from machine import Pin, time_pulse_us
import time

# Define pins for Trig and Echo
trig_pin = Pin(27, Pin.OUT)
echo_pin = Pin(26, Pin.IN)

# Define LED output
led_red = Pin(25, Pin.OUT)

def measure_distance(): # function definition
    # Send a 10us pulse to trigger the sensor
    trig_pin.off() # Sets the trigger pin low. 
    time.sleep_us(2) # Very short pause to stabilize the low signal
    trig_pin.on()# Sets the trigger pin high.
    time.sleep_us(10) # Keeps the trigger pin high for 10 microseconds 
    trig_pin.off() #Sets the trigger pin low again.

    # Measure the pulse duration on the Echo pin
    pulse_duration = time_pulse_us(echo_pin, 1, 30000)  # Timeout after 30ms
    # 30000: This is a timeout value specified in microseconds. 
    # It means that time_pulse_us() will wait for a maximum of 30,000 microseconds (30 milliseconds) for the rising edge on the echo_pin.  If 
    #no pulse is detected within this time, the function will return -1. 


    # Calculate distance in centimeters
    distance_cm = (pulse_duration / 2) / 29.1  # Speed of sound is 343 m/s or 29.1 cm/us
    return distance_cm

while True:
    distance = measure_distance()
    print("Distance:", distance, "cm")

    #coding for simple ultrasonic sensor
    #if distance < 200, red LED is ON
    #if distance > 201, red LED is OFF
    if (distance < 200):
        led_red.value(1)
        time.sleep(1)  # Wait for 1 second before the next measurement
    if (distance > 201):
        led_red.value(0)
        time.sleep(1)  # Wait for 1 second before the next measurement