Read out heartbeat sensor

(Deutsche Version) If you look for "heartbeat sensor" on ebay, you find a small circuit board which contains an LED and a photo transistor. This board claims to be able to measure the heart beat. I bought one sensor and tried it. Unfortunately, there is very few (ok, basically absolutely no) documentation about the sensor and not even something like a product key, only xinda and Lcup can be found on the sensor.

The heartbeat sensor

The sensor has 3 pins which are labelled very good. One of them has a -, here you have to connect GND. The next pin has an S, this is the pin for the sensor output. The last pin (in the middle) which has no label is connected to the LED. This pin has to be connected to +5V and the sensor output has to be connected to an analog input of the arduino.

The program reads the sensor value and sends it to the computer via the serial port where you can further process the data (e.g. with Matlab, maybe I will write a post about that). Here is a picture of the plotted heartbeat data on the computer and below the code for the Arduino.

//Define pins for LED and sensor.
int ledPin = 13;
int sensorPin = 0;
//alpha is used in the original proposed code by the company (see below).
double alpha = 0.75;
//lasttime is used to have a very precise measurement of the time so the calculated pulse is correct.
unsigned long lasttime;

void setup ()
{
  //Switch on the LED.
  pinMode (ledPin, OUTPUT);
  digitalWrite(ledPin,HIGH);
  Serial.begin (9600);
  lasttime = micros();
}

void loop ()
{
  //Also used for smoothening the signal.
  static double oldValue = 0;
  
  //Wait 10 ms between each measurement.
  while(micros() - lasttime < 10000)
  {
    delayMicroseconds(100);
  }
  
  //Read the signal.
  int rawValue = analogRead (sensorPin);
  lasttime += 10000;
  
  //In the "original" code example, "value" was sent to the computer. This calculation basically smoothens "rawValue".
  //double value = alpha * oldValue + (1 - alpha) * rawValue;
  
  //Send back the measured value.
  Serial.println (rawValue);
  oldValue = value;
}