HC-SR04 on a Pico with MicroPython

# An HC-SR04 on a Raspberry Pi Pico, in MicroPython.
# The HC-SR04 needs 5 V, from VBUS (pin 40). TRIG is on GP3 (pin 5). ECHO is a
# 5 V pulse, so it reaches GP2 (pin 4) through a 1 k and 2 k divider: 5 V x 2/3
# is 3.3 V, all the RP2040 can take.
from machine import Pin, time_pulse_us
import time

trig = Pin(3, Pin.OUT)
echo = Pin(2, Pin.IN)
led = Pin("LED", Pin.OUT)       # the Pico's own LED: lit closer than 20 cm

def distance_cm():
    trig.low()
    time.sleep_us(2)
    trig.high()
    time.sleep_us(10)           # a 10 us pulse starts a measurement
    trig.low()
    t = time_pulse_us(echo, 1, 30000)   # how long ECHO stays high, in us
    if t < 0:
        return None             # no echo within 30 ms: nothing in range
    return t * 0.0343 / 2       # sound goes 0.0343 cm a microsecond, there and back

while True:
    d = distance_cm()
    if d is None:
        print("Out of range")
    else:
        print("Distance: %.1f cm" % d)
        led.value(d < 20)
    time.sleep(0.5)