Digital I/O with Arduino

Digital Input/Output (I/O) is a fundamental aspect of Arduino programming that allows you to interact with the physical world by reading digital signals from sensors and controlling actuators such as LEDs, motors, and relays. Digital I/O pins on the Arduino board can be configured as either inputs or outputs, depending on the requirements of your project. In this section, we will explore how to use digital I/O pins in Arduino programming, including how to read digital inputs and control digital outputs.

digitalRead()

The digitalRead() function is used to read the state of a digital input pin. It returns either HIGH (1) or LOW (0) depending on the voltage level at the pin. To use digitalRead(), you first need to configure the pin as an input using the pinMode() function in the setup() function of your sketch. For example:

const int buttonPin = 2; // the number of the pushbutton pin
void setup()
{
  pinMode(buttonPin, INPUT); // initialize the pushbutton pin as an input
}
void loop()
{
  int buttonState = digitalRead(buttonPin); // read the state of the pushbutton
  if (buttonState == HIGH) {
    // do something when the button is pressed
  } else {
    // do something when the button is not pressed
  }
}

In this example, we configure pin 2 as an input for a pushbutton. The digitalRead() function reads the state of the button, and we can use an if statement to perform different actions based on whether the button is pressed (HIGH) or not pressed (LOW).

digitalWrite()

The digitalWrite() function is used to write a HIGH or LOW value to a digital output pin. The pin must have been previously configured as an output using pinMode().

const int ledPin = 13; // the number of the LED pin

void setup()
{
  pinMode(ledPin, OUTPUT); // initialize the LED pin as an output
}

void loop()
{
  digitalWrite(ledPin, HIGH); // turn the LED on
  delay(1000);                // wait for a second
  digitalWrite(ledPin, LOW);  // turn the LED off
  delay(1000);                // wait for a second
}

In this example, we configure pin 13 as an output and then alternately write HIGH and LOW to it with a one-second delay. This causes the built-in LED on most Arduino boards to blink.

pinMode()

The pinMode() function is used to configure a specific pin to behave either as an input or an output. It must be called before using digitalRead() or digitalWrite() on a pin, typically in the setup() function.

const int ledPin = 13;   // the number of the LED pin
const int buttonPin = 2; // the number of the pushbutton pin

void setup()
{
  pinMode(ledPin, OUTPUT);   // initialize the LED pin as an output
  pinMode(buttonPin, INPUT); // initialize the pushbutton pin as an input
}

pinMode() takes two arguments: the pin number and the mode (OUTPUT, INPUT, or INPUT_PULLUP). INPUT_PULLUP enables the internal pull-up resistor on the pin, which is useful when connecting a button without an external resistor.

Complete Example: Button-Controlled LED

The example below combines pinMode(), digitalRead(), and digitalWrite() to control an LED with a pushbutton:

const int ledPin = 13;   // the number of the LED pin
const int buttonPin = 2; // the number of the pushbutton pin

void setup()
{
  pinMode(ledPin, OUTPUT);   // initialize the LED pin as an output
  pinMode(buttonPin, INPUT); // initialize the pushbutton pin as an input
}

void loop()
{
  int buttonState = digitalRead(buttonPin); // read the state of the pushbutton

  if (buttonState == HIGH) {
    digitalWrite(ledPin, HIGH); // turn LED on when button is pressed
  } else {
    digitalWrite(ledPin, LOW);  // turn LED off when button is not pressed
  }
}