Wrong sign when converting

Forum 2005-2010 (read only) Software Syntax & Programs
1

I have a temp sensor (LM335A) connected to analog pin 0. When converting the values to degress Celsius I got wrong sign if the temperature is -0,xx. For example:
-1,xx will be displayed as -1,xx (OK) but
-0,xx will be displayed as 0,xx (wrong, missing - sign when temp is 0, something)
I have picked up the code from different examples in Arduinos examples, but because of lack of skills in this area I can't find the error in the code:

// *************************************************************************************
unsigned int precision =100;
unsigned int frac;
double val;
double valr;

client.print("Analog 0: ");
val = analogRead(0);
val = val - 10; // calibration of LM335
val = val * 0.48875; // 5Volt/1023 * 100
valr = val - 273.15; // convert Kelvin to Celsius

client.print(int(valr)); // prints the int part
client.print(","); // print the decimal point

if(valr >= 0)
frac = (valr - int(valr)) * precision;
else
frac = (int(valr)- valr ) * precision;
int frac1 = frac;
while( frac1 /= 10 )
precision /= 10;
precision /= 10;
while( precision /= 10)
client.print("0");
client.print(frac,DEC) ;
client.print(" &#176C");
client.println("
");

2

try replacing:
if(valr >= 0)
frac = (valr - int(valr)) * precision;
else
frac = (int(valr)- valr ) * precision;

with

if(valr < 0.0){
client.print('-');
valr = -valr;
}
frac = (valr - int(valr)) * precision;

3

Now I got a - but missplaced .... 0,-42

4

Can you post a version of the code that uses Serial so I can have a look and try running it? Just replace your sensor code with a hard coded floating point value

5

Here it comes, you have to change the different val statement:

void setup()
{

Serial.begin(9600);

}

void loop()
{
unsigned int precision =100;
unsigned int frac;
double val;
double valr;

val = 567; // expected result = -0,91 (actual 0,-91)
// val = 560; // expected result = -4,33 (actual -4,-33)
// val = 618; // expected result = 24,01 (actual 24,01)
val = val - 10; // calibration of LM335
val = val * 0.48875; // 5/1023 * 100
valr = val - 273.15; // Kelvin to Celsius
Serial.print("Analog 0: ");
Serial.print(int(valr)); //prints the int part
Serial.print(","); // print the decimal point

if(valr < 0.0){
Serial.print('-');
valr = -valr;
}
frac = (valr - int(valr)) * precision;
int frac1 = frac;
while( frac1 /= 10 )
precision /= 10;
precision /= 10;
while( precision /= 10)
Serial.print("0");
Serial.print(frac,DEC) ;
Serial.print(" &#176C");

}

6

you need to print the integer part after you print the sign. try this:

void setup(){
  Serial.begin(9600);
}

void loop()
{
  unsigned int precision =100;
  unsigned int frac;
  double val;
  double valr;   


  val = 567;                // expected result = -0,91 (actual 0,-91)
  //  val = 560;                // expected result = -4,33   (actual -4,-33) 
  //  val = 618;                // expected result = 24,01   (actual 24,01) 
  val = val - 10;          // calibration of LM335
  val = val * 0.48875;     // 5/1023 * 100
  valr = val - 273.15;     // Kelvin to Celsius
  Serial.print("Analog 0: ");
  if(valr < 0.0){
    Serial.print('-');
    valr = -valr;
  }
  Serial.print(int(valr));  //prints the int part
  Serial.print(","); // print the decimal point
  frac = (valr - int(valr)) * precision; 
  int  frac1 = frac;
  while( frac1 /= 10 )
    precision /= 10;
  precision /= 10;
  while(  precision /= 10)
    Serial.print("0");
  Serial.print(frac,DEC) ;
  Serial.print(" &#176C"); 
}
7

It works, Thank you! ;D