Strain gages wheat-stone bridge

Hi everyone, it's the first time that I'm writing here, and I'm a beginner in the Arduino's world, so I apologize for every fault I'll could do.
I'm working on a project to measure the deformation of a object. I'm using two strain gages (350 Ohm), with a half-bridge, but when I'm going to run che code no difference in output as been recorded when I apply a force on the object. I posting the sketch of my circuit, where maybe I did an error. Thanks in advance.

You have not provided your circuit diagram and your code.
You should at least amplify the signal, using precision instrumentation amplifiers.
Read the following to get started.
Strain gauge + Arduino scale
HX711 Load cell or Strain Gauge and Arduino

First of all thanks you for the answer. This is what's missing, I'm already using an amplifier (Hx711).
The only difference from the diagram is that I have the resistor of 350 Ohm

#include <HX711.h>

#define DOUT1 7
#define CLK1 8

HX711 scale1; //(DOUT1, CLK1);

float mvRead1;
float strain1;
float data_in1;
float gageFactor1 = 2.105;

void setup() {
  scale1.begin(DOUT1, CLK1);

void loop() {
  data_in1 = scale1.get_value();
  mvRead1 =(0.00000000238 * data_in1)*1000.0;  
  strain1 = (2.0*mvRead1)/(5000.0*gageFactor1);
  Serial.print(-mvRead1, 10);
  Serial.println(-strain1, 10);
1 Like

Show the type of your load cell (link to seller web-site). If the strain gauges are installed on one side of the beam, then they should be included in the opposite arms of the bridge, and if they are installed on both sides of the beam, then the strain gauges should be included in the adjacent arms, as in your diagram.

the strain gauges have to be set up so one is in compression while the other is in tension. or, as non geeks would say it: on the opposite sides of the stick. one of them increases in resistance while the other decreases. start by verifying that; the input to the HX711.

Thanks again. I don't have a load cell, I'm applying a compressional force (vertical), and I positioned the strain gauges on two opposite face of the sample.

Well, then your scheme will not work, since both datasets are compressed (stretched) at the same time.
Place your strain gauges on opposite shoulders of the bridge. Uce Case 2.

Still not works unfortunately. Is this the correct understanding of the circuit?

If you see anything on yellow stickers, you are a happy person. I can't see, even with glasses.

I am not using Fritzing. If you do, make the diagram easy to read.

Well the problem with that circuit is how do you match the voltages of the two arms? They need to be within a few millivolts of each other to avoid saturating the

In all these circuits that mix strain guages and fixed resistors you have to worry about thermal drift.

Far better to use extra matching strain guages to complete the bridge as these will be thermally matched to the live ones.

If you do use fixed resistors you may have to add small series trimmer pots to set the value precisely, and you need precision, low-tempco, resistors, standard 1% or 5% resistors will drift all over the place with temperature change.

And to the OP its Wheatstone bridge, named after Charles Wheatstone.

this is the first good diagram I found:

a Wheatstone bridge is two potentiometers. they act as voltage dividers. you read the difference between the two. the voltage between A & B must be 0 when the device under test is at rest. this state is "balanced"

the next sentence is crucial:

R1 X R4 must equal R2 X R3 within 1%

if you exceed 1% you have to feed current into it to balance the voltage. this current heats the resistors, which changes the resistance, which changes the balance, and the gage "drifts"

you can change any one resistor and read a voltage across A & B. for more effect, arrange the strain gages so the same strain makes R1 show more resistance while R3 shows less, due to the same strain. for maximum effect, do the same with R2 decreasing resistance and R4 increasing resistance

if you put 4 strain gages on a diaphragm, arranged properly, you get a pressure gage. if you put 4 strain gages on a flat bar, mounted on one side and cantilevered, arranged and wired properly, you get an accelerometer.

An engineer named Murphy came up with Murphys Law when a technician wired two strain gages on a seat belt, but not on opposite sides. the strain gage measured nothing, because both strain gages increased and decreased resistance with strain, so A & B did not unbalance. I am inclined to believe that Murphy did not explain that properly, and it was his screwup, not the techs.

Be carefull with the breadboard: the contact resistance between the pins of your resistor and the board can have serious influence on your measurement. Soldering can be a necessary alternative.

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