Lesson 12 Button

Introduction

In this lesson, we will learn how to use buttons.

Components

– 1 * Raspberry Pi

– 1 * Breadboard

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

– 1 * Button module

– 1 * Dual-color Common-Cathode LED module

– Several jumper wires

Experimental Principle

Use a normally open button as an input device of Raspberry Pi. When the button is pressed, the General Purpose Input/Output (GPIO) 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. Then you can run the corresponding code accordingly.

In this experiment, we will print a string on the screen and control an LED.

Experimental Procedures

Step 1: Build the circuit

Button Module                Raspberry Pi

 S ——————————– GPIO0

+ ——————————-   3.3V

– ——————————–  GND

Dual-color LED module connection: connect pin R on dual-color LED module to GPIO1 of the Raspberry Pi; GND to GND

Step 2: Edit and save the code (see path/Rpi_SensorKit_code/13_button/button.c)

Step 3: Compile

              gcc  button.c  -lwiringPi

Step 4: Run

              ./a.out

Press the button and you will see a string “Button is pressed” displayed on the screen, and the state of the LED will be switched ON/OFF.

 button.c

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

#define BtnPin    0
#define LedPin    1

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

	pinMode(BtnPin, INPUT);
	pinMode(LedPin, OUTPUT);

	while(1){
		if(0 == digitalRead(BtnPin)){
			delay(10);
			if(0 == digitalRead(BtnPin)){
				while(!digitalRead(BtnPin));
				digitalWrite(LedPin, !digitalRead(LedPin));	
				printf("Button is pressed\n");	
			}
		}
	}

	return 0;
}

Python Code

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

BtnPin = 11
LedPin = 12

Led_status = 0

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.LOW) # Set LedPin low to off led

def swLed(ev=None):
	global Led_status
	Led_status = not Led_status
	GPIO.output(LedPin, Led_status)  # switch led status(on-->off; off-->on)
	print "LED: on " if Led_status else "LED: off"

def loop():
	GPIO.add_event_detect(BtnPin, GPIO.FALLING, callback=swLed, bouncetime=200) # wait for falling
	while True:
		pass   # Don't do anything

def destroy():
	GPIO.output(LedPin, GPIO.LOW)     # 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()