This heartbeat sensor uses a bright infrared (IR) LED and a phototransistor to detect the pulse of a fingertip. A red LED flashes with each pulse. The IR LED lights up the top of the finger, and a phototransistor on the bottom of the finger measures the light emitted. When the heartbeat pulses the resistance of the phototransistor will be slightly changed. The heartbeat sensor uses a very high resistance resistor because most of the light is absorbed by the finger and the phototransistor must be sensitive enough to detect these small changes in light. The resistance can be adjusted to get the best results.

It is important to keep stray light from entering the phototransistor. For home lighting that is particularly important because the lights at home are generally 50HZ or 60HZ and thus fluctuate, so a faint heartbeat will add considerable noise When running the program the measured values are printed. It could be challenging to measure an actual heart rate value using this sensor.

Test Code

int sensorPin = 0;
double alpha = 0.75;
int period = 100;
double change = 0.0;
double minval = 0.0;

void setup () {
Serial.begin (9600);
}

void loop () {
static double oldValue = 0;
static double oldChange = 0;
int rawValue = analogRead (sensorPin);
double value = alpha * oldValue + (1 - alpha) * rawValue;
Serial.print (rawValue);
Serial.print (",");
Serial.println (value);
oldValue = value;
delay (period);
}