Heartbeat Sensor 

Hardware

Python

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
# Define function to measure charge time
def RC_Analog (Pin):
   counter = 0
   # Discharge capacitor
   GPIO.setup(Pin, GPIO.OUT)
   GPIO.output(Pin, GPIO.LOW)
   time.sleep(1)
   GPIO.setup(Pin, GPIO.IN)
   # Count loops until voltage across capacitor reads high on GPIO
   while(GPIO.input(Pin)==GPIO.LOW):  
         counter =counter+1
   return counter
# Main program loop
while True:
  print RC_Analog(4) # Measure timing using GPIO4

Test and Result

Pulse Number display, you should compare and convert data with real heartbeat machine

Reference