import RPi.GPIO as GPIO
import time

#IO pin numbers
SENSOR_PIN = 18

#Setup IO ports
GPIO.setwarnings(False)
GPIO.setmode(GPIO.BCM)
GPIO.setup(SENSOR_PIN, GPIO.IN)

pulse_start = 0
pulse_end = 0

try:
	#Main loop
	while True :
		while (GPIO.input(SENSOR_PIN)==1):
			pulse_start = time.time() 

		timeout = 0
		while (GPIO.input(SENSOR_PIN)==0) and (timeout < 50):
			timeout = timeout + 1		
			pulse_end = time.time() 

		if timeout < 50:
			pulse_width = pulse_end - pulse_start 
			outputString = "pulse_width: " + str(pulse_width)
			print outputString

except KeyboardInterrupt:
    pass
p.stop()