Programming in Arduino

1Telling a chip what to do

A microcontroller does nothing until it is given a program. On an Arduino the program is called a sketch; you write it on a computer in the Arduino IDE (or in a simulator), the IDE translates it into machine code, and the code is copied into the board's flash memory, where it runs every time the board is powered. In this lesson you write your first sketches: blinking an LED, reading a button, and doing both at once without one blocking the other.

2The shape of every sketch

Every sketch has two functions. setup() runs once when the board starts or is reset; loop() then runs again and again, for as long as the board has power.

3The circuit

An LED is a diode: current flows only from its anode (+, the longer leg) to its cathode (−, the shorter leg, beside the flat edge of the case). Put it in backwards and it simply stays dark. The resistor limits the current; without it the LED and the pin can be damaged. On a breadboard, the five holes in each short row are connected to each other, and the long rails along the edges carry 5 V and GND.

4Sketch 1: blink and report

All the sketches in this lesson were compiled for the Arduino Uno with arduino-cli without errors.

// Blink an LED on pin 8 and report each change to the Serial Monitor
const int LED_PIN = 8;

void setup() {
  pinMode(LED_PIN, OUTPUT);     // pin 8 will drive the LED
  Serial.begin(9600);           // open serial at 9600 bits per second
}

void loop() {
  digitalWrite(LED_PIN, HIGH);  // 5 V on pin 8: LED on
  Serial.println("LED on");
  delay(500);                   // wait 500 ms
  digitalWrite(LED_PIN, LOW);   // 0 V: LED off
  Serial.println("LED off");
  delay(500);
}

Click Upload, then open the Serial Monitor (set to 9600 baud): it prints "LED on" and "LED off" every half second, while the red LED blinks. Printing to the Serial Monitor is your main tool for finding out what a sketch is really doing.

5Sketch 2: a button

// Light the LED on pin 8 while the button on pin 2 is pressed
const int BUTTON_PIN = 2;
const int LED_PIN = 8;

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);  // internal resistor holds pin 2 HIGH
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  bool pressed = (digitalRead(BUTTON_PIN) == LOW);  // pressing connects pin 2 to GND
  if (pressed) {
    digitalWrite(LED_PIN, HIGH);
  } else {
    digitalWrite(LED_PIN, LOW);
  }
}

INPUT_PULLUP switches on a resistor inside the chip that connects pin 2 to 5 V. With the button up, pin 2 reads HIGH. Pressing the button connects pin 2 straight to GND, so it reads LOW. This is why "pressed" means LOW: the logic is active low. Without a pull-up, an unconnected input pin "floats" and reads random values.

6Sketch 3: doing two things at once

While delay(500) runs, the Uno does nothing else at all: a button pressed during the delay is not noticed. The fix is to check the clock instead of waiting:

// Blink without delay(): the loop never stops, so the button is always read
const int BUTTON_PIN = 2;
const int LED_PIN = 8;
const int LED2_PIN = 7;            // second LED, lit while the button is held
const unsigned long INTERVAL = 500;   // ms between changes

unsigned long lastChange = 0;         // millis() needs unsigned long
bool ledOn = false;

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  pinMode(LED2_PIN, OUTPUT);
}

void loop() {
  unsigned long now = millis();       // ms since the board started
  if (now - lastChange >= INTERVAL) { // time for the next change?
    ledOn = !ledOn;
    digitalWrite(LED_PIN, ledOn ? HIGH : LOW);
    lastChange = now;
  }
  // this runs thousands of times a second, between LED changes
  digitalWrite(LED2_PIN, digitalRead(BUTTON_PIN) == LOW ? HIGH : LOW);
}

Each pass of loop() takes a few microseconds. Most passes only read the button; once every 500 ms the if condition is true and the red LED changes. The green LED follows the button at once, while the red LED keeps blinking. This pattern, comparing millis() with a stored time, is how real controllers handle several jobs.

7Hands-on: a pedestrian crossing

1. In Tinkercad or Wokwi, build the circuit above: red LED on pin 8, green LED on pin 7, each with a 220 Ω resistor, and a push button on pin 2 to GND.
2. Upload Sketch 1, then Sketch 2, and check that each behaves as described.
3. Write a new sketch: the red LED stays on until the button is pressed; then, after 2 seconds, red goes off and green comes on for 5 seconds, then back to red. Print each change to the Serial Monitor.
4. Rewrite it with millis() instead of delay(), so that a second press during the green phase is noticed (and printed as "already crossing").

Check: the lights follow the sequence every time, the Serial Monitor shows each change, and your millis() version notices a press at any moment.