Lesson 3 Controlling an LED by a Button

Introduction

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

Experimental Conditions

– 1*Raspberry Pi

– 1*Breadboard

– 1*Network cable (or USB wireless network adapter)

– 1*LED

– 1*Button

– 1*Resistor (220Ω)

– Jumper wires

Experimental Principle

Use a normally open button as an input of Raspberry Pi, when the button is pressed, the GPIO (General Purpose Input/Output) connected to the button will turn into low level (0V). We can detect the state of the GPIO connected to the button through programming. That is, if the GPIO turns into low level, it means the button is pressed, you can run the corresponding code according to this condition. In this experiment, we make the LED light up.

Experimental Procedures

Step 1: Connect the circuit as shown in the following diagram

Step 2: Edit and save the code with vim (see path/Rpi_UniversalStartKit /03_BtnAndLed/BtnAndLed.c)

Step 3: Compile the code

            gcc  BtnAndLed.c  -lwiringPi

Step 4: Run the program

            ./a.out

Press Enter, when you press the button, the LED will light up; when you release the button, the LED will go out.

Summary

Through this experiment, you have basically mastered the Input and Output programming operation of Raspberry Pi GPIOs. I hope you can make persistent efforts and continue to learn the next contents.

BtnAndLed.c

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

#define LedPin    0
#define ButtonPin 1

int main(void)
{
	if(wiringPiSetup() == -1){ //when initialize wiring failed,print messageto screen
		printf("setup wiringPi failed !");
		return 1; 
	}
	
	pinMode(LedPin, OUTPUT); 
	pinMode(ButtonPin, INPUT);

	pullUpDnControl(ButtonPin, PUD_UP);  //pull up to 3.3V,make GPIO1 a stable level
	while(1){
		digitalWrite(LedPin, HIGH);
		if(digitalRead(ButtonPin) == 0){ //indicate that button has pressed down
			digitalWrite(LedPin, LOW);   //led on
		}
	}

	return 0;
}

Python Code

#!/usr/bin/env python
import RPi.GPIO as GPIO

LedPin = 11    # pin11 --- led
BtnPin = 12    # pin12 --- button

def setup():
	GPIO.setmode(GPIO.BOARD)       # Numbers GPIOs by physical location
	GPIO.setup(LedPin, GPIO.OUT)   # Set LedPin's mode is output
	GPIO.setup(BtnPin, GPIO.IN, pull_up_down=GPIO.PUD_UP)    # Set BtnPin's mode is input, and pull up to high level(3.3V)
	GPIO.output(LedPin, GPIO.HIGH) # Set LedPin high(+3.3V) to off led

def loop():
	while True:
		if GPIO.input(BtnPin) == GPIO.LOW: # Check whether the button is pressed or not.
			print '...led on'
			GPIO.output(LedPin, GPIO.LOW)  # led on
		else:
			print 'led off...'
			GPIO.output(LedPin, GPIO.HIGH) # led off

def destroy():
	GPIO.output(LedPin, GPIO.HIGH)     # led off
	GPIO.cleanup()                     # Release resource

if __name__ == '__main__':     # Program start from here
	setup()
	try:
		loop()
	except KeyboardInterrupt:  # When 'Ctrl+C' is pressed, the child program destroy() will be  executed.
		destroy()