Blink LED
setup() and loop() program structure
Blink an external LED on a half breadboard through a current-limiting resistor connected to pin 13.
Pin connections
| Part 1 | Part 2 | |
|---|---|---|
Arduino pin 13 | → | Breadboard 13t e |
Resistor pin 1 | → | LED anode (+) |
LED cathode (-) | → | Breadboard tn 16 |
Breadboard tn 3 | → | Arduino GND |
See it
Every program has setup() and loop()!
Your first sketch: turn an LED on and off forever.
Blinking lights show power or status on almost every gadget.
The story
The problem
You need to understand how Arduino code is organized before anything else.
Think of it like
setup() is like getting dressed once; loop() is your daily routine on repeat.
Meet the parts
Connects components in rows like a real breadboard
Breadboard
Workspace
Decides when the LED is on or off
Arduino
Brain
Shows output from the code
LED
Light
Limits current to the LED
Resistor
Safety
How it works
setup() runs once
When power turns on, setup() configures pin 13 — then never runs again.
void setup() {
pinMode(LED_PIN, OUTPUT);
}Turn LED on
Pin 13 goes HIGH and current flows through the LED.
digitalWrite(LED_PIN, HIGH);
Wait a moment
The light stays on for half a second.
delay(500);
Turn off and repeat
LED off, another pause, then loop() jumps back to step 2 forever.
digitalWrite(LED_PIN, LOW); delay(500);
Then loop back to step 2
Build the circuit
Follow these steps in order. Match the wires to the colors shown.
- 1
Place Breadboard
Place the Breadboard (bb1) on the breadboard.
Breadboard placed — build like the real world!
Loading part… - 2
Place Arduino
Place the Arduino (uno) on the breadboard.
Arduino placed — ready to build!
Loading part… - 3
Place Resistor
Place the Resistor (r1) on the breadboard.
Resistor added across breadboard rows 13 and 14!
Loading part… - 4
Place LED
Place the LED (led1) on the breadboard.
LED ready on the breadboard!
Loading part… - 5
Connect Arduino pin 13 to Breadboard (bb1) 13t.e
Arduino pin 13 to breadboard row 13 e
Tip: Arduino pin 13 to breadboard row 13 e
Signal on the breadboard!
- 6
Connect Resistor (r1) 1 to LED (led1) anode (+)
Resistor to LED anode on the breadboard
Tip: Resistor to LED anode on the breadboard
LED linked to resistor!
- 7
Connect LED (led1) cathode (-) to Breadboard (bb1) tn.16
LED cathode to breadboard ground rail at column 16
Tip: LED cathode to breadboard ground rail at column 16
Ground rail linked!
- 8
Connect Breadboard (bb1) tn.3 to Arduino GND
Breadboard ground rail to Arduino GND
Tip: Breadboard ground rail to Arduino GND
Circuit complete!
Try it
- Press Run — the LED should blink in the simulator.
- Remember: setup() once, loop() forever!
Peek at code
Pin name constant
const int LED_PIN = 13;
LED_PIN is a friendly name for pin 13 — use it everywhere instead of repeating the number.
setup() — runs once
void setup() {
pinMode(LED_PIN, OUTPUT);
}setup() runs one time when the board starts. pinMode says pin 13 is an output.
loop() — blink forever
void loop() {
digitalWrite(LED_PIN, HIGH);
delay(500);
digitalWrite(LED_PIN, LOW);
delay(500);
}loop() turns the LED on, waits, off, waits — then Arduino runs loop() again automatically.
Show full sketch (blink-led.ino)
const int LED_PIN = 13;
void setup() {
pinMode(LED_PIN, OUTPUT);
}
void loop() {
digitalWrite(LED_PIN, HIGH);
delay(500);
digitalWrite(LED_PIN, LOW);
delay(500);
}
Quick quiz
Q1. What does delay(500) do?
- A. Waits half a second ✓
- B. Sets brightness to 500
- C. Turns pin 500 on
Q2. Which function runs only once when the board starts?
- A. setup() ✓
- B. loop()
- C. delay()
Code lab — try on your own
Make the LED blink faster by lowering both delay() numbers.
Hint: Try 200 or 300 instead of 500.
Add a short comment on the line that turns the LED on (digitalWrite).
Hint: Use // at the end or start of that line.
Add a comment on line 1 explaining what LED_PIN means.
Hint: Example: // external LED on pin 13
Code walkthrough
A line-by-line tour of the sketch — the same steps as in Robo Gurukul Studio.
Program overview
Technical
Sketches have globals, then setup() once, then loop() forever.
In this project
Blink an external LED on a half breadboard through a current-limiting resistor connected to pin 13.
Why here
Read from top to bottom. Hover words or lines for help!
const int LED_PIN = 13;
setup()
Technical
Runs one time when the board turns on.
In this project
Sets up pins and libraries for Blink LED.
Why here
One-time setup belongs here—not in loop().
void setup() {
pinMode(LED_PIN, OUTPUT);
}loop()
Technical
Runs again and again after setup() is done.
In this project
This is the main action you see in Blink LED.
Why here
Repeating work (blink, read sensors) goes here.
void loop() {
digitalWrite(LED_PIN, HIGH);
delay(500);
digitalWrite(LED_PIN, LOW);
delay(500);
}
Try this: Change numbers in loop(), then compile and run the simulator.
pinMode
Technical
Tells a pin if it listens or drives something.
In this project
Gets the Blink LED circuit ready in the simulator.
Why here
Goes in setup() because we only set pins once at the start.
pinMode(LED_PIN, OUTPUT);
digitalWrite
Technical
Turns a pin ON or OFF.
In this project
Controls lights, motors, or buzzers in Blink LED.
Why here
Goes in loop() so it can keep changing while the program runs.
digitalWrite(LED_PIN, HIGH);
delay
Technical
Waits for some time. Nothing else runs during the wait.
In this project
Controls speed so you can see Blink LED in the simulator.
Why here
Right after an action that should stay the same for a moment.
delay(500);
