# Strain Measurument With Quarter Bridge Strain gauge

Hi,i am working on a project which i want to detect strain with a stran gauge.I am using HX711 adc to improve resolution but i dont know how to connect quarter bridge configration and how to code it i have just seen people using load cells

the bridge in question is a wheatstone bridge.

a full bridge is, practically speaking, two potentiometers composed of four individual sensors. if you had a full bridge gage, R1 & R2 would effectively be one potentiometer, and R3 & Rx would be a different potentiometer. all four resistors would be variable resistors that respond to what the gage measures. they would be arranged so all four contribute to the output: An increse in what the gage measures would cause R1 & Rx to decrease resistance while R2 & R3 increse resistance.

R1 * Rx should equal R2 * R3. if they are more than 2% different, one of these resistors will carry current, get hot, change value as it heats, and the baseline of the gage will drift.

the wheatstone bridge illustrated here is a quarter wave bridge, only one active element.

you will get an analog value between C and D. if one leg of the power source is grounded this will be floating, not referenced to ground. you read this with an analog input pin, which is referenced to ground

you need to research how people get around this. If you can use a battery for the excite voltage, or a power supply with a floating ground ( a wall wart that does not have a connection between the negative lead and ground ) you can ground C or D and measure the resulting voltage

A quarter bridge is basically a full bridge with only one active element.
The other three are fixed resistors of the same value as the active element.
The resistors most be stable and have a low tolerance (1% metalfilm).

OP mentioned a HX711
That board has a buildin reference of ~4.25volt between E+ and E-
Leo..

Guys ty for your informative replies , now it looks i am passed this electronic problems i have complete my circuit and now i am getting stable output but i cant read them on the serial port bcs they are two small like 20 milivolt and serial port shows two digits and i need at least 5-6 digit resolution could u help me coding this ? btw this is the code i have used also

int voltage1=0; //"Değer" adlı 0 başlangıçlı bir değişken tanımlıyoruz
int voltage2=0;

int voltagedifference1;
int voltagedifference2;
float voltagedifferencea;

void setup()
{
Serial.begin(9600); //9600 Baund bir seri haberleşme başlatıyoruz
Serial.println("voltage_difference"); //Seri monitörde bir kez gönderilen bir mesaj tanımlıyoruz
}
void loop()
{
voltage1 = analogRead(A0); //"Değer" değişkeni potansiyometrenin değerini okuyup buna göre değişir

voltagedifference1 = (voltage1-voltage2);
voltagedifferencea = ((float)5/16777216)*voltagedifference1;
Serial.println(voltagedifferencea); //Okunan değer seri monitörde mesaj olarak gönderilir

delay(1000); //Bu işlem 100 milisaniye aralıklarla yapılır
}

Ofcourse you can't use the Arduino A/D, because of the small voltage differences involved.
That's where the HX711 fits in.
Excitation voltage supply, instrumentation amplifier, and 24-bit A/D. All in one.
The HX711 outputs digital data. Lots of examples on the web.
Leo..

i am aware that arduino's adc cant handle this job i am just having problem with using hx711 should i use its library or just basicly measure it's voltage on the dt and sck pins and when i am trying to convert hx711 output to voltage on arduino is this the correct way to do it (5/16777216)*voltagedifference1;

HuseyinCanSuluk:
i am aware that arduino's adc cant handle this job i am just having problem with using hx711 should i use its library or just basicly measure it's voltage on the dt and sck pins...

As said, the HX711 outputs DIGITAL DATA, not a voltage.

HuseyinCanSuluk:
when i am trying to convert hx711 output to voltage...

Makes no sense.
Why would you want to do this.
Leo..

almost all of the examples about measuring weight but i need raw data of the voltage difference of the wheatstone bridge to calculate stress

HX711 libraries have AFAIK a raw data output function.

Use that value to convert to anything you like.
Onces, kg, pounds, tons, stone, mV, etc.
Must ofcourse calibrate/experiment with a multiplication factor to get the right converted values.

float millivolt = reading * 0.012345; // calibrate!
Leo..

@HuseyinCanSuluk

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