[ SOLVED ] Beehives electronic scale with HX711 --> adjusting Wheatstone bridge

[Joel14]

Hi all,

I am french, Sory for my English.

I work on a Beehives electronic scale with HX711 (not for sale, only for my beehives). I use HX711 with 4 load cells. the schematic is standard see picture. I have wired 4 systems "4 load cells systems" with this schematic except for the electronic that is à STM32F411 - Black pill.

.

With 2 of " 4 load cells system" , the result is ok and i can mesure my body weight. the value of the output HX711 at 0kg . Tare is equal to "380 000" .

With the 2 others "4 load cells system", the result is bad. The value of the output HX711 without weight is equal to "66 380 000" = 712kg .

With my body weight the output value is decreasing at "9543290" . the différence = -57092680 .

I have verified. I don't find wiring difference between good "4 load cells system" and bad "4 load cells system". How is possible that value of the output is decreasing on the 2 bad "4 load cells system" ?

Thanks for your answers.

Joël

[hammy]

You may have some load cells wired backwards or just wrong - example below plus see if you can see with a DVM the voltage produced .
Also what range are the load cells ? I see it says upper and lower - if the top ones are in tension they will need wiring in a reversed sense

Example

[Joel14]

Ok Hammy,

I had bought cells here : 4 cells + 1HX711 module
modèle of cells --> Modèle: 925119

I measured the Wheatstone bridge at 2000ohms between E+ and E- and also 2000 ohms between A+ and A- . May be , is it possible that wire color on cells is internally wrong connected.
I try to confirm if the cells RV and RC have the same colors for all cells.

I made a voltage measurement between A+ and A- ( not connected to HX711) .

with one bad "4 cells systems", weight off = V = -0.35mv and with my body weight V = 4.5mV . This is not normal . I need to have always positive values.
on second bad "4 cells systems", weight off = V = -0.46mv and with my body weight V = 4.5mV

with both good 4 cells systems, weight off = V = 0.52mv and with my body weight V =5.5mV.

I search tomorrow.
Joël

[hammy]

4 of those load cells will weigh 200kg - big beehive , you might not get much resolution

[sterretje]

@Joel14, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advise on) your project.

You don't have to apologise for being French If you feel more comfortable posting in your mother language, there is a dedicated French section on the forum under the heading International.

[Joel14]

Hi all,

no problem to move the topic.

I don't have problem with the resolution. I see a change on display when I add 10g on the jig.
for doing this I adapt the gain of the HX711 and I take the average on 10 read values. for doing this I made my own C function.

I forgot to say yesterday that all the measurement are sending on a blynk application. for this I use an ESP32 dev board that receive data with it serial interface. see picture -->blynk display

void HX711_mesure_Masse (uint8_t ChanelGain)
{
  uint8_t i=0;
  PD_SCK_Reset ;
  Masse_HX711 = 0;

  if (DOUT_status == 1 ) {return;}	// sort de la mesure si la mesure n'est pas prete

  for (i=1 ; i <= ChanelGain; i++)	// chanelGain = 25, 26 ou 27
    {
      PD_SCK_Reset ;
      delais;

      PD_SCK_Set ;
      delais;

      if (DOUT_Mesure == GPIO_PIN_SET)
	{ Masse_HX711|= BitStatus;}
      else
	{ Masse_HX711 &= ~BitStatus;}

      Masse_HX711  = Masse_HX711 << 1;       	// décalage à gauche de 1

    }

  Masse_HX711 = Masse_HX711 - Tare_HX711 ;	// tare
  if (Masse_HX711 <= 0) { Masse_HX711 = 0;}
  Masse_HX711 = Masse_HX711/ echelle_HX711 ;	// echelle affichage à 10g
  Masse_moyenne = Masse_moyenne + Masse_HX711;	// cumul de 10 mesure de masse
  nbMesuresMasse ++;

  if (nbMesuresMasse >=10)
    {
      Masse_moyenne = Masse_moyenne / 10;
      Masse = Masse_moyenne ;
      M_Masse[NumeroRuche]	= Masse;
      Masse_moyenne = 0;
      nbMesuresMasse = 0;
    }

  PD_SCK_Reset ;
  DOUT_status = 1 ;

  return;

}

void HX711_mesure_Masse_init(uint8_t ChanelGain)
{
  uint8_t i=0;
  PD_SCK_Reset ;

  while (DOUT_Mesure == GPIO_PIN_SET) {}	// attendre que le HX711 ait fini une mesure

  for (i=1 ; i <= ChanelGain; i++)		// demande de chanelGain = 25, 26 ou 27 pour la prochaine mesure
    {
      PD_SCK_Reset ;
      delay1ms(1);
      PD_SCK_Set ;
      delay1ms(1);
    }

  PD_SCK_Reset ;
  DOUT_status = 1 ;
  return;
}

see under the 4 load cell system and my designed board.

4 load systems

my PCB board with STM32F411 black pill

[Joel14]

I solve the problem by adding 2 x 0.4 ohm in the Wheatstone bridge . see picture to see where I put the two resistors. Wheatstone bridge modification.

With this 2 resistors , V is positive =0.15mV without load weight compare without resistors I had V negative = -0.30mV. now calibration is ok and my body weight is correct .

Joël

[Wawa]

I hope you're using a Sparkfun HX711 board for your STM processor.
Because that's the only one that can to be used, without modifications, with a 3.3volt processor.
Using a different HX711 board will result in instability or could damage your processor,
depending how you have hooked it up.
Common green HX711 boards are 5volt logic only.
Leo..

[b_spi]

Also check, if there is a connection between GND and E- on your HX711 board. There are some boards where this connection is missing - mostly because they use only single-layer pcbs.