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.. _2.1.1_c:

2.1.1 Button
==================

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

In this project, we will learn how to turn on or off the LED by using a
button.

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

In this project, we need the following components. 

.. image:: ../img/list_2.1.1_Button.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_resistor`
        - |link_resistor_buy|
    *   - :ref:`cpn_led`
        - |link_led_buy|
    *   - :ref:`cpn_button`
        - |link_button_buy|

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

Use a normally open button as the input of Raspberry Pi, the connection
is shown in the schematic diagram below. When the button is pressed, the
GPIO18 will turn into low level (0V). We can detect the state of the
GPIO18 through programming. That is, if the GPIO18 turns into low level,
it means the button is pressed. You can run the corresponding code when
the button is pressed, and then the LED will light up.

.. note::
    The longer pin of the LED is the anode and the shorter one is
    the cathode.

.. image:: ../img/image302.png


.. image:: ../img/image303.png


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

**Step 1**: Build the circuit.

.. image:: ../img/image152.png

**Step 2**: Open the code file.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/c/2.1.1/

.. note::
    Change directory to the path of the code in this experiment via **cd**.

**Step 3**: Compile the code.

.. raw:: html

   <run></run>

.. code-block::

    gcc 2.1.1_Button.c -lwiringPi

**Step 4**: Run the executable file.

.. raw:: html

   <run></run>

.. code-block::

    sudo ./a.out

After the code runs, press the button, the LED lights up; otherwise,
turns off.

.. note::

    If it does not work after running, or there is an error prompt: \"wiringPi.h: No such file or directory\", please refer to :ref:`install_wiringpi`.

**Code**

.. code-block:: c

    #include <wiringPi.h>
    #include <stdio.h>

    #define LedPin      0
    #define ButtonPin   1

    int main(void){
        // When initialize wiring failed, print message to screen
        if(wiringPiSetup() == -1){
            printf("setup wiringPi failed !");
            return 1;
        }
        
        pinMode(LedPin, OUTPUT);
        pinMode(ButtonPin, INPUT);
        digitalWrite(LedPin, HIGH);
        
        while(1){
            // Indicate that button has pressed down
            if(digitalRead(ButtonPin) == 0){
                // Led on
                digitalWrite(LedPin, LOW);
            //  printf("...LED on\n");
            }
            else{
                // Led off
                digitalWrite(LedPin, HIGH);
            //  printf("LED off...\n");
            }
        }
        return 0;
    }

**Code Explanation**

.. code-block:: c

    #define LedPin      0

Pin GPIO17 in the T_Extension Board is equal to the GPIO0 in the
wiringPi.

.. code-block:: c

    #define ButtonPin   1

ButtonPin is connected to GPIO1.

.. code-block:: c

    pinMode(LedPin, OUTPUT);

Set LedPin as output to assign value to it.

.. code-block:: c

    pinMode(ButtonPin, INPUT);

Set ButtonPin as input to read the value of ButtonPin.

.. code-block:: C

    while(1){
            // Indicate that button has pressed down
            if(digitalRead(ButtonPin) == 0){
                // Led on
                digitalWrite(LedPin, LOW);
            //  printf("...LED on\n");
            }
            else{
                // Led off
                digitalWrite(LedPin, HIGH);
            //  printf("LED off...\n");
            }
        }


``if (digitalRead (ButtonPin) == 0)`` : check whether the button has been
pressed. Execute ``digitalWrite(LedPin, LOW)`` when button is pressed to
light up LED.

``digitalRead()`` function is to read HIGH (high level) or LOW (low level) of 
the input parameter word pin, it returns 1 when pin is HIGH and returns 0 
when pin is LOW.

``digitalWrite()`` function is to write HIGH (high level) or LOW (low level) to 
the input parameter word pin.

Phenomenon Picture
^^^^^^^^^^^^^^^^^^

.. image:: ../img/image153.jpeg