Digital Input: Reading a Push Button

Read a physical button press into your sketch, using the pull-down wiring from Lesson cb-08 and a new function: digitalRead.

You'll need: a push-button, a 10kΩ resistor, jumper wires, a breadboard, and your Uno.

Wire the button exactly as the pull-down circuit from Lesson cb-08: one side of the button to 5V, a 10kΩ resistor from the same side down to GND, and connect that junction to digital pin 2. This guarantees pin 2 reads a clean LOW when the button is open and a clean HIGH when it's pressed — reading a switch with nothing else connected leaves the pin floating (Lesson cb-08 explains exactly why that's unreliable).

void setup() {
  pinMode(2, INPUT);
  pinMode(13, OUTPUT);
}

void loop() {
  int buttonState = digitalRead(2);
  digitalWrite(13, buttonState);
}

digitalRead(2) checks pin 2's current voltage and returns either HIGH or LOW, storing it in a variable named buttonState. Feeding that same value straight into digitalWrite(13, buttonState) means the onboard LED mirrors the button in real time — lit exactly while the button is held, dark otherwise.

This read-a-pin, act-on-the-value pattern — check an input, then do something based on what it says — is the shape behind nearly every interactive sketch from here on, however much more complex the "act on it" part eventually gets.