Digital I/O

Digital means two states: HIGH (5V, on, 1) or LOW (0V, off, 0). No in-between. Buttons, motion sensors, and LEDs live in this world.

Output: lighting an LED

Wire: Uno pin 13 → 220Ω resistor → LED long leg (+); LED short leg (−) → GND. (The built-in LED needs no wiring, but wiring your own teaches the circuit.)

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);   // LED on
  delay(500);
  digitalWrite(13, LOW);    // LED off
  delay(500);
}

Why the resistor? An LED without one draws as much current as it can — briefly very bright, then dead, possibly taking the pin with it. The 220Ω resistor limits current to ~15mA, safely under the pin's ~20mA limit. No LED without a resistor (except the built-in one — it has its own).

Input: reading a button

Wire: button between pin 2 and GND. No extra resistor needed — we use the chip's built-in one:

const int BUTTON_PIN = 2;
const int LED_PIN = 13;

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

void loop() {
  int buttonState = digitalRead(BUTTON_PIN);
  if (buttonState == LOW) {
    digitalWrite(LED_PIN, HIGH);  // pressed → LED on
  } else {
    digitalWrite(LED_PIN, LOW);
  }
}

Why does pressed read LOW? INPUT_PULLUP connects the pin internally to 5V through a resistor, so it reads HIGH when idle. Pressing the button connects it to GND, dragging it LOW. Backwards from intuition, but it means the button needs no external parts — one wire to GND and you're done.

Debouncing: buttons lie

A button press isn't one clean on/off — the metal contacts bounce, producing a burst of rapid on/off flickers over a few milliseconds. Your loop runs fast enough to see each bounce as separate presses. For "LED follows button" you won't notice; for "count presses" you must.

The simple fix: after seeing a change, ignore further changes for ~50ms:

const int BUTTON_PIN = 2;
int pressCount = 0;
int lastState = HIGH;
unsigned long lastDebounceTime = 0;

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

void loop() {
  int reading = digitalRead(BUTTON_PIN);
  if (reading != lastState) {
    lastDebounceTime = millis();
  }
  if ((millis() - lastDebounceTime) > 50) {
    if (reading == LOW && lastState == HIGH) {
      pressCount++;
      Serial.print("Presses: ");
      Serial.println(pressCount);
    }
  }
  lastState = reading;
}

(Uses Serial — covered properly in Serial. For now: it prints the count to your computer.)

digitalRead / digitalWrite cheat sheet

Call	Does
pinMode(pin, OUTPUT)	Pin will drive HIGH/LOW
pinMode(pin, INPUT)	Pin reads external signal (needs external resistor)
pinMode(pin, INPUT_PULLUP)	Pin reads, idle = HIGH
digitalWrite(pin, HIGH/LOW)	Set output
digitalRead(pin)	Returns HIGH or LOW

Next: Analog