Different voltages between arduino UNO and multimeter

Other Hardware Science and Measurement
7

Most any range you want by using a voltage divider. For best results use precision resistors and in your calculation use the actual resistor values.

Voltage Divider Calculator (ohmslawcalculator.com)

Say you set a 2.6v reference. Put your output voltage to 2.6. I use 10K for R2, 5V for the input , calculate. And you'll find a 10ishK works.

Here is an example of code I use to measure volts used on a ESP32.

void fReadBattery( void * parameter )
{
  float adcValue = 0.0f;
  const float r1 = 50500.0f; // R1 in ohm, 50K
  const float r2 = 10000.0f; // R2 in ohm, 10k potentiometer
  float Vbatt = 0.0f;
  int printCount = 0;
  float vRefScale = (3.3f / 4096.0f) * ((r1 + r2) / r2);
  uint64_t TimePastKalman  = esp_timer_get_time(); // used by the Kalman filter UpdateProcessNoise, time since last kalman calculation
  SimpleKalmanFilter KF_ADC_b( 1.0f, 1.0f, .01f );
  TickType_t xLastWakeTime = xTaskGetTickCount();
  const TickType_t xFrequency = 1000; //delay for mS
  for (;;)
  {
    adc1_get_raw(ADC1_CHANNEL_0); //read and discard
    adcValue = float( adc1_get_raw(ADC1_CHANNEL_0) ); //take a raw ADC reading
    KF_ADC_b.setProcessNoise( (esp_timer_get_time() - TimePastKalman) / 1000000.0f ); //get time, in microsecods, since last readings
    adcValue = KF_ADC_b.updateEstimate( adcValue ); // apply simple Kalman filter
    Vbatt = adcValue * vRefScale;
    xSemaphoreTake( sema_CalculatedVoltage, portMAX_DELAY );
    CalculatedVoltage = Vbatt;
    xSemaphoreGive( sema_CalculatedVoltage );
    /*
      printCount++;
      if ( printCount == 3 )
      {
      log_i( "Vbatt %f", Vbatt );
      printCount = 0;
      }
    */
    TimePastKalman = esp_timer_get_time(); // time of update complete
    xLastWakeTime = xTaskGetTickCount();
    vTaskDelayUntil( &xLastWakeTime, xFrequency );
    //log_i( "fReadBattery %d",  uxTaskGetStackHighWaterMark( NULL ) );
  }
  vTaskDelete( NULL );
}

If you notice I use a Kalman Filter to smooth the readings.

GitHub - denyssene/SimpleKalmanFilter: A basic implementation of Kalman Filter for single variable models.

Updating the process noise before each reading, improves filtering.
KF_ADC_b.setProcessNoise( (esp_timer_get_time() - TimePastKalman) / 1000000.0f ); //get time,

Oh. I found that if R2 is a !)K pot, you can zoom in that reading.

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