temp_test.arduino

float temp;
float target_temp = 21.0;
const int tempPin = A0;    // the analog pin used to read temp
const int ledPin =  3;      // the number of the LED pin

/*

LM 35
5v    --|\
A0    --| |
GND   --|/


LED

GND    -------------------|\  (-)
PIN3   ---1k-/\/\/\-------|/  (+)

*/
void setup()
{
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);   
  digitalWrite(ledPin, HIGH);  
  delay(200);
  digitalWrite(ledPin, LOW);  
  delay(200);
  digitalWrite(ledPin, HIGH);  
  delay(200);
  digitalWrite(ledPin, LOW);  
  delay(200);
}

float measure_temp()
{
  float tempK = (((analogRead(tempPin)/ 1023.0) * 5.0) * 100.0);  
  //Converts Kelvin to Celsius minus 2.5 degrees error
  float tempC = tempK - 273.0;   
//  float tempF = ((tempK - 2.5) * 9 / 5) - 459.67;
  return tempK;
}


void loop()
{
  // check what works best

  temp = measure_temp();
  if( temp > target_temp ) {
    digitalWrite(ledPin, LOW);
  } else {
    digitalWrite(ledPin, HIGH);
  }
  
  Serial.print("temperature = ");
  Serial.print(temp);
  Serial.print("*C");
  Serial.println();
  delay(1000);
}