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Topic: Reading Small voltages (Read 4 times) previous topic - next topic

zach_ina_box

Thanks for the input everyone.  I connected my circuit to a dmm and checked the output voltage from the bridge (unbalanced) and am getting around 29 mV and when measuring the output from the op amp I am getting around 2.47 V.  This is about what I want but there are still errors.  When measuring the bridge voltage there is a voltage increase when the load cell is loaded but when measuring the op amp output there is no discernible change in voltage when loading the cell.  I then connect the output from pin six of the op amp to a analog input pin on the Arduino board and print to serial the output voltage is a constant 2.47 V but again no change when loading the cell.  I am running out of time on this project and am very worried at this point.  My code is below.

/*
Load Cell data collection program
*/
const double ArduinoVoltage = 5.00; // CHANGE THIS FOR 3.3v Arduinos
const double ArduinoResolution = ArduinoVoltage / 1024;

int vPin = 0;     // Assign input pin from load cell circuit


void setup()
{
  Serial.begin(9600);
  pinMode(vPin, INPUT);
}

void loop()
{
double val = 0.0;              // Variable for storing vPin input
double voltage = 0.0;          // Variable for storing voltage input
val = analogRead(vPin);       // Read the voltage from vPin
voltage = val*ArduinoResolution;
Serial.println(voltage);
delay(100);
}

Grumpy_Mike

Have you also connected the ground of your amplifier supply to the ground of the arduino?

zach_ina_box

The bridge and the op amp have separate voltage sources.  The arduino is providing a 5 volt excitation voltage to the bridge and the op amp has a 9 v external excitation voltage.

backbone

Never to old to learn and I learn every day

DVDdoug

Quote
I am getting around 2.47 V.  This is about what I want but there are still errors.  When measuring the bridge voltage there is a voltage increase when the load cell is loaded but when measuring the op amp output there is no discernible change in voltage when loading the cell.
I assume you need more gain in your op-amp, or another gain stage as Terry suggested.  But, you need to design your circuit so that you don't amplify that ~2.5V bias (or feed-in a lower bias and amplify it along with the signal).

Quote
I am running out of time on this project and am very worried at this point.
On a "big project", isn't there an electronics enginner available to help?   And once you get this working,  I'd build two of 'em.  The last thing you need is for a custom ond-of-a-kind gizmo failing and shutting everything down! 

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