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[on:]

Hi,

Im currently working on this project in which i need to measure dV/dt at the terminals of a battery. Is there  a function i can use? I am now able to get the voltage by using the analogRead(), but how can i get the rate of change in that voltage?

Thanks
Shirley

[Reply #1 on:]

Measure it and check the time. Store the values.
Measure it and check the time again. Store the values someplace else.
Repeat a few times to get more data to work with.

Use millis to get the time.

[Reply #2 on:]

Measure it and check the time. Store the values.
Measure it and check the time again. Store the values someplace else.
Repeat a few times to get more data to work with.

Use millis to get the time.

I modified a piece of code from the tuorial. Here's what i did:
/*
  ReadAnalogVoltage
  Reads an analog input on pin 0, converts it to voltage, and prints the result to the serial monitor.
  Attach the center pin of a potentiometer to pin A0, and the outside pins to +5V and ground.
 
 This example code is in the public domain.
 */

// the setup routine runs once when you press reset:
void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}
float voltage;
float LastVoltage;
float rate;
// the loop routine runs over and over again forever:
void loop() {
  // read the input on analog pin 0:
  int sensorValue = analogRead(A0);
  LastVoltage = voltage;
  Serial.print("Last voltage: ");
  Serial.println(LastVoltage);
  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):
  float voltage = sensorValue * (5.0 / 1023.0);
  // print out the value you read:
  Serial.println(voltage);
  // Calculate the rate of change in voltage, in V/s
  rate = (voltage-LastVoltage);
  Serial.print("dV: ");
  Serial.println(rate);
}

However the LastVoltage is always 0, do you know why?

Thanks
Shirley

[Reply #3 on:]

Hi, look at this code fragment:

Code:

float voltage;
float LastVoltage;
float rate;
// the loop routine runs over and over again forever:
void loop() {
  // read the input on analog pin 0:
  int sensorValue = analogRead(A0);
  LastVoltage = voltage;
  Serial.print("Last voltage: ");
  Serial.println(LastVoltage);
  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):

  float voltage = sensorValue * (5.0 / 1023.0); // voltage is redeclared here, masking the global variable

Also note that you should divide by 1024. And you are not calculating a rate since there is no reference to millis() or micros().
Besides, you don't really need to convert to voltage: an int value ranging from 0 to 1023 is as good to detect change as a float value ranging from 0 to 5, but the conversion from int to float is computationally expensive and you'll get less samples.
Finally, don't forget to put your code in code tags.

[Reply #4 on:]

Read the how to use this forum sticky, go back and put code tags in that post.

Code:

float voltage = sensorValue * (5.0 / 1023.0);

Creates a new variable called voltage that is not the same as the global one you created outside the loop. Therefore when you do

Code:

LastVoltage = voltage;

voltage is always zero because it is never being set to anything.

[Reply #5 on:]

It is assigned 0 as voltage did not get its value yet.
This is caused by the "float voltage" in the loop which create a second variable inside the loop called voltage .

slightly modified your code  

Code:

/*
  ReadAnalogVoltage
 Reads an analog input on pin 0, converts it to voltage, and prints the result to the serial monitor.
 Attach the center pin of a potentiometer to pin A0, and the outside pins to +5V and ground.
 
 This example code is in the public domain.
 */


float voltage = 0;
float LastVoltage = 0;
float rate = 0;
unsigned long lastTime = 0;

unsigned long dt = 100; // dt in milliseconds

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  if (millis() - lastTime  >= dt)   // wait for dt milliseconds
  {
    lastTime = millis();
    int sensorValue = analogRead(A0);
    voltage = sensorValue * (5.0 / 1023.0); // this line changed !!

    Serial.print("SensorValue: ");
    Serial.println(sensorValue);
    Serial.print("Last voltage: ");
    Serial.println(LastVoltage, 4);
    Serial.print("Current voltage: ");
    Serial.println(voltage, 4 );

    rate = (voltage-LastVoltage);
    Serial.print("dV/dt: ");
    Serial.println(1000*rate/dt, 4);
   Serial.println();

    LastVoltage = voltage;
  }
}