Wiring the MLX90614 with the arduino should be straightforward. Just follow the hookup image file provided in this project.

In a nutshell, power the sensor using the 3.3V rail, wire up SDA to analog input pin 4 and SCL to analog pin 5, and add 4.7kΩ pull-up resistors to both the data line and the clock line. The sensor datasheet advises the use of a .1uf capacitor between power and ground but I did not use it and the circuit works well without it.

Before anything else you need to have the ic2master library into your Arduino library folder for any of the code below to work. For convenience I've added the library to this project but you can go download it from where I got it or simply search for "ic2master library".

Once that is out of the way dive right into the code below. Be advised that initial parameters might have to be adjusted for it to work properly. That is something that is intrinsic to the technique due to the wide variations in blood flow and skull thickness from one person to another.

#include <i2cmaster.h>
//Parameters
int del_noise = 10; //take the average of 10 temp
float led_sensitivity = 15000; //255...1000000
float sleep = 20; //read temp every 20ms
int led_pin = 11; //LED Pin absolute
int equi_step = 100; //1000ms between each equilibration temperature
int equi_times = 20; //take 20 steps for equilibration that last *equi_step*
float norm[10]; //has to be the same as del_noise
float treshold = 10; ///treshold led analog intensity
//Variables
float avrg = 0; //do not change
float celsius_old = 0;//do not change
float celsius_old2 = 0;//do not change
float analog = 0;//do not change
 //Functions
float i2c(){
  int dev = 0x5A<<1;
  int data_low = 0;
  int data_high = 0;
  int pec = 0;
 
  i2c_start_wait(dev+I2C_WRITE);
  i2c_write(0x07);
 
  // read
  i2c_rep_start(dev+I2C_READ);
  data_low = i2c_readAck(); //Read 1 byte and then send ack
  data_high = i2c_readAck(); //Read 1 byte and then send ack
  pec = i2c_readNak();
  i2c_stop();
 
  //This converts high and low bytes together and processes temperature, MSB is a error bit and is ignored for temps
  double tempFactor = 0.02; // 0.02 degrees per LSB (measurement resolution of the MLX90614)
  double tempData = 0x0000; // zero out the data
  int frac; // data past the decimal point
 
  // This masks the error bit of the high byte, then moves it left 8 bits and adds the low byte.
  tempData = (double)(((data_high & 0x007F) << 8) + data_low);
  tempData = (tempData * tempFactor)-0.01;
 
  float cels = tempData - 273.15;
 
  return cels;
}
void setup(){
    pinMode(led_pin, OUTPUT);
    digitalWrite(led_pin, HIGH);
    
    Serial.begin(9600);
    Serial.println("Setup...");
    
    digitalWrite(led_pin, LOW);
    i2c_init(); //Initialise the i2c bus
    PORTC = (1 << PORTC4) | (1 << PORTC5);//enable pullups
    digitalWrite(led_pin, HIGH);
    
  digitalWrite(led_pin, LOW);
  digitalWrite(led_pin, HIGH);
  digitalWrite(led_pin, LOW);
  Serial.println("Equilibrate...");
 
  for (int i = 0; i < equi_times; i++) {
      digitalWrite(led_pin, HIGH);
      delay(equi_step/2);
      digitalWrite(led_pin, LOW);
      delay(equi_step/2);
    }
    
  Serial.println("Norming...");
  for (int i = 0; i < del_noise; i++) {
    norm[i] = i2c();
    delay(sleep);
  }  
}
int i = 0;
void loop(){
  i++;
  float percent = 0;
  float celsius = 0;
  for (int i = 0; i < (del_noise-1); i++) {
      norm[i] = norm[i+1];
      celsius += norm[i];
  }
  norm[del_noise-1]=i2c();
  celsius += norm[del_noise-1];  
  celsius = celsius/del_noise;
  Serial.println("Celsius");
  Serial.println(celsius);
  Serial.println(celsius_old);
  if ((celsius_old != 0) && (celsius_old2 != 0)) {
    percent = (celsius/celsius_old2)-1;
    analog = percent * led_sensitivity;
  }
  celsius_old = celsius;
  celsius_old2 = celsius_old;
  if (i == 10){ //write percent each 10th value to serial port
  Serial.println(percent);
  i = 0;
  }
  if (analog <= treshold) {
    analog = 0;
  }
  if (analog > 255) {
    analog = 255;
  }
  analogWrite(led_pin, analog);
  delay(sleep);
}