Keyes 039 – Pulse Rate Sensor Module


This post has been updated in May 2016, now I have one of these units to test

This project uses bright infrared (IR) LED and a phototransistor to detect the pulse in the finger. A red LED flashes and phototransistor placed on the other side of the finger detects the pulse. The circuit uses a very high resistance to obtain the best reading, but this makes the circuit susceptible to stray light. Try to keep stray light, particularly mains lighting, to a minimum to reduce interference  to the small signal representing your pulse.

The output from the IR LED is normally invisible, but point the camera from your mobile phone at it to see it glowing brightly. As provided the light output will never get to the sensor, so carefully bend the LED so that it points at the photodiode. I spent some time trying to find a sketch that worked and eventually found this one in the Arduino Forum. Many thanks to the originator. The on board LED pulses to your heartbeat reasonably well and the output from the Serial Monitor gives a very rough reading that all in all at least makes this unit kind of usable.

My advice – buy the heat beat monitor shown in my blog here and use the Keyes 039 for spares!

////////////////////////////////////////////////////////////////////////
/// Copyright (c)2015 Dan Truong
/// Permission is granted to use this software under the MIT
/// licence, with my name and copyright kept in source code
/// http://http://opensource.org/licenses/MIT
///
/// KY039 Arduino Heartrate Monitor V1.0 (April 02, 2015)
////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////
/// @param[in] IRSensorPin Analog pin on which IR detector is connected
/// @param[in] delay (msec) delay between calls to this method. It is
///                  best to call it at least 5 times per beat, aka
///                  no slower than 150msec for 70bpm. An ideal value
///                  is 60ms or faster to handle up to 200 BPM.
///
/// @brief
/// True if heartbeat is detected on the sensor.
/// This code is trivial and just does a peak detection, instead of
/// trying to detect the heart’s pulse waveform.
/// Note: I am fudging sensor data with the delay to make the integer
/// math after that uses constants, somewhat independant of the sleep
/// delay used in the main loop. Otherwise if maxValue decays too slow
/// or too fast, it causes glitches and false beat detection.
////////////////////////////////////////////////////////////////////////
//#define HBDEBUG(i) i
#define HBDEBUG(i)

bool
heartbeatDetected(int IRSensorPin, int delay)
{
static int maxValue = 0;
static bool isPeak = false;
int rawValue;
bool result = false;

rawValue = analogRead(IRSensorPin);
// Separated because analogRead() may not return an int
rawValue *= (1000/delay);
HBDEBUG(Serial.print(isPeak); Serial.print(“p, “));
HBDEBUG(Serial.print(rawValue); Serial.print(“r, “));
HBDEBUG(Serial.print(maxValue); Serial.print(“m, “));

// If sensor shifts, then max is out of whack.
// Just reset max to a new baseline.
if (rawValue * 4L < maxValue) {
maxValue = rawValue * 0.8;
HBDEBUG(Serial.print(“RESET, “));
}

// Detect new peak
if (rawValue > maxValue – (1000/delay)) {
// Only change peak if we find a higher one.
if (rawValue > maxValue) {
maxValue = rawValue;
}
// Only return true once per peak.
if (isPeak == false) {
result = true;
HBDEBUG(Serial.print(result); Serial.print(“,  *”));
}
isPeak = true;
} else if (rawValue < maxValue – (3000/delay)) {
isPeak = false;
// Decay max value to adjust to sensor shifting
// Note that it may take a few seconds to re-detect
// the signal when sensor is pushed on meatier part
// of the finger. Another way would be to track how
// long since last beat, and if over 1sec, reset
// maxValue, or to use derivatives to remove DC bias.
maxValue-=(1000/delay);
}
HBDEBUG(Serial.print(“\n”));
return result;
}

////////////////////////////////////////////////////////////////////////
// Arduino main code
////////////////////////////////////////////////////////////////////////
int ledPin=13;
int analogPin=0;

void setup()
{
// Built-in arduino board pin for the display LED
pinMode(ledPin,OUTPUT);

// Init serial console
Serial.begin(9600);
Serial.println(“Heartbeat detection sample code.”);
}

const int delayMsec = 60; // 100msec per sample

// The main loop blips the LED and computes BPMs on serial port.
void loop()
{
static int beatMsec = 0;
int heartRateBPM = 0;

if (heartbeatDetected(analogPin, delayMsec)) {
heartRateBPM = 60000 / beatMsec;
digitalWrite(ledPin,1);

// Print msec/beat and instantaneous heart rate in BPM
Serial.print(beatMsec);
Serial.print(“, “);
Serial.println(heartRateBPM);

beatMsec = 0;
} else {
digitalWrite(ledPin,0);
}
// Note: I assume the sleep delay is way longer than the
// number of cycles used to run the code hence the error
// is negligible for math.
delay(delayMsec);
beatMsec += delayMsec;
}

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