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/c/2.1.1/

Note

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

Step 3: Compile the code.

gcc 2.1.1_Button.c -lwiringPi

Step 4: Run the executable file.

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 Install and Check the WiringPi.

Code

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

#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

#define LedPin      0

Pin GPIO17 in the T_Extension Board is equal to the GPIO0 in the wiringPi.

#define ButtonPin   1

ButtonPin is connected to GPIO1.

pinMode(LedPin, OUTPUT);

Set LedPin as output to assign value to it.

pinMode(ButtonPin, INPUT);

Set ButtonPin as input to read the value of ButtonPin.

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.

digitalRead() function is to read HIGH (high level) or LOW (low level) of the input parameter word pin, it returns 1 when pin is HIGH and returns 0 when pin is LOW.

digitalWrite() function is to write HIGH (high level) or LOW (low level) to the input parameter word pin.

Phenomenon Picture