Project: blink and button

Learn

What you’ll build

An LED you control with code, then with a push button, then a toggle switch made from code: press once for on, press again for off. This is exactly how a robot reads a bump sensor.

Parts

Part	Qty
Arduino UNO (or compatible) + USB cable	1
Breadboard	1
LED (any colour)	1
Resistor: 330 Ω (UNO R3 / clones) or 470 Ω–1 kΩ (UNO R4)	1
Push button (4-leg tactile)	1
Jumper wires	4

Note

UNO R4 pins supply less current than R3 pins, so use a bigger resistor. The LED is a little dimmer, but the board stays safe.

Wiring

 Arduino pin 8 ──── resistor ──── LED long leg (+)
                                  LED short leg (−) ──── GND

 Arduino pin 2 ──── button ──── GND

1. Unplug the USB cable while wiring.
2. Connect an Arduino GND pin to the breadboard’s blue − rail.
3. Put the LED in the breadboard. Connect its short leg to the − rail.
4. Connect the resistor from the LED’s long leg to an empty row, then a jumper from that row to pin 8.
5. Place the button across the centre gap so its legs sit in two rows, say rows 20 and 22. Connect row 20 to pin 2 and row 22 to the − rail, both on the same side of the gap. (The legs facing each other across the gap are always joined; pressing connects row 20 to row 22.)

If it doesn’t work

Symptom	Check
LED never lights	LED backwards? GND wired? Code uses pin 8 and the wire is on pin 8?
LED always on (step 2)	Button wired to 5V instead of GND, or INPUT instead of INPUT_PULLUP
Button does nothing	Button rotated 90°. Its legs are joined in pairs, so try rotating it
Toggle flips twice or randomly	Increase the debounce delay to 100

Still stuck? Work through My robot doesn’t work.

Examples

Step 1: blink an external LED

const int LED = 8;

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

void loop() {
  digitalWrite(LED, HIGH);
  delay(500);
  digitalWrite(LED, LOW);
  delay(500);
}

You should see: your LED blinking twice per second.

Step 2: light while the button is held

const int LED = 8;
const int BUTTON = 2;

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);   // pin is HIGH until the button connects it to GND
}

void loop() {
  if (digitalRead(BUTTON) == LOW) {  // LOW means pressed
    digitalWrite(LED, HIGH);
  } else {
    digitalWrite(LED, LOW);
  }
}

Why INPUT_PULLUP, and why is pressed LOW?

An input pin connected to nothing “floats” and reads random values. INPUT_PULLUP turns on a resistor inside the Arduino that gently pulls the pin up to 5 V (HIGH). Pressing the button connects the pin to GND, so it reads LOW. It feels backwards at first, but it means no extra resistor is needed.

You should see: the LED is on only while you hold the button.

Step 3: a toggle switch in code

const int LED = 8;
const int BUTTON = 2;

bool ledOn = false;
bool wasPressed = false;

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
}

void loop() {
  bool pressed = (digitalRead(BUTTON) == LOW);

  if (pressed && !wasPressed) {   // only at the moment the button goes down
    ledOn = !ledOn;               // flip: on becomes off, off becomes on
    digitalWrite(LED, ledOn ? HIGH : LOW);
    delay(50);                    // ignore "bounce" for 50 ms
  }
  wasPressed = pressed;
}

Two new ideas:

• Edge detection: we act only when the button changes from not-pressed to pressed, not the whole time it’s held.
• Debouncing: metal contacts inside a button bounce for a few milliseconds, which looks like several fast presses. The short delay(50) ignores them.

You should see: each press flips the LED on or off.

Practice

Simulate it first (optional)

No parts, or want to check your wiring? Build the whole project in the virtual Arduino:

1. Press Start simulator below.
2. Click the diagram.json tab, select everything in it, and replace it with the code below. The LED, resistor and button appear already wired, exactly like the diagram above.
3. Paste each step’s code into sketch.ino and press the green ▶. Click the virtual button to press it.

diagram.json

{
  "version": 1,
  "author": "RoboLearn",
  "editor": "wokwi",
  "parts": [
    { "type": "wokwi-arduino-uno", "id": "uno", "top": 0, "left": 0, "attrs": {} },
    { "type": "wokwi-led", "id": "led1", "top": -120, "left": 120, "attrs": { "color": "red" } },
    { "type": "wokwi-resistor", "id": "r1", "top": -40, "left": 170, "attrs": { "value": "330" } },
    { "type": "wokwi-pushbutton", "id": "btn1", "top": -110, "left": 260, "attrs": { "color": "green" } }
  ],
  "connections": [
    [ "uno:8", "r1:2", "green", [] ],
    [ "r1:1", "led1:A", "green", [] ],
    [ "led1:C", "uno:GND.1", "black", [] ],
    [ "btn1:1.l", "uno:2", "blue", [] ],
    [ "btn1:2.l", "uno:GND.1", "black", [] ]
  ],
  "dependencies": {}
}

Virtual Arduino UNOOpen full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

SOSEasy

Blink · · · — — — · · · in Morse code (short = 200 ms, long = 600 ms) on the pin 8 LED. See the SOS answer in the Arduino UNO lesson’s Practice tab, and change LED_BUILTIN to 8.

Speed controlMedium

Each button press makes the blink faster; after the fastest speed it goes back to slow. Keep the blink time in a variable and change it on each new press.

Show an answer

const int LED = 8, BUTTON = 2;
int wait = 800;
bool wasPressed = false;

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
}

void loop() {
  bool pressed = digitalRead(BUTTON) == LOW;
  if (pressed && !wasPressed) {
    wait = wait / 2;
    if (wait < 50) wait = 800;
  }
  wasPressed = pressed;
  digitalWrite(LED, HIGH); delay(wait);
  digitalWrite(LED, LOW);  delay(wait);
}

The button is only checked between blinks, so at slow speeds you may need to hold it briefly. The traffic-light and reaction-game challenges fix this with millis().

Traffic lightMedium

Add yellow and green LEDs on pins 9 and 10 and cycle red (3 s) → green (3 s) → yellow (1 s).

Reaction gameHard

The LED lights after a random wait of 1–5 s. Print how many milliseconds you took to press the button. random(1000, 5000) picks the wait; millis() gives the time now.

Show an answer

const int LED = 8, BUTTON = 2;

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
  Serial.begin(9600);
  randomSeed(analogRead(A0));
}

void loop() {
  digitalWrite(LED, LOW);
  delay(random(1000, 5000));
  digitalWrite(LED, HIGH);
  unsigned long start = millis();
  while (digitalRead(BUTTON) == HIGH) {}   // wait for the press
  Serial.print("Reaction time: ");
  Serial.print(millis() - start);
  Serial.println(" ms");
  delay(1500);
}

Quick check

1. With INPUT_PULLUP, what does digitalRead return while the button is pressed?

HIGHLOWA random value5

2. Why does the toggle code check pressed && !wasPressed?

To act only at the moment the button goes down, not the whole time it is heldTo make the LED brighterBecause C++ requires two variablesTo save battery

3. What is button "bounce"?

The button springing back upContacts rapidly connecting and disconnecting for a few ms, looking like many pressesA broken buttonA type of LED

Check my answers