Add the following declarations on the top of the sketch:

#define SENSOR A0
#define HEARTBEAT_POLL_PERIOD50
#define SECONDS 10
constint TIMESLOTS = SECONDS * 1000 / HEARTBEAT_POLL_PERIOD;
int sensorReading = 0;

We define the object-like macro SENSOR with a A0 value that is the pin we will use for the analog read. We set HEARTBEAT_POLL_PERIOD to specify how many milliseconds the microcontroller should wait between consecutive sensor readings. With the SECONDS parameter, we define the seconds that should elapse before we use collected data to process and estimate the heart rate. Indeed, we multiply SECONDS by 1000 to convert this value into milliseconds and then we divide it by the...