Can anyone explain the algorithm on this code? Heart Rate

Other Hardware Sensors
1

This code is what I found on the Internet and I want to understand how it works.
The main thing I want to understand is how he calculates bpm.
#include <Adafruit_SSD1306.h>

#define OLED_Address 0x3C
Adafruit_SSD1306 oled(1);

int x=0;
int lastx=0;
int lasty=0;
int LastTime=0;
bool BPMTiming=false;
bool BeatComplete=false;
int BPM=0;

#define UpperThreshold 550
#define LowerThreshold 500

void setup() {
oled.begin(SSD1306_SWITCHCAPVCC, OLED_Address);
oled.clearDisplay();
oled.setTextSize(2);
}

void loop()
{
if(x>127)
{
oled.clearDisplay();
x=0;
lastx=x;
}

int value=analogRead(0);
oled.setTextColor(WHITE);
int y=60-(value/16);
oled.writeLine(lastx,lasty,x,y,WHITE);
lasty=y;
lastx=x;
// calc bpm

if(value>UpperThreshold)
{
if(BeatComplete)
{
BPM=millis()-LastTime;
BPM=int(60/(float(BPM)/1000));
BPMTiming=false;
BeatComplete=false;
}
if(BPMTiming==false)
{
LastTime=millis();
BPMTiming=true;
}
}
if((value<LowerThreshold)&(BPMTiming))
BeatComplete=true;
// display bpm
oled.writeFillRect(0,50,128,16,BLACK);
oled.setCursor(0,50);
oled.print(BPM);
oled.print(" BPM");
oled.display();
x++;
}
2

Have you tried to work out how the algorithm works? It's pretty simple. What parts are giving difficulty?

It might help if you Auto-Format the code. That will make understanding the structure a little easier.

4

I try to write some comments.
did I have some misunderstanding?

if(a>127)                // Clear EEG
{
display.clearDisplay();
a=0;
lasta=a;
}

ThisTime=millis(); //Get the time 
int value=analogRead(0); //read anlog
display.setTextColor(WHITE);
int b=60-(value/16);why

display.writeLine(lasta,lastb,a,b,WHITE); //write a Line (EEG)
lastb=b;
lasta=a;
if(value>UpperThreshold)     // Heart beat leading edge detected.
{
if(BeatComplete) //calculate the bpm 
{
BPM=ThisTime-LastTime;
BPM=int(1/float(BPM)*1000*60);   //Divide by 60x1000 to change from millisecond to minute.
BPMTiming=false;
BeatComplete=false;

}
if(BPMTiming==false) //  Reset BPMTiming
LastTime=millis();
BPMTiming=true; // this switches BPMTiming as set in if (BeatComplete) from false to true
}
}
if((value<LowerThreshold)&(BPMTiming))  // Heart beat trailing edge detected.
BeatComplete=true;                                        // edge detected,beat is complete
display.writeFillRect(0,50,128,16,BLACK);
display.setCursor(0,50);
display.print("BPM:");
display.print(BPM);
display.display();

a++;

   }
5

Here, you are multiplying by 60x1000, not dividing.

Otherwise your comments are correct.

I think "why" here was meant to be a comment. It is not valid C code. What this line is doing is calculating a vertical pixel position on the oled from the analog input value.

6 
  // If the value is high...
  if (value > UpperThreshold)
  {
    // and it's the first time it was high since the last time it was low...
    if (BeatComplete)
    {
      // This is the start of a beat.

      // Record the number of milliseconds since the start of 
      // the previous beat
      unsigned long millisecondsPerBeat = millis() - LastTime;

      // divide the number of milliseconds into the number of
      // milliseconds in a minute (60000) to get beats per minute
      BPM = 60000.0 / millisecondsPerBeat;
      BPMTiming = false;
      BeatComplete = false;
    }
    if (BPMTiming == false)
    {
      LastTime = millis();
      BPMTiming = true;
    }
  }
  if ((value < LowerThreshold) & (BPMTiming))
    BeatComplete = true;

[/quote]

1 Like
7

You can always add a few serial monitor prints, and maybe delays to help you work out program flow and the value of certain variables.
You can comment out sections too to help

8

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