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2.1.1 Button
============
Introduction
-----------------
In this lesson, we will learn how to turn on or off the LED by using a
button.
Components
---------------
.. image:: img/list_2.1.1_Button.png
Principle
-------------
**Button**
Button is a common component used to control electronic devices. It is
usually used as switch to connect or break circuits. Although buttons
come in a variety of sizes and shapes, the one used here is a 6mm
mini-button as shown in the following pictures.
Two pins on the left are connected, and the one on the right is similar
to the left, which is shown below:
.. image:: img/image148.png
:width: 400
:align: center
The symbol shown as below is usually used to represent a button in
circuits.
.. image:: img/image301.png
:width: 400
:align: center
When the button is pressed, the 4 pins are connected, thus closing the
circuit.
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
:width: 600
:align: center
.. image:: img/image303.png
:width: 400
:align: center
Experimental Procedures
---------------------------
**Step 1**: Build the circuit.
.. image:: img/image152.png
:width: 800
For C Language Users
^^^^^^^^^^^^^^^^^^^^
**Step 2**: Open the code file.
.. raw:: html
.. code-block::
cd ~/davinci-kit-for-raspberry-pi/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
.. code-block::
gcc 2.1.1_Button.c -lwiringPi
**Step 4**: Run the executable file.
.. raw:: html
.. 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:`C code is not working?`.
**Code**
.. code-block:: c
#include
#include
#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::
#define LedPin 0
Pin GPIO17 in the T_Extension Board is equal to the GPIO0 in the
wiringPi.
.. code-block::
#define ButtonPin 1
ButtonPin is connected to GPIO1.
.. code-block::
pinMode(LedPin, OUTPUT);
Set LedPin as output to assign value to it.
.. code-block::
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.
For Python Language Users
^^^^^^^^^^^^^^^^^^^^^^^^^
**Step 2**: Open the code file.
.. raw:: html
.. code-block::
cd ~/davinci-kit-for-raspberry-pi/python
**Step 3**: Run the code.
.. raw:: html
.. code-block::
sudo python3 2.1.1_Button.py
Now, press the button, and the LED will light up; press the button
again, and the LED will go out. At the same time, the state of the LED
will be printed on the screen.
**Code**
.. note::
You can **Modify/Reset/Copy/Run/Stop** the code below. But before that, you need to go to source code path like ``davinci-kit-for-raspberry-pi/python``.
.. raw:: html
.. code-block:: python
import RPi.GPIO as GPIO
import time
LedPin = 17 # Set GPIO17 as LED pin
BtnPin = 18 # Set GPIO18 as button pin
Led_status = True # Set Led status to True(OFF)
# Define a setup function for some setup
def setup():
# Set the GPIO modes to BCM Numbering
GPIO.setmode(GPIO.BCM)
GPIO.setup(LedPin, GPIO.OUT, initial=GPIO.HIGH) # Set LedPin's mode to output, and initial level to high (3.3v)
GPIO.setup(BtnPin, GPIO.IN) # Set BtnPin's mode to input.
# Define a callback function for button callback
def swLed(ev=None):
global Led_status
# Switch led status(on-->off; off-->on)
Led_status = not Led_status
GPIO.output(LedPin, Led_status)
if Led_status:
print ('LED OFF...')
else:
print ('...LED ON')
# Define a main function for main process
def main():
# Set up a falling detect on BtnPin,
# and callback function to swLed
GPIO.add_event_detect(BtnPin, GPIO.FALLING, callback=swLed)
while True:
# Don't do anything.
time.sleep(1)
# Define a destroy function for clean up everything after
# the script finished
def destroy():
# Turn off LED
GPIO.output(LedPin, GPIO.HIGH)
# Release resource
GPIO.cleanup()
# If run this script directly, do:
if __name__ == '__main__':
setup()
try:
main()
# When 'Ctrl+C' is pressed, the program
# destroy() will be executed.
except KeyboardInterrupt:
destroy()
**Code Explanation**
.. code-block:: python
LedPin = 17
Set GPIO17 as LED pin
.. code-block:: python
BtnPin = 18
Set GPIO18 as button pin
.. code-block:: python
GPIO.add_event_detect(BtnPin, GPIO.FALLING, callback=swLed)
Set up a falling detect on BtnPin, and then when the value of BtnPin
changes from a high level to a low level, it means that the button is
pressed. The next step is calling the function, swled.
.. code-block:: python
def swLed(ev=None):
global Led_status
# Switch led status(on-->off; off-->on)
Led_status = not Led_status
GPIO.output(LedPin, Led_status)
Define a callback function as button callback. When the button is
pressed at the first time，and the condition, not Led_status is false,
GPIO.output() function is called to light up the LED. As the button is
pressed once again, the state of LED will be converted from false to
true, thus the LED will turn off.
Phenomenon Picture
------------------------
.. image:: img/image153.jpeg