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6.1 Measuring Distance with an Ultrasonic Sensor

In this lesson, we’ll learn how to use an ultrasonic sensor module with the Raspberry Pi Pico 2 to measure the distance to an object. Ultrasonic sensors are commonly used in robotics and automation systems for object detection and distance measurement.

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Ultrasonic Module	1	BUY

Understanding the Ultrasonic Sensor

The ultrasonic sensor works by emitting a short ultrasonic pulse from the Trig pin and listening for the echo on the Echo pin. By measuring the time it takes for the echo to return, we can calculate the distance to an object using the speed of sound.

ultrasonic_prin

Trigger Pulse: A 10-microsecond high pulse on the Trig pin initiates the measurement.
Ultrasonic Burst: The sensor emits an 8-cycle ultrasonic burst at 40 kHz.
Echo Reception: The Echo pin goes high, and stays high until the echo is received back.
Time Measurement: By measuring the time the Echo pin stays high, we can calculate the distance.

Circuit Diagram

sch_ultrasonic

Wiring Diagram

wiring_ultrasonic

Writing the Code

Let’s write a MicroPython program to measure distance using the ultrasonic sensor.

Note

Open the 6.1_measuring_distance.py from newton-lab-kit/micropython or copy the code into Thonny, then click “Run” or press F5.
Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

import machine
import utime

# Define the pins connected to the sensor
TRIG = machine.Pin(17, machine.Pin.OUT)
ECHO = machine.Pin(16, machine.Pin.IN)

def measure_distance():
    # Ensure the trigger pin is low
    TRIG.low()
    utime.sleep_us(2)
    # Send a 10µs pulse to trigger the measurement
    TRIG.high()
    utime.sleep_us(10)
    TRIG.low()

    # Wait for the echo pin to go high (start of echo pulse)
    while ECHO.value() == 0:
        pass
    start_time = utime.ticks_us()

    # Wait for the echo pin to go low (end of echo pulse)
    while ECHO.value() == 1:
        pass
    end_time = utime.ticks_us()

    # Calculate the duration of the echo pulse
    duration = utime.ticks_diff(end_time, start_time)

    # Calculate the distance (speed of sound is 34300 cm/s)
    distance = (duration * 0.0343) / 2
    return distance

while True:
    dist = measure_distance()
    print("Distance: {:.2f} cm".format(dist))
    utime.sleep(0.5)

Once the code is running, the Thonny Shell should display the distance readings in centimeters. Move an object closer or farther from the sensor to see the readings change.

Understanding the Code

Import necessary modules and set up the trigger and echo pins:

import machine
import utime

TRIG = machine.Pin(17, machine.Pin.OUT)
ECHO = machine.Pin(16, machine.Pin.IN)

Measuring Distance:

Sends a trigger pulse to initiate measurement.
Waits for the echo response.
Calculates the duration of the echo pulse.
Computes the distance using the speed of sound.

def measure_distance():
    # Ensure trigger is low
    TRIG.low()
    utime.sleep_us(2)
    # Trigger a 10µs pulse
    TRIG.high()
    utime.sleep_us(10)
    TRIG.low()

    # Wait for echo to start
    while ECHO.value() == 0:
        pass
    start_time = utime.ticks_us()

    # Wait for echo to end
    while ECHO.value() == 1:
        pass
    end_time = utime.ticks_us()

    # Calculate duration
    duration = utime.ticks_diff(end_time, start_time)
    # Calculate distance
    distance = (duration * 0.0343) / 2
    return distance

Main Loop:

Continuously measures and prints the distance.
Pauses for half a second between measurements.

while True:
    dist = measure_distance()
    print("Distance: {:.2f} cm".format(dist))
    utime.sleep(0.5)

Understanding Limitations

Blocking Code:
- The while loops used to wait for the echo can block other code from running.
- For more advanced applications, consider using interrupts or asynchronous programming to avoid blocking.
Measurement Range:
- The HC-SR04 sensor typically has a range of 2 cm to 400 cm.
- Objects closer than 2 cm or farther than 400 cm may not be detected accurately.
Environmental Factors:
- Temperature and humidity can affect the speed of sound.
- For precise measurements, adjust the speed of sound based on ambient conditions.

Conclusion

You’ve successfully used an ultrasonic sensor to measure distance with the Raspberry Pi Pico 2. This fundamental skill is widely applicable in robotics, automation, and interactive projects.