What you'll learn

Timing a pulse by hand
Turning echo time into distance
A resistor divider for a 3.3 V input
Timeouts that stop the loop from hanging

Difficulty: Intermediate
Time: About 20 minutes
Board: Raspberry Pi Pico
Parts: 5

How it works

The HC-SR04 sends out a burst of sound when TRIG (GP19) gets a short pulse. ECHO then goes high until the sound comes back. The sensor runs on 5 V, so ECHO goes through a 1k and 2k divider to bring it down to 3.3 V for GP18.

The MicroPython code sends a 20 microsecond pulse and times how long ECHO stays high with ticks_us. Sound goes there and back at 343 m/s, so the distance is the time times 0.0343 centimeters per microsecond, divided by 2. It prints a result twice a second.

Parts list

5 parts, plus the jumper wires. They retail for about $7.10; on Mokxi they're free.

1 × Raspberry Pi PicoIn Fight Stick Kit.
1 × Ultrasonic rangerIn Halloween Prop Kit.
1 × Resistor, 1k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × Resistor, 2k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × Power, 5 V

Kits that can build it

This starter kit has every part in the circuit. Each kit's page lists what else it can build.

Freenove Ultimate Starter Kit for Raspberry Pi Pico

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pin	Connects to
Ultrasonic ranger pin TRIG	Raspberry Pi Pico pin GP19
Resistor, 1k Ω pin 2	Raspberry Pi Pico pin GP18
Resistor, 2k Ω pin 1	Raspberry Pi Pico pin GP18
Ultrasonic ranger pin VCC	5 V
Resistor, 1k Ω pin 1	Ultrasonic ranger pin ECHO
Ultrasonic ranger pin GND	Ground
Resistor, 2k Ω pin 2	Ground

The code

The MicroPython code exactly as the editor opens it. Change a line there and press Run: the board runs it on the official MicroPython firmware, with nothing to compile.

main.py · Raspberry Pi Pico

# Ultrasonic ranging by hand: a 20 us pulse on TRIG (GP19), then time how
# long ECHO (GP18) stays high. Sound goes there and back at 343 m/s.
from machine import Pin
import time

trig = Pin(19, Pin.OUT, value=0)
echo = Pin(18, Pin.IN)

def distance_cm():
    trig.value(1)
    time.sleep_us(20)                         # 20 us: the datasheet asks for at least 10
    trig.value(0)
    start = time.ticks_us()
    while echo.value() == 0:                 # wait for the echo to start
        if time.ticks_diff(time.ticks_us(), start) > 30000:
            return None
    began = time.ticks_us()
    while echo.value() == 1:                 # and time it
        if time.ticks_diff(time.ticks_us(), began) > 30000:
            return None
    return time.ticks_diff(time.ticks_us(), began) * 0.0343 / 2

while True:
    d = distance_cm()
    print("distance: %.1f cm" % d if d is not None else "nothing in range")
    time.sleep_ms(500)

Code walkthrough

The key functions of the sketch, and what each one does.

`distance_cm` main.py

Sets trig high for 20 us, then low. It waits for echo to go high, then times how long it stays high. If either wait lasts more than 30000 us, it returns None, which the main loop prints as "nothing in range". Otherwise it returns the time times 0.0343 divided by 2.

def distance_cm():
    trig.value(1)
    time.sleep_us(20)                         # 20 us: the datasheet asks for at least 10
    trig.value(0)
    start = time.ticks_us()
    while echo.value() == 0:                 # wait for the echo to start
        if time.ticks_diff(time.ticks_us(), start) > 30000:
            return None
    began = time.ticks_us()
    while echo.value() == 1:                 # and time it
        if time.ticks_diff(time.ticks_us(), began) > 30000:
            return None
    return time.ticks_diff(time.ticks_us(), began) * 0.0343 / 2

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

It always says "nothing in range" This project
Check TRIG on GP19, ECHO through the divider to GP18, and 5 V on the sensor's VCC.
ECHO goes straight to the Pico This project
Keep the divider: 1k from ECHO to GP18, and 2k from GP18 to ground.
Distance reads zero, or never changes. Ultrasonic ranger
Power it from 5 V and share ground. Send a 10 microsecond HIGH pulse on TRIG, then time ECHO with pulseIn. Check TRIG and ECHO are not swapped.
Readings jump around or are wrong on a 3.3 V board. Ultrasonic ranger
ECHO outputs 5 V, so divide it down for 3.3 V boards. Aim at flat, hard surfaces. The range is about 2 cm to 4 m, and soft or angled targets bounce the sound away.
The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
The Pico does not show up as a drive, or the program will not load. Raspberry Pi Pico
Hold the BOOTSEL button while you plug in USB. A drive named RPI-RP2 appears. Then copy the UF2 file onto it.
A 5 V signal or sensor is wired to a GPIO pin. Raspberry Pi Pico
Pico GPIO pins are 3.3 V only and are not 5 V tolerant. Use a level shifter or a voltage divider for any 5 V output going into the board.