# Digital Pins

So far everything we have seen does not involve controlling any electronics hardware using the Arduino.

Let's start by writing a simple sketch that turns on the built-in LED on the Uno board. It is the small LED labeled L on the board.

There are two kinds of pins: digital and analog. We will not cover the analog pins for now. The digital pins can be set to either HIGH or LOW. This means we either output 5V through that pin or 0V.

A pin can also be either INPUT or OUTPUT. If we are outputting voltage from the Arduino board to an electronic component connected to that pin, then our pin is an OUTPUT pin. If the electronic component is producing voltage through that pin, and the Arduino board is reading that voltage then the pin is an INPUT pin. The INPUT or OUTPUT mode is determined with respect to the Arduino board. If we want to connect an LED to the Arduino, we will want to output voltage from the Arduino to the LED. Therefore, the pin we connect the LED to will be in OUTPUT mode. The digital pins can be seen on the top side of the Uno board labeled from 0 to 13.

On the Uno board, there is a built-in LED next to the L letter. This LED is connected to pin 13 internally. In the Arduino platform, if you use LED_BUILTIN in your code, it will be translated into 13 as that is the pin for the built-in LED.



The above program turns on the built-in LED and then waits for 3 seconds.

# pinMode

In the setup function, the first thing we do is specify for each pin we are using whether that pin is going to be used as an output or input pin. We do that by calling the Arduino function pinMode(pinNumber, mode);. The pinNumber is the number of the pin we want to specify its mode, and the mode is whether it is OUTPUT or INPUT. On line 2 of the program, we are specifying that pin 13 is an output pin. Which means we will be deciding if the pin has 5V or 0V. If we output 5V then the LED will turn on.

# digitalWrite

Line 7 specifies the output to be HIGH to the LED_BUILTIN pin. Which is the same if we use 13 instead of LED_BUILTIN. We specify the output to a digital pin by using the Arduino function digitalWrite(pinNumber, HIGHorLOW);.

Exercise

In the above code, can you turn the built in LED off after 3 seconds?

Answer 
void setup() {
  // initialize digital pin 
  //LED_BUILTIN as an output.
  pinMode(LED_BUILTIN, OUTPUT);

  // turn the LED on (HIGH is the voltage level)
  digitalWrite(LED_BUILTIN, HIGH);  
  // wait for 3 second 
  delay(3000);    

  digitalWrite(LED_BUILTIN, LOW);

}
//loop function is empty and is 
//doing nothing after the setup
void loop() {
}

# Blinking LED

We want to continuously blink the build-in LED every 1 second. This sounds like a task for the loop function!

We can also connect LEDs other than the built-in one to the Arduino board. In the above environment, we have a red LED connected to the Arduino board on pin1 11.

Exercise

Modify the code above to blink the red LED opposite to the built-in LED.

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