • Hi! Thanks for your response. I have tried many and currently am using the following HX711 library - github.com/queuetue/Q2-HX711-Arduino-Library.git And the code is simply reading raw values from ADC using hx711.read(). If I use a full-bridge of 4 load cells, I get continuously changing values and if I connect a full-bridge of one load cell and two external resistors, I only read 16777215 and nothing else.
    – PSharma
    Commented Jan 7, 2020 at 0:34
  • I have connected them both ways. If I use a full-bridge of 4 load cells, I get continuously changing values and if I connect a full-bridge of one load cell and two external resistors, I only read 16777215 and nothing else, even if I put weights on the load cell.
    – PSharma
    Commented Jan 7, 2020 at 0:35
  • My last piece of advice is to go over your connections for stray what ever. Look at all the solder joints on EVERYTHING and see if you can find a bridge or something touching that should not.
    – Jeff A
    Commented Jan 7, 2020 at 1:30
  • Thanks for your time. I have checked it a number of times. Couldn't find any unintended connections plus all GND connections are also good.
    – PSharma
    Commented Jan 7, 2020 at 2:04

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