from machine import Pin, time_pulse_us
import utime

# Change these if you wired the sensor to different GPIOs
TRIG_PIN = 3
ECHO_PIN = 2

trig = Pin(TRIG_PIN, Pin.OUT)
echo = Pin(ECHO_PIN, Pin.IN)

SOUND_SPEED = 343      # metres per second (20 °C, sea-level)
MAX_DIST_CM = 500      # ≈5 m max range you care about
TIMEOUT_US  = int((MAX_DIST_CM / 100) * 2 / SOUND_SPEED * 1_000_000)

def distance_cm() -> float | None:
    """Return distance in centimetres, or None on timeout."""
    # 1 ) Make sure the trigger is LOW for ≥2 µs
    trig.value(0)
    utime.sleep_us(2)

    # 2 ) Send a 10 µs HIGH pulse to start measurement
    trig.value(1)
    utime.sleep_us(10)
    trig.value(0)

    # 3 ) Measure echo HIGH pulse width (µs)
    try:
        pulse_len = time_pulse_us(echo, 1, TIMEOUT_US)
    except OSError:         # time_pulse_us timed out
        return None

    # 4 ) Distance = (pulse time × speed of sound) / 2
    return (pulse_len * SOUND_SPEED / 1_000_000) * 100 / 2  # → centimetres

# ──────────────────────────────  MAIN LOOP  ────────────────────────────── #
while True:
    dist = distance_cm()
    if dist is None:
        print("Out of range or no echo")
    else:
        print("Distance: {:.2f} cm".format(dist))
    utime.sleep(1)