2.1.1 Button

Introduction

In this beginner-friendly lesson, you will learn how to control an LED light using a simple push button. This is a fundamental skill in electronics that teaches you about digital input and output.

Components

What is a Button?

A button (also called a push button or tactile switch) is one of the most basic input devices in electronics. Think of it like a doorbell button - when you press it, it completes an electrical connection, and when you release it, the connection is broken.

The button we’re using is a small 6mm tactile button. Here’s how it works:

Looking at the button from above, you can see it has 4 metal pins. The two pins on the left side are always connected to each other, and the two pins on the right side are always connected to each other. When you press the button down, all 4 pins get connected together.

In circuit diagrams, we represent a button with this symbol:

How does it work?

When the button is not pressed: The circuit is open (broken)
When the button is pressed: The circuit is closed (complete), allowing electricity to flow

Connect

T-Board Name	physical	wiringPi	BCM
GPIO17	Pin 11	0	17
GPIO18	Pin 12	1	18

Now let’s connect our button to the Raspberry Pi. We’ll use what’s called a “pull-up” circuit configuration. Here’s what happens:

When the button is not pressed: GPIO18 reads HIGH (3.3V)
When the button is pressed: GPIO18 reads LOW (0V)

Our program will monitor GPIO18 and detect when it changes from HIGH to LOW, which tells us the button was pressed.

Note

LED Tip for Beginners: LEDs (Light Emitting Diodes) have two legs of different lengths. The longer leg is the positive side (anode) and the shorter leg is the negative side (cathode). Always connect the longer leg to the positive voltage!

Code

For C Language User

Go to the code folder compile and run.

cd ~/Basic-Starter-Kit-for-Raspberry-Pi/c/2.1.1/

gcc 2.1.1_Button.c -lwiringPi

sudo ./a.out

After the code runs, press the button, the LED lights up; otherwise, turns off.

This is the complete code

#include <wiringPi.h>    // Library for GPIO control on Raspberry Pi
#include <stdio.h>       // Standard input/output library

// Define GPIO pin numbers (using WiringPi numbering scheme)
#define LED_PIN         0    // LED connected to WiringPi pin 0
#define BUTTON_PIN      1    // Button connected to WiringPi pin 1

// Define logic states for better readability
#define LED_ON          LOW   // LED turns on when GPIO outputs LOW
#define LED_OFF         HIGH  // LED turns off when GPIO outputs HIGH
#define BUTTON_PRESSED  0     // Button reads LOW when pressed (pull-up resistor)
#define BUTTON_RELEASED 1     // Button reads HIGH when released

int main(void)
{
    printf("Starting Button Control LED Demo...\n");

    // Initialize WiringPi library
    // This must be called before using any GPIO functions
    if(wiringPiSetup() == -1){
        printf("Error: Failed to initialize WiringPi library!\n");
        printf("Make sure you are running with proper permissions.\n");
        return 1;
    }

    printf("WiringPi initialized successfully.\n");

    // Configure GPIO pins
    pinMode(LED_PIN, OUTPUT);       // Set LED pin as output
    pinMode(BUTTON_PIN, INPUT);     // Set button pin as input

    // Set initial state: LED off
    digitalWrite(LED_PIN, LED_OFF);
    printf("LED pin configured as OUTPUT, Button pin configured as INPUT.\n");
    printf("Press the button to control the LED. Press Ctrl+C to exit.\n\n");

    // Variable to track previous button state for change detection
    int previousButtonState = BUTTON_RELEASED;  // Assume button starts released

    // Main control loop - runs continuously
    while(1)
    {
        // Read the current state of the button
        int currentButtonState = digitalRead(BUTTON_PIN);

        // Only act when button state changes (avoid continuous printing)
        if(currentButtonState != previousButtonState)
        {
            if(currentButtonState == BUTTON_PRESSED)
            {
                // Turn on the LED when button is pressed
                digitalWrite(LED_PIN, LED_ON);
                printf("Button pressed - LED ON\n");
            }
            else
            {
                // Turn off the LED when button is released
                digitalWrite(LED_PIN, LED_OFF);
                printf("Button released - LED OFF\n");
            }

            // Update previous state for next comparison
            previousButtonState = currentButtonState;
        }

        // Small delay to prevent excessive CPU usage and debounce button
        delay(50);  // 50ms delay (reduced for better responsiveness)
    }

    // This code will never be reached due to infinite loop above
    // But it's good practice to include cleanup code
    return 0;
}

For Python Language User

Go to the code folder and run.

cd ~/Basic-Starter-Kit-for-Raspberry-Pi/python

python 2.1.1_Button.py