File content as of revision 10:ad2548407023:

#include "mbed.h"
#include "uLCD_4DGL.h"

//Calculates the BPM using a photoresistor as input and outputs the BPM to an LCD screen.
uLCD_4DGL uLCD(p28,p27,p30); // serial tx, serial rx, reset pin;
AnalogIn photoresistor(p15);
Timer t; //running timer
Ticker interrupt; //recurring 2 ms interrupt 

volatile float beat;  //raw analog signal data from photoresistor
volatile int BPM = 60;   //beats per minute
volatile int beatCount = 0; //number of beats found
volatile float currIBI = 0;   // current InterBeat Interval: the time since the last beat
volatile float lastIBI = 1; //last InterBeat Interval: the time between beats
volatile float pastIBI[10];   //past ten IBI values
volatile float sumIBI = 0;    ///sum of past ten IBI values
volatile unsigned long lastBeatTime = 0;    //time of last beat 
volatile float peak = 25;    //peak of heartbeat wave; initilized to 20 to avoid noise at startup
volatile float trough = 15;  //trough of heartbeat wave
volatile float lowerThresh = 10;  // used to reduce noice; change these values based on amount of light in room
volatile float upperThresh = 30;  // used to reduce noice; change these values based on amount of light in room
volatile float amp = 5; // amplitude of heartbeat wave
volatile bool firstBeat = true; //keeps track of first beat found
volatile bool beatFound = false;    //true when a beat is found, false in between beats
volatile bool BPMfound = false; //true when BPM is found, false otherwise.

void findBPM(){   
    __disable_irq();  // disable interrupts
    beat = photoresistor * 100;   // read the analog data from the photoresistor and multiply by 100 (amplifies small values)          
    currIBI = t.read() - lastBeatTime;  
    // avoid noise by waiting 3/5 of last IBI     
    if((beat < lowerThresh) && (currIBI > (lastIBI*3)/5)){       
        if (beat < trough){                       
            trough = beat;                         
        }
    }
    if((beat > upperThresh) && (beat > peak)){          
        peak = beat;                            
    }                                      
    // photoresistor signal surges when there is a heartbeat
    if ((currIBI > 0.25) && (beat > upperThresh) && (beatFound == false)){ 
        beatFound = true;  
        beatCount++;
        lowerThresh = beat - 0.7; //adjust threshold to account for any change in lighting and avoid noise                          
        lastIBI = t.read() - lastBeatTime;  
        lastBeatTime = t.read();                
        //if this is  the first beat found, fill pastIBI values with this value until more beats are found
        if(firstBeat){                        
            firstBeat = false;                
            for(int i=0; i< 10; i++){       
                pastIBI[i] = lastIBI;                      
            }
        }
        sumIBI = lastIBI;       
        //shift data over one value in pastIBI array to make room for next IBI value and discard oldest value             
        for(int i=0; i< 9; i++){               
            pastIBI[i] = pastIBI[i+1];              
            sumIBI += pastIBI[i];          
        }
        pastIBI[9] = lastIBI; //add most recent IBI value to array                          
        sumIBI = sumIBI/10; //average the last 10 IBI values 
        BPM = 60/sumIBI; //BPM = 1 minute / average IBI
        if(beatCount == 5){ //Only report BPM after at least 5 beats are found
            BPMfound = true;      
            beatCount = 0;
        }                                             
    }
    //when the signal is less than the threshold, the beat is over 
    if (((beat < lowerThresh) && beatFound) || (currIBI > 2)){     
        beatFound = false;                     // reset the beatFound flag to find the next beat
        amp = peak - trough;                   // calculate amplitude of heartbeat wave
        upperThresh = amp/2 + trough;               // set upper threshold to 50% of the amplitude
        peak = upperThresh;                         
        trough = lowerThresh;
    }
    __enable_irq();   //enable interrupts
} 

int main()
{
    uLCD.color(RED);
    uLCD.printf("\nHeart Beat Monitor\n");
    wait(1);
    uLCD.cls();
    t.start();
    interrupt.attach(&findBPM, 0.002); //throws interrupt every 2ms to read sensor data and calculate BPM
    while(1){
        if(t.read() > 10){ //wait 10 seconds to get an accurate average 
            if(BPMfound){ 
                BPMfound = false; //start calculating next BPM
                uLCD.locate(1,2);
                uLCD.printf("\nYour BPM is:\n");
                uLCD.printf("%2D", BPM);
                wait(3);
                uLCD.cls();
                lastBeatTime = 0;
                beatCount = 0;
                t.start(); //restart timer
            }
            //No heartbeat detected wtihin 10 seconds
            else{
                uLCD.printf("\nNo heartbeat detected!");
                wait(3);
                uLCD.cls();
                lastBeatTime = 0;
                beatCount = 0;
                t.start(); //restart timer
            }
        }
        else{
            uLCD.locate(1,2);
            uLCD.printf("\nCalculating...\n");
            wait(.1);
            uLCD.cls();
        }
    }
}