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.. _2.2.8_js:

2.2.8 Ultrasonic Sensor Module
==============================

Introduction
--------------

The ultrasonic sensor uses ultrasonic to accurately detect objects and
measure distances. It sends out ultrasonic waves and converts them into
electronic signals.

Required Components
------------------------------

In this project, we need the following components. 

.. image:: ../img/list_2.2.5.png

It's definitely convenient to buy a whole kit, here's the link: 

.. list-table::
    :widths: 20 20 20
    :header-rows: 1

    *   - Name	
        - ITEMS IN THIS KIT
        - LINK
    *   - Raphael Kit
        - 337
        - |link_Raphael_kit|

You can also buy them separately from the links below.

.. list-table::
    :widths: 30 20
    :header-rows: 1

    *   - COMPONENT INTRODUCTION
        - PURCHASE LINK

    *   - :ref:`cpn_gpio_board`
        - |link_gpio_board_buy|
    *   - :ref:`cpn_breadboard`
        - |link_breadboard_buy|
    *   - :ref:`cpn_wires`
        - |link_wires_buy|
    *   - :ref:`cpn_ultrasonic_sensor`
        - |link_ultrasonic_buy|

Schematic Diagram
-----------------

.. image:: ../img/image329.png


Experimental Procedures
-----------------------

**Step 1:** Build the circuit.

.. image:: ../img/image220.png

**Step 2:** Go to the folder of the code.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/nodejs/

**Step 3:** Run the code.

.. raw:: html

   <run></run>

.. code-block::

    sudo node ultrasonic_sensor.js

With the code run, the ultrasonic sensor module detects the distance
between the obstacle ahead and the module itself, then the distance
value will be printed on the screen.

**Code**

.. code-block:: js

    const Gpio = require('pigpio').Gpio;

    // The number of microseconds it takes sound to travel 1cm at 20 degrees celcius
    const MICROSECDONDS_PER_CM = 1e6/34321;

    const trigger = new Gpio(23, {mode: Gpio.OUTPUT});
    const echo = new Gpio(24, {mode: Gpio.INPUT, alert: true});

    trigger.digitalWrite(0); // Make sure trigger is low

    const watchHCSR04 = () => {
      let startTick;

      echo.on('alert', (level, tick) => {
        if (level === 1) {
          startTick = tick;
        } else {
          const endTick = tick;
          const diff = (endTick >> 0) - (startTick >> 0); // Unsigned 32 bit arithmetic
          console.log(diff / 2 / MICROSECDONDS_PER_CM);
        }
      });
    };

    watchHCSR04();

    // Trigger a distance measurement once per second
    setInterval(() => {
      trigger.trigger(10, 1); // Set trigger high for 10 microseconds
    }, 1000);




**Code Explanation**

The ``trigger`` function can be used to generate a pulse on a GPIO and 
``alerts`` can be used to determine the time of a GPIO state change 
accurate to a few microseconds. 

These two features can be combined to measure distance using a HC-SR04 
ultrasonic sensor.

.. code-block:: js

    setInterval(() => {
      trigger.trigger(10, 1); // Set trigger high for 10 microseconds
    }, 1000);

This is to periodically send out a 10us ultrasonic pulse.

.. code-block:: js

  const watchHCSR04 = () => {

    echo.on('alert', (level, tick) => {
        if (level === 1) {
          startTick = tick;
        } else {
          const endTick = tick;
          const diff = (endTick >> 0) - (startTick >> 0); // Unsigned 32 bit arithmetic
          console.log(diff / 2 / MICROSECDONDS_PER_CM);
        }    
    });
  };

This function sets an alert that will record the time between sending the pulse (level is 1) and receiving the echo (level is 0).
By multiplying the time difference by the speed of sound (and dividing by 2), you can get the distance to the obstacle ahead.



.. https://github.com/fivdi/pigpio

Phenomenon Picture
------------------

.. image:: ../img/image221.jpeg