====== HC-SR04 - Ultrasonic Distance Measure ======
Ultrasonic Distance Measuring Sensor

{{:components:hc-sr04.png?nolink&200|}}

**Module main technical parameters**
  * Working Voltage : 3.3 - 5V(DC)
  * Static current: Less than 2mA.
  * Output signal: Electric frequency signal, high level 5V, low level 0V.
  * Sensor angle: Not more than 15 degrees.
  * Detection distance: 2cm-450cm.
  * High precision: Up to 0.3cm
  * Input trigger signal: 10us TTL impulse
  * Echo signal : output TTL PWL signal

**Module Working Principle**
  * Adopt IO trigger through supplying at least 10us sequence of high level signal.
  * The module automatically send eight 40khz square wave and automatically detect whether receive the returning pulse signal.
  * If there is signals returning, through outputting high level and the time of high level continuing is the time of that from the ultrasonic transmitting to receiving. 


  Test distance =  (high level time * sound velocity (340M/S) / 2.

====== Code ======
<code python>
from HCSR04 import HCSR04
import time

sensor = HCSR04(trigger_pin=12, echo_pin=14, echo_timeout_us=10000)
while True:
    distance = sensor.distance_cm()
    print('Distance:', distance, 'cm')
    time.sleep(0.1)
</code>

===== Library =====
<code python HCSR04.py>
import machine, time
from machine import Pin

__version__ = '0.2.0'
__author__ = 'Roberto Sánchez'
__license__ = "Apache License 2.0. https://www.apache.org/licenses/LICENSE-2.0"

class HCSR04:
    """
    Driver to use the untrasonic sensor HC-SR04.
    The sensor range is between 2cm and 4m.
    The timeouts received listening to echo pin are converted to OSError('Out of range')
    """
    # echo_timeout_us is based in chip range limit (400cm)
    def __init__(self, trigger_pin, echo_pin, echo_timeout_us=500*2*30):
        """
        trigger_pin: Output pin to send pulses
        echo_pin: Readonly pin to measure the distance. The pin should be protected with 1k resistor
        echo_timeout_us: Timeout in microseconds to listen to echo pin. 
        By default is based in sensor limit range (4m)
        """
        self.echo_timeout_us = echo_timeout_us
        # Init trigger pin (out)
        self.trigger = Pin(trigger_pin, mode=Pin.OUT, pull=None)
        self.trigger.value(0)

        # Init echo pin (in)
        self.echo = Pin(echo_pin, mode=Pin.IN, pull=None)

    def _send_pulse_and_wait(self):
        """
        Send the pulse to trigger and listen on echo pin.
        We use the method `machine.time_pulse_us()` to get the microseconds until the echo is received.
        """
        self.trigger.value(0) # Stabilize the sensor
        time.sleep_us(5)
        self.trigger.value(1)
        # Send a 10us pulse.
        time.sleep_us(10)
        self.trigger.value(0)
        try:
            pulse_time = machine.time_pulse_us(self.echo, 1, self.echo_timeout_us)
            return pulse_time
        except OSError as ex:
            if ex.args[0] == 110: # 110 = ETIMEDOUT
                raise OSError('Out of range')
            raise ex

    def distance_mm(self):
        """
        Get the distance in milimeters without floating point operations.
        """
        pulse_time = self._send_pulse_and_wait()

        # To calculate the distance we get the pulse_time and divide it by 2 
        # (the pulse walk the distance twice) and by 29.1 becasue
        # the sound speed on air (343.2 m/s), that It's equivalent to
        # 0.34320 mm/us that is 1mm each 2.91us
        # pulse_time // 2 // 2.91 -> pulse_time // 5.82 -> pulse_time * 100 // 582 
        mm = pulse_time * 100 // 582
        return mm

    def distance_cm(self):
        """
        Get the distance in centimeters with floating point operations.
        It returns a float
        """
        pulse_time = self._send_pulse_and_wait()

        # To calculate the distance we get the pulse_time and divide it by 2 
        # (the pulse walk the distance twice) and by 29.1 becasue
        # the sound speed on air (343.2 m/s), that It's equivalent to
        # 0.034320 cm/us that is 1cm each 29.1us
        cms = (pulse_time / 2) / 29.1
        return cms
</code>