Mission 06 · Stage 1

Alternate LEDs

alternating output states

Turn two LEDs on and off in opposite states.

Pin connections

Part 1		Part 2
Arduino pin 12	→	Red resistor pin 2
Red resistor pin 1	→	Red LED anode (+)
Red LED cathode (-)	→	Arduino GND
Arduino pin 13	→	Green resistor pin 2
Green resistor pin 1	→	Green LED anode (+)
Green LED cathode (-)	→	Arduino GND

See it

When one is ON, the other is OFF!

Two LEDs take opposite states — like a seesaw.

Direction indicators and turn signals alternate outputs.

The story

The problem

You need two outputs that never match at the same time.

Think of it like

Like a seesaw — one side up means the other is down.

Meet the parts

Runs your sketch

Arduino

Brain

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First alternating light

Red LED

Output A

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Second alternating light

Green LED

Output B

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Protects the red LED

Red resistor

Safety

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Protects the green LED

Green resistor

Safety

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How it works

LED A wins

Pin 12 is HIGH while pin 13 is LOW — opposite states on purpose.

digitalWrite(LED_A, HIGH);
digitalWrite(LED_B, LOW);

Hold

Both states stay visible for 300 ms so you can see who is on.

delay(300);

LED B wins

Now the roles flip — A off, B on. Like a seesaw.

digitalWrite(LED_A, LOW);
digitalWrite(LED_B, HIGH);

Hold and repeat

Another pause, then loop() swaps back to A again.

delay(300);

Then loop back to step 1

Build the circuit

Follow these steps in order. Match the wires to the colors shown.

1
Place Arduino
Place the Arduino (uno) on the breadboard.
Arduino placed!
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2
Place Red LED
Place the Red LED (ledA) on the breadboard.
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3
Place Green LED
Place the Green LED (ledB) on the breadboard.
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4
Place Red resistor
Place the Red resistor (rA) on the breadboard.
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5
Place Green resistor
Place the Green resistor (rB) on the breadboard.
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6
Connect Arduino pin 12 to Red resistor (rA) 2
Connect Arduino pin 12 to Red resistor (rA) 2.
7
Connect Red resistor (rA) 1 to Red LED (ledA) anode (+)
Connect Red resistor (rA) 1 to Red LED (ledA) anode (+).
8
Connect Red LED (ledA) cathode (-) to Arduino GND
Connect Red LED (ledA) cathode (-) to Arduino GND.
9
Connect Arduino pin 13 to Green resistor (rB) 2
Connect Arduino pin 13 to Green resistor (rB) 2.
10
Connect Green resistor (rB) 1 to Green LED (ledB) anode (+)
Connect Green resistor (rB) 1 to Green LED (ledB) anode (+).
11
Connect Green LED (ledB) cathode (-) to Arduino GND
Connect Green LED (ledB) cathode (-) to Arduino GND.

Try it

Run — red and green should never both be bright together.
Watch them take turns like a blinker.

Peek at code

Two output pins

const int LED_A = 12;
const int LED_B = 13;

LED_A and LED_B name the two pins we alternate between.

Setup both pins

void setup() {
  pinMode(LED_A, OUTPUT);
  pinMode(LED_B, OUTPUT);
}

Both pins are outputs so we can drive two LEDs independently.

Alternate loop

void loop() {
  digitalWrite(LED_A, HIGH);
  digitalWrite(LED_B, LOW);
  delay(300);
  digitalWrite(LED_A, LOW);
  digitalWrite(LED_B, HIGH);
  delay(300);
}

Each half of loop() sets opposite HIGH/LOW pairs, with delay() between swaps.

Show full sketch (alternate-leds.ino)

const int LED_A = 12;
const int LED_B = 13;
void setup() {
  pinMode(LED_A, OUTPUT);
  pinMode(LED_B, OUTPUT);
}
void loop() {
  digitalWrite(LED_A, HIGH);
  digitalWrite(LED_B, LOW);
  delay(300);
  digitalWrite(LED_A, LOW);
  digitalWrite(LED_B, HIGH);
  delay(300);
}

Quick quiz

Q1. Where does repeating work belong?

A. loop() ✓
B. setup()
C. pinMode only

Why: Correct—loop() runs again and again.

Q2. When LED A is HIGH, LED B should be…

A. LOW (opposite) ✓
B. HIGH (same)
C. Unconnected

Why: Correct — alternating means opposite ON/OFF states.

Code lab — try on your own

Swap faster — change both delay(300) to delay(150).
Hint: There are two matching delay lines in loop().
Add a comment on the first digitalWrite(LED_A, HIGH) line explaining “A on”.
Hint: Use // at the end of line 8.

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

Turn two LEDs on and off in opposite states.

Why here

Read from top to bottom. Hover words or lines for help!

const int LED_A = 12;
const int LED_B = 13;

setup()

Technical

Runs one time when the board turns on.

In this project

Sets up pins and libraries for Alternate LEDs.

Why here

One-time setup belongs here—not in loop().

void setup() {
  pinMode(LED_A, OUTPUT);
  pinMode(LED_B, OUTPUT);
}

loop()

Technical

Runs again and again after setup() is done.

In this project

This is the main action you see in Alternate LEDs.

Why here

Repeating work (blink, read sensors) goes here.

void loop() {
  digitalWrite(LED_A, HIGH);
  digitalWrite(LED_B, LOW);
  delay(300);
  digitalWrite(LED_A, LOW);
  digitalWrite(LED_B, HIGH);
  delay(300);
}

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 Alternate LEDs circuit ready in the simulator.

Why here

Goes in setup() because we only set pins once at the start.

  pinMode(LED_A, OUTPUT);

digitalWrite

Technical

Turns a pin ON or OFF.

In this project

Controls lights, motors, or buzzers in Alternate LEDs.

Why here

Goes in loop() so it can keep changing while the program runs.

  digitalWrite(LED_A, HIGH);

delay

Technical

Waits for some time. Nothing else runs during the wait.

In this project

Controls speed so you can see Alternate LEDs in the simulator.

Why here

Right after an action that should stay the same for a moment.

  delay(300);

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