Ultrasonic Sensor

see also https://tutorials-raspberrypi.de/entfernung-messen-mit-ultraschallsensor-hc-sr04/

# WF 2019-06-11
# Distance Measuring with HC-SR04 Ultrasonic Sensor
#
# You need to modify the GPIO_TRIGGER and GPIO_ECHO settings
# according to your setup for this code to work properly.

# Needed Libraries
import RPi.GPIO as GPIO
import time

#GPIO Modus (BOARD / BCM)
GPIO.setmode(GPIO.BCM)
      
#GPIO Pins to be used 
GPIO_TRIGGER = 15
GPIO_ECHO = 14 
       
# set direction of GPIO pins (IN / OUT)
GPIO.setup(GPIO_TRIGGER, GPIO.OUT)
GPIO.setup(GPIO_ECHO, GPIO.IN)
        
def distance():
  # set Trigger to HIGH
  GPIO.output(GPIO_TRIGGER, True)

  # set Trigger to Low after 0.01ms 
  time.sleep(0.00001)
  GPIO.output(GPIO_TRIGGER, False)
                                  
  InitTime = time.time() 
  TriggerTime = time.time()
  # Sound travels 6 m in 1/40 of a second the 
  # HC-SR04 can measure up to some 3m only so 6m is the maximum echo 
  # travel distance
  MaxTime = 1/40.0;
  EchoTime = time.time()
         
  # wait for trigger and set time 
  while GPIO.input(GPIO_ECHO) == 0:
    TriggerTime = time.time()
    if InitTime-TriggerTime > MaxTime:
      break;
                                                                
  # wait for echo and set time
  while GPIO.input(GPIO_ECHO) == 1:
    EchoTime = time.time()
    if EchoTime-TriggerTime > MaxTime:
      break;
                                                          
  # calc time difference between trigger and echo
  TimeElapsed = EchoTime - TriggerTime
  # multiply by speed fo sound (34300 cm/s)
  # and divide by two (sound went back and forth) 
  distance = (TimeElapsed * 34300) / 2
     
  return distance
                                                                                   
if __name__ == '__main__':
    try:
        print("Starting ultrasound distance measurement ...")
        while True:
            dist = distance()
            if dist <= 300:
                print ("distance = %.1f cm" % dist)
            else:
                print ("distance = ?");
            time.sleep(1)


    # on CTRL-C reset GPIO
    except KeyboardInterrupt:
        print("Aborted ultrasound distance measurement by User ... cleaning up GPIO")
        GPIO.cleanup()