# Arduino project: read a digital input

> Learn how to read a digital input on an Arduino with the digitalRead() function, wiring a button to pin 3 with INPUT_PULLUP to detect when it is pressed.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2021-03-22 | Topics: [Arduino](https://flaviocopes.com/tags/electronics/) | Canonical: https://flaviocopes.com/arduino-project-read-digital-input/

In this project you'll learn **how to read from a digital I/O pin**, using the `digitalRead()` function.

Digital I/O pins on the Arduino Uno board are the ones on the USB port side of the board, and they might also be marked as digital on the pins, like in my case:

![Arduino Uno board showing digital pins numbered 0-13 with DIGITAL(PWM~) labels on the USB side](https://flaviocopes.com/images/arduino-project-read-digital-input/IMG_3189.jpg)

They are numbered from `0` to `13`, but you will almost always skip pins `0` and `1` unless you run out of pins, because they are used for serial communication (see the `rx` and `tx` labels printed on them).

We'll first build the circuit, then we'll switch to the Arduino IDE on our computer.

The circuit will be simple. We'll have a button that has one lead connected to GND on the Arduino, one lead to the digital pin #3 (any other digital pin would work).

![Arduino Uno connected to breadboard with button wired between digital pin 3 and GND using red and blue jumper wires](https://flaviocopes.com/images/arduino-project-read-digital-input/IMG_3243.jpg)

![Close-up of black pushbutton on Arduino board showing digital pins 0-13 with DIGITAL(PWM~) label](https://flaviocopes.com/images/arduino-project-read-digital-input/IMG_3244.jpg)

We'll detect the voltage level on pin #3 every second. We define the pin number as a constant to avoid having a [magic number](https://flaviocopes.com/magic-numbers/) in our code:

```c
#define BUTTON_PIN 3
```

In the `setup()` function we call pinMode() to set this pin as an input pin, with the `INPUT_PULLUP` option:

```c
pinMode(BUTTON_PIN, INPUT_PULLUP);
```

The `INPUT_PULLUP` option is needed because by default if nothing is connected to the pin (if the button is not clicked, the circuit is open and it's like nothing is connected to the pin), the input pin is **floating**, which means it's subject to interference and it can randomly pick a 0 or a 1.

In the circuit we could add a pull-up or a pull-down resistance, but it's easier to pass the `INPUT_PULLUP` flag to Arduino. If nothing is connected, it defaults to 1.

Then in the `loop()` function we get the value of the input pin using the `digitalRead()` function:

```c
int value = digitalRead(BUTTON_PIN);
```

This value can be 0 or 1, depending on the input.

If the button is pressed, Arduino will detect 0.

If the button is not pressed, Arduino will detect 1.

You can also use the constants `LOW`, which maps to 0, and `HIGH` that maps to 1.

This value can be printed to the serial output, like in this program:

```c
#define BUTTON_PIN 3

void setup() {
    pinMode(BUTTON_PIN, INPUT_PULLUP);
    Serial.begin(9600);
}

void loop() {
    int value = digitalRead(BUTTON_PIN);
    Serial.print(value);
    delay(1000);
}
```

Save and upload the program to the Arduino, then click the top-right button "Serial Monitor" in the Arduino IDE:

![Arduino IDE showing Serial Monitor button highlighted in blue circle in the top-right toolbar](https://flaviocopes.com/images/arduino-project-read-digital-input/Screen_Shot_2020-12-11_at_14.40.15.png)

It will show the serial interface monitor. Make sure it's tuned to 9600 baud at the bottom, and you should see a bunch of `1` until you press the button for a second, at that point you should get a `0`.

![Serial Monitor window showing stream of 1s and 0s output with 9600 baud setting at bottom](https://flaviocopes.com/images/arduino-project-read-digital-input/Screen_Shot_2020-12-11_at_14.39.20.png)

We can also light a LED. We could add it to the circuit, but we can also use the Arduino Uno **built-in LED**, which maps to the digital I/O pin #13. We can write a program in this way:

```c
#define BUTTON_PIN 3

void setup() {
    pinMode(BUTTON_PIN, INPUT_PULLUP);
    pinMode(13, OUTPUT);
    digitalWrite(13, LOW);
}

void loop() {
    int value = digitalRead(BUTTON_PIN);
    digitalWrite(13, value);
}
```

to immediately reflect the button input value, without waiting 1 second. Previously we did that to avoid writing too many values to the serial interface, making it unmanageable.

See? The built-in LED (near pin #13 / GND at the top) lights up when the program starts, and it's turned down when we press the button

![Arduino Uno with built-in LED illuminated near pin 13 and button circuit on breadboard](https://flaviocopes.com/images/arduino-project-read-digital-input/IMG_3245.jpg)

![Finger pressing button on breadboard connected to Arduino with built-in LED turned off](https://flaviocopes.com/images/arduino-project-read-digital-input/IMG_3246.jpg)

You can also do the opposite (keep it off and only turn it on when we press the button) with

```c
digitalWrite(13, !value);
```

And instead of referencing the magic number `13`, you can use the build-in constant `LED_BUILTIN`:

```c
#define BUTTON_PIN 3

void setup() {
    pinMode(BUTTON_PIN, INPUT_PULLUP);
    pinMode(LED_BUILTIN, OUTPUT);
    digitalWrite(LED_BUILTIN, LOW);
}

void loop() {
    int value = digitalRead(BUTTON_PIN);
    digitalWrite(LED_BUILTIN, !value);
}
```