Push Button & LED Control

• 1× Arduino Uno R3
• 1× USB programming cable
• 1× Breadboard  (MB-102)
• 1× LED  (red, yellow, or blue)
• 1× 220Ω resistor  red–red–brown · protects the LED
• 1× Tactile push button  (the 12×12 clicky square, 4 legs)
• 1× 10kΩ resistor  brown–black–orange · the button's pull-down
• 4–6× Male-to-male jumper wires

This is the very first circuit of the course, so take it one line at a time. Build the LED half first and get it blinking, then add the button — its four legs are the part that trips people up, so save them for last.

Do the LED half first — it's your warm-up. Push the LED into the breadboard so its two legs land in different rows, run pin 12 through the 220Ω resistor to the LED's long leg, and the short leg down to the − (GND) rail. Add a wire from the Arduino's GND pin to that rail. Plug in USB only after you've checked the LED's long leg goes to the resistor side. Once it's blinking, add the button: straddle it across the centre gap, feed 5V into leg A, and run the diagonal leg B to both pin 2 and the 10kΩ resistor down to GND.

Open the Arduino IDE, type this in, and upload it:

// Blink warm-up: flash the LED on and off
#define LED_PIN 12          // the LED is wired to pin 12

void setup() {
  pinMode(LED_PIN, OUTPUT); // pin 12 is an OUTPUT (sends power out)
}

void loop() {
  digitalWrite(LED_PIN, HIGH); // LED ON  (HIGH = 5 volts)
  delay(500);                  // wait half a second
  digitalWrite(LED_PIN, LOW);  // LED OFF (LOW = 0 volts)
  delay(500);                  // wait, then repeat
}

If it blinks on for half a second, off for half a second, your output half is perfect. Now teach the board to listen.

Add the button wiring from the table, then upload this:

// --- Pin setup ---
#define BUTTON_PIN 2    // pin 2 "listens" to the button (digital INPUT)
#define LED_PIN    12   // pin 12 drives the LED (digital OUTPUT)

void setup() {
  pinMode(BUTTON_PIN, INPUT);   // pin 2 will READ voltage: is it pushed?
  pinMode(LED_PIN, OUTPUT);     // pin 12 will SEND voltage to the LED
}

void loop() {
  int buttonState = digitalRead(BUTTON_PIN);  // HIGH (pushed) or LOW (released)

  if (buttonState == HIGH) {        // pushed -> 5V reaches pin 2
    digitalWrite(LED_PIN, HIGH);    // ...turn the LED ON
  } else {                          // released -> pull-down holds pin 2 at 0V
    digitalWrite(LED_PIN, LOW);     // ...turn the LED OFF
  }
}

Press and hold — the LED should light the instant you press and go dark the instant you let go. Tap it fast and watch the light keep up: your Arduino is now reading your finger thousands of times a second and reacting every time.

Now make it yours. Try these before peeking at the answers:

1. Flip the logic. LED normally ON, goes dark only while you hold the button — a “hold to turn off” night light.
2. Make it latch. One press turns it on and it stays on; press again to turn it off — a proper on/off switch.
3. Press to blink. Bring back the Step-1 blink, but only while the button is held. Output and input working together.

In Step 1 you made the Arduino talk. digitalWrite(LED_PIN, HIGH) told pin 12 to push out 5 volts; that shoves a tiny current through the resistor, through the LED, and out to GND — a glowing LED is just what an LED does when current flows the right way. LOW sets the pin back to 0 volts and the light goes out. That's a digital output — the Arduino flipping a pin on and off like a switch it controls with code.

In Step 2 you taught it to listen. digitalRead(BUTTON_PIN) asks one question: is there voltage on pin 2 right now? “Yes” = HIGH, “no” = LOW. That single yes/no is a digital input — the simplest way it senses the world outside itself.

Here's the surprise: an input pin is nosy. If it isn't firmly wired to something, it floats — picking up stray voltage from your hand, nearby wires, even the air — and flips between HIGH and LOW on its own, making the LED flicker for no reason.

That's the whole job of the 10kΩ resistor. Button not pressed → it quietly ties pin 2 to GND, so the pin reads a rock-solid LOW. Press → 5V rushes in through leg A and overpowers that gentle pull, so the pin jumps to HIGH. We call it a pull-down resistor because it pulls the pin down to ground.