Lesson 2 Controlling an LED by a Button

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Introduction

In this lesson, we will learn how to turn an LED on or off by a button.

Components

– 1 * Raspberry Pi

– 1 * Breadboard

– 1 * LED

– 1 * Button

– 1 * Resistor (220Ω)

– Jumper wires

– 1 * T-Extension Board

– 1 * 40-Pin Cable

Principle

Button

Buttons are a common component used to control electronic devices. They are usually used as switches to connect or disconnect 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. Pin 1 is connected to pin 2 and pin 3 to pin 4. So you just need to connect either of pin 1 and pin 2 to pin 3 or pin 4. 

The following is the internal structure of a button. Since the pin 1 is connected to pin 2, and pin 3 to pin 4. The symbol on the right below is usually used to represent a button in circuits.

When the button is pressed, the 4 pins are connected, thus closing the circuit.

Use a normally open button as the input of Raspberry Pi, the detailed connection is as shown in the schematic diagram below. When the button is pressed, the B18 will turn into low level (0V). We can detect the state of the B18 through programming. That is, if the B18 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

For C language users:

Step 2: Open the code file:

    cd /home/pi/SunFounder_Super_Kit_V3.0_for_Raspberry_Pi/C

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

Step 3: Compile the Code

gcc 02_buttonControlLed.c -o 02_buttonControlLed -lwiringPi

or

make 02_buttonControlLed

Step 4: Run the executable file above

     sudo ./02_buttonControlLed

Step 5: Check the code

     nano 02_buttonControlLed.c 

Press Ctrl+X to exit, if you have modified the code, there will be a prompt asking whether to save the changes or not. Type in Y (save) or N (don’t save). Then press Enter to exit. Repeat Step 3 and Step 4 to see the effect after modifying.

For Python users:

Step 2: Open the code file          

    cd /home/pi/SunFounder_Super_Kit_V3.0_for_Raspberry_Pi/Python

Step 3Run the code

    sudo python 02_buttonControlLed.py

Step 4: Check the code

     nano 02_buttonControlLed.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. 

Summary

Through this experiment, you have learnt how to control the GPIOs of the Raspberry Pi by programming. 

C Code

/**********************************************************************
* Filename    : buttonControlLed.c
* Description : Controlling an led with button.
* Author      : Robot
* E-mail      : support@sunfounder.com
* website     : www.sunfounder.com
* Update      : Cavon    2016/07/01
**********************************************************************/
#include <wiringPi.h>
#include <stdio.h>

#define LedPin		0
#define ButtonPin 	1

int main(void)
{
	// When initialize wiring failed, print messageto screen
	if(wiringPiSetup() == -1){
		printf("setup wiringPi failed !");
		return 1; 
	}
	
	pinMode(LedPin, OUTPUT); 
	pinMode(ButtonPin, INPUT);
	// Pull up to 3.3V,make GPIO1 a stable level
	pullUpDnControl(ButtonPin, PUD_UP);

	printf("\n");
	printf("\n");
	printf("========================================\n");
	printf("|          Button control LED          |\n");
	printf("|    ------------------------------    |\n");
	printf("|         LED connect to GPIO0         |\n");
	printf("|        Button connect to GPIO1       |\n");
	printf("|                                      |\n");
	printf("|     Press button to turn on LED.     |\n");
	printf("|                                      |\n");
	printf("|                            SunFounder|\n");
	printf("========================================\n");
	printf("\n");
	printf("\n");

	digitalWrite(LedPin, HIGH);
	printf("LED off...\n");

	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;
}

Python Code

#!/usr/bin/env python

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

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

# Define a function to print message at the beginning
def print_message():
	print ("========================================")
	print ("|          Button control LED          |")
	print ("|    ------------------------------    |")
	print ("|         LED connect to GPIO0         |")
	print ("|        Button connect to GPIO1       |")
	print ("|                                      |")
	print ("|   Press button to turn on/off LED.   |")
	print ("|                                      |")
	print ("|                            SunFounder|")
	print ("========================================\n")
	print 'Program is running...'
	print 'Please press Ctrl+C to end the program...'
	raw_input ("Press Enter to begin\n")

# 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, pull_up_down=GPIO.PUD_UP)
	# Set up a falling detect on BtnPin, 
	# and callback function to swLed
	GPIO.add_event_detect(BtnPin, GPIO.FALLING, callback=swLed)

# 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():
	# Print messages
	print_message()
	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 child program 
	# destroy() will be  executed.
	except KeyboardInterrupt:
		destroy()