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.

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

Name	ITEMS IN THIS KIT	LINK
Raphael Kit	337	Raphael Kit

You can also buy them separately from the links below.

COMPONENT INTRODUCTION	PURCHASE LINK
GPIO Extension Board	BUY
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
LED	BUY
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.

Experimental Procedures¶

Step 1: Build the circuit.

Step 2: Open the code file.

cd ~/raphael-kit/python

Step 3: Run the code.

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 raphael-kit/python. After modifying the code, you can run it directly to see the effect.

import RPi.GPIO as GPIO
import time
LedPin = 17  # Set GPIO17 as LED pin
BtnPin = 18  # Set GPIO18 as button pin

# Set Led status to True(OFF)
Led_status = True

# Define a setup function for some setup
def setup():
    # Set the GPIO modes to BCM Numbering
    GPIO.setmode(GPIO.BCM)
    # Set LedPin's mode to output,
    # and initial level to high (3.3v)
    GPIO.setup(LedPin, GPIO.OUT, initial=GPIO.HIGH)
    # Set BtnPin's mode to input,
    # and pull up to high (3.3V)
    GPIO.setup(BtnPin, GPIO.IN)


# 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

LedPin = 17

Set GPIO17 as LED pin

BtnPin = 18

Set GPIO18 as button pin

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.

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.