Cảm biến nhịp tim

Cảm biến nhịp tim Pulse Sensor có giá thành thấp, kích thước rất nhỏ, là cảm biến nhịp tim có thể cắm và chạy cho các bo mạch tương thích Arduino.Cảm biến này bổ sung mạch khuếch đại và khử nhiễu cho phần cứng. Rõ ràng là nhanh hơn và dễ dàng hơn để có được kết quả đọc xung đáng tin cậy. Thường được sử dụng trong các lĩnh vực như: thiết bị thể dục, thiết bị y tế,...

Thông số kỹ thuật:

• Nguồn: 3~5 VDC
• Dòng tiêu thụ: < 4mA
• Ngõ ra: Analog.
• Đường kính cảm biến : 1.6 cm ( 0.625 inch).
• Độ dày: 0.125 inch

Sơ đồ chân:

Code tham khảo Arduino: 

//code copied from arduino.cc 
int pulsePin = A0;                 // Pulse Sensor purple wire connected to analog pin A0
int blinkPin = 13;                // pin to blink led at each beat

// Volatile Variables, used in the interrupt service routine!
volatile int BPM;                   // int that holds raw Analog in 0. updated every 2mS
volatile int Signal;                // holds the incoming raw data
volatile int IBI = 600;             // int that holds the time interval between beats! Must be seeded! 
volatile boolean Pulse = false;     // "True" when User's live heartbeat is detected. "False" when not a "live beat". 
volatile boolean QS = false;        // becomes true when Arduoino finds a beat.

static boolean serialVisual = true;   // Set to 'false' by Default.  Re-set to 'true' to see Arduino Serial Monitor ASCII Visual Pulse 

volatile int rate[10];                      // array to hold last ten IBI values
volatile unsigned long sampleCounter = 0;          // used to determine pulse timing
volatile unsigned long lastBeatTime = 0;           // used to find IBI
volatile int P = 512;                      // used to find peak in pulse wave, seeded
volatile int T = 512;                     // used to find trough in pulse wave, seeded
volatile int thresh = 525;                // used to find instant moment of heart beat, seeded
volatile int amp = 100;                   // used to hold amplitude of pulse waveform, seeded
volatile boolean firstBeat = true;        // used to seed rate array so we startup with reasonable BPM
volatile boolean secondBeat = false;      // used to seed rate array so we startup with reasonable BPM

void setup()
{
  pinMode(blinkPin,OUTPUT);         // pin that will blink to your heartbeat!
  Serial.begin(115200);             // we agree to talk fast!
  interruptSetup();                 // sets up to read Pulse Sensor signal every 2mS 
                                    // IF YOU ARE POWERING The Pulse Sensor AT VOLTAGE LESS THAN THE BOARD VOLTAGE, 
                                    // UN-COMMENT THE NEXT LINE AND APPLY THAT VOLTAGE TO THE A-REF PIN
                                    //   analogReference(EXTERNAL);   
}


//  Where the Magic Happens
void loop()
{
   serialOutput();  
   
  if (QS == true) // A Heartbeat Was Found
    {     
      // BPM and IBI have been Determined
      // Quantified Self "QS" true when arduino finds a heartbeat
      serialOutputWhenBeatHappens(); // A Beat Happened, Output that to serial.     
      QS = false; // reset the Quantified Self flag for next time    
    }
     
  delay(20); //  take a break
}


void interruptSetup()
{     
  // Initializes Timer2 to throw an interrupt every 2mS.
  TCCR2A = 0x02;     // DISABLE PWM ON DIGITAL PINS 3 AND 11, AND GO INTO CTC MODE
  TCCR2B = 0x06;     // DON'T FORCE COMPARE, 256 PRESCALER 
  OCR2A = 0X7C;      // SET THE TOP OF THE COUNT TO 124 FOR 500Hz SAMPLE RATE
  TIMSK2 = 0x02;     // ENABLE INTERRUPT ON MATCH BETWEEN TIMER2 AND OCR2A
  sei();             // MAKE SURE GLOBAL INTERRUPTS ARE ENABLED      
} 

void serialOutput()
{   // Decide How To Output Serial. 
 if (serialVisual == true)
  {  
     arduinoSerialMonitorVisual('-', Signal);   // goes to function that makes Serial Monitor Visualizer
  } 
 else
  {
      sendDataToSerial('S', Signal);     // goes to sendDataToSerial function
   }        
}

void serialOutputWhenBeatHappens()
{    
 if (serialVisual == true) //  Code to Make the Serial Monitor Visualizer Work
   {            
     Serial.print(" Heart-Beat Found ");  //ASCII Art Madness
     Serial.print("BPM: ");
     Serial.println(BPM);
   }
 else
   {
     sendDataToSerial('B',BPM);   // send heart rate with a 'B' prefix
     sendDataToSerial('Q',IBI);   // send time between beats with a 'Q' prefix
   }   
}

void arduinoSerialMonitorVisual(char symbol, int data )
{    
  const int sensorMin = 0;      // sensor minimum, discovered through experiment
  const int sensorMax = 1024;    // sensor maximum, discovered through experiment
  int sensorReading = data; // map the sensor range to a range of 12 options:
  int range = map(sensorReading, sensorMin, sensorMax, 0, 11);
  // do something different depending on the 
  // range value:
}


void sendDataToSerial(char symbol, int data )
{
   Serial.print(symbol);
   Serial.println(data);                
}

ISR(TIMER2_COMPA_vect) //triggered when Timer2 counts to 124
{  
  cli();                                      // disable interrupts while we do this
  Signal = analogRead(pulsePin);              // read the Pulse Sensor 
  sampleCounter += 2;                         // keep track of the time in mS with this variable
  int N = sampleCounter - lastBeatTime;       // monitor the time since the last beat to avoid noise
                                              //  find the peak and trough of the pulse wave
  if(Signal < thresh && N > (IBI/5)*3) // avoid dichrotic noise by waiting 3/5 of last IBI
    {      
      if (Signal < T) // T is the trough
      {                        
        T = Signal; // keep track of lowest point in pulse wave 
      }
    }

  if(Signal > thresh && Signal > P)
    {          // thresh condition helps avoid noise
      P = Signal;                             // P is the peak
    }                                        // keep track of highest point in pulse wave

  //  NOW IT'S TIME TO LOOK FOR THE HEART BEAT
  // signal surges up in value every time there is a pulse
  if (N > 250)
  {                                   // avoid high frequency noise
    if ( (Signal > thresh) && (Pulse == false) && (N > (IBI/5)*3) )
      {        
        Pulse = true;                               // set the Pulse flag when we think there is a pulse
        digitalWrite(blinkPin,HIGH);                // turn on pin 13 LED
        IBI = sampleCounter - lastBeatTime;         // measure time between beats in mS
        lastBeatTime = sampleCounter;               // keep track of time for next pulse
  
        if(secondBeat)
        {                        // if this is the second beat, if secondBeat == TRUE
          secondBeat = false;                  // clear secondBeat flag
          for(int i=0; i<=9; i++) // seed the running total to get a realisitic BPM at startup
          {             
            rate[i] = IBI;                      
          }
        }
  
        if(firstBeat) // if it's the first time we found a beat, if firstBeat == TRUE
        {                         
          firstBeat = false;                   // clear firstBeat flag
          secondBeat = true;                   // set the second beat flag
          sei();                               // enable interrupts again
          return;                              // IBI value is unreliable so discard it
        }   
      // keep a running total of the last 10 IBI values
      word runningTotal = 0;                  // clear the runningTotal variable    

      for(int i=0; i<=8; i++)
        {                // shift data in the rate array
          rate[i] = rate[i+1];                  // and drop the oldest IBI value 
          runningTotal += rate[i];              // add up the 9 oldest IBI values
        }

      rate[9] = IBI;                          // add the latest IBI to the rate array
      runningTotal += rate[9];                // add the latest IBI to runningTotal
      runningTotal /= 10;                     // average the last 10 IBI values 
      BPM = 60000/runningTotal;               // how many beats can fit into a minute? that's BPM!
      QS = true;                              // set Quantified Self flag 
      // QS FLAG IS NOT CLEARED INSIDE THIS ISR
    }                       
  }

  if (Signal < thresh && Pulse == true)
    {   // when the values are going down, the beat is over
      digitalWrite(blinkPin,LOW);            // turn off pin 13 LED
      Pulse = false;                         // reset the Pulse flag so we can do it again
      amp = P - T;                           // get amplitude of the pulse wave
      thresh = amp/2 + T;                    // set thresh at 50% of the amplitude
      P = thresh;                            // reset these for next time
      T = thresh;
    }

  if (N > 2500)
    {                           // if 2.5 seconds go by without a beat
      thresh = 512;                          // set thresh default
      P = 512;                               // set P default
      T = 512;                               // set T default
      lastBeatTime = sampleCounter;          // bring the lastBeatTime up to date        
      firstBeat = true;                      // set these to avoid noise
      secondBeat = false;                    // when we get the heartbeat back
    }

  sei();                                   // enable interrupts when youre done!
}// end isr

Tài liệu tham khảo:

• Heartbeat Sensor using Arduino (Heart Rate Monitor)

Video tham khảo: