hcsr04 with a fix for the stupid signal being frozen

hcsr04_ultrasonic.py

# http://stackoverflow.com/questions/32300000/galileo-and-ultrasonic-error-when-distance-less-than-4cm
import mraa
import time

trig = mraa.Gpio(3)
echo = mraa.Gpio(4)

trig.dir(mraa.DIR_OUT)
echo.dir(mraa.DIR_IN)


def distance(measure='cm'):
    trig.write(0)
    time.sleep(0.1)

    trig.write(1)
    time.sleep(0.00001)
    trig.write(0)

    while echo.read() == 0:
            nosig = time.time()
    sig = None
    while echo.read() == 1:
            sig = time.time()

    if sig is None:
        return -1
    # et = Elapsed Time
    et = sig - nosig

    if measure == 'cm':
            distance =  et * 17150
    elif measure == 'in':
            distance = et / 0.000148
    else:
            print('improper choice of measurement!!')
            distance = None

    return distance if distance < 80 else -1


while True:
    print(distance('cm'))
    time.sleep(1)