// These constants won't change.  They're used to give names
// to the pins used:
const int analogInPin = 2;  // Analog input pin that the pulse sensor is attached to (physical pin 2, Arduino pin 4, analogRead(2) according to blog)
const int analogOutPin = 1; // Analog output pin that the LED is attached to

int sensorValue = 0;        // value read from the pulse sensor
int outputValue = 0;        // value output to the PWM (analog out)

int maxval = 0; // want a maximum set according to the incoming values (which will change due to many outside factors). Start at 0 to allow incoming data to define maximum
int minval = 1023; //start at 1023 (max possible analog value) to allow incoming data to set the minimum
int counter = 0; //counter to allow forced reset every few seconds (to prevent outliers/times when you aren't wearing the sensor from creating permanent outlier max and min vals.

void setup() {
  pinMode(analogInPin, INPUT);
  pinMode(analogOutPin, OUTPUT);
}

void loop() {
  if(counter==5000){ //Reset max and min values approximately every 5 seconds
    counter = 0;
    maxval = 0;
    minval = 1023;
  }
  // read the analog in value:
  sensorValue = analogRead(analogInPin);
  //update mins and maxes if necessary:
  if(sensorValue > maxval){
    maxval = sensorValue;
  }
  if(sensorValue < minval){
    minval = sensorValue;
  }      
  // map it to the range of the analog out:
  outputValue = map(sensorValue, minval, maxval, 0, 255);  
  // change the analog out value:
  analogWrite(analogOutPin, outputValue);          

  // wait 2 milliseconds before the next loop
  // for the analog-to-digital converter to settle
  // after the last reading:
  delay(2);        
  counter++; //increment counter
}