3.8. Sonar Distance Sensor

The SR04 Ultrasonic distance sensor allows us to measure distances to objects. It has the following properties:

• Minimum rangę: 20mm

• Maximum range: 4000mm

• Accuracy: 3mm

• Angle (horizontal field of view): 15 degrees

Note

We’ll learn more about how ultrasonic sensors work later in Touch and Proximity.

3.8.1. Reading the Distance

Reading the distance requires knowledge of how the sensor works. For the moment we’ll ignore that and just use the code below to read the distances:

import utime


def ultrasound_measure():
    pin1.write_digital(1)
    utime.sleep_us(10)
    pin1.write_digital(0)

    timeout = utime.ticks_us()
    while True:
        pulseBegin = utime.ticks_us()
        if 1 == pin2.read_digital():
            break
        if (pulseBegin - timeout) > 5000:
            return -1

    while True:
        pulseEnd = utime.ticks_us()
        if 0 == pin2.read_digital():
            break
        if (pulseEnd - pulseBegin) > 5000:
            return -2

    x = pulseEnd - pulseBegin
    d = x / 58
    return int(d)

Example

To read the distance in our main loop we might do:

while True:
    dist = ultrasound_measure()
    if dist >= 0:
        print(dist)
        sleep(100)

Note

The ultrasonic sensor reads will fail occasionally, therefore we check that we get a positive distance before accepting the reading. We also keep reading until we get a good measurement.