Mission 14 · Stage 2

Toggle Button

edge detection and toggle latch

Each button press toggles an LED on or off using edge detection.

Pin connections

Part 1		Part 2
Arduino pin 2	→	Button pin 1
Button pin 2	→	Arduino GND
Arduino pin 13	→	Resistor pin 1
Resistor pin 2	→	LED anode (+)
LED cathode (-)	→	Arduino GND

See it

One press, flip the state!

Detect the moment of press (edge) to toggle the LED on or off.

Power buttons on devices often toggle with edge detection.

The story

The problem

Holding a button on is not a toggle — you need to catch the press instant.

Think of it like

Like flipping a coin once per tap — not while your finger rests.

Meet the parts

Runs your sketch

Arduino

Brain

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Each press toggles state

Button

Input

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Protects the LED

Resistor

Safety

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Stays on until next press

LED

Output

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How it works

Remember last reading

We compare today’s reading with lastReading to spot a change.

bool reading = digitalRead(BUTTON_PIN);

Detect press edge

HIGH→LOW means the button was just pressed — one edge, one toggle.

if (lastReading == HIGH && reading == LOW)

Flip stored state

ledOn latches the LED state between presses.

ledOn = !ledOn;
digitalWrite(LED_PIN, ledOn ? HIGH : LOW);

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 Button
Place the Button (btn1) on the breadboard.
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3
Place Resistor
Place the Resistor (r1) on the breadboard.
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4
Place LED
Place the LED (led1) on the breadboard.
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5
Connect Arduino pin 2 to Button (btn1) 1.l
Connect Arduino pin 2 to Button (btn1) 1.l.
6
Connect Button (btn1) 2.l to Arduino GND
Connect Button (btn1) 2.l to Arduino GND.
7
Connect Arduino pin 13 to Resistor (r1) 1
Connect Arduino pin 13 to Resistor (r1) 1.
8
Connect Resistor (r1) 2 to LED (led1) anode (+)
Connect Resistor (r1) 2 to LED (led1) anode (+).
9
Connect LED (led1) cathode (-) to Arduino GND
Connect LED (led1) cathode (-) to Arduino GND.

Try it

Tap the button — LED toggles each press.
Hold it down — should NOT flicker on/off rapidly.

Peek at code

Toggle state variables

bool ledOn = false;
bool lastReading = HIGH;

ledOn remembers the LED; lastReading remembers the previous button read.

Edge detection

void loop() {
  bool reading = digitalRead(BUTTON_PIN);
  if (lastReading == HIGH && reading == LOW) {
    ledOn = !ledOn;
    digitalWrite(LED_PIN, ledOn ? HIGH : LOW);
  }
  lastReading = reading;
  delay(10);
}

Only on a fresh press does the LED flip — holding the button does nothing extra.

Show full sketch (toggle-button.ino)

const int BUTTON_PIN = 2;
const int LED_PIN = 13;
bool ledOn = false;
bool lastReading = HIGH;
void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
}
void loop() {
  bool reading = digitalRead(BUTTON_PIN);
  if (lastReading == HIGH && reading == LOW) {
    ledOn = !ledOn;
    digitalWrite(LED_PIN, ledOn ? HIGH : LOW);
  }
  lastReading = reading;
  delay(10);
}

Quick quiz

Q1. Where does repeating work belong?

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

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

Q2. Why check lastReading == HIGH && reading == LOW?

A. To detect the instant of pressing ✓
B. To make the LED brighter
C. To run setup() again

Why: Correct — edge detection fires once per press, not while held.

Code lab — try on your own

Make edge checks snappier — change delay(10) to delay(5).
Hint: Last line of loop().
Comment the line ledOn = !ledOn explaining “flip state”.
Hint: Line 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

Each button press toggles an LED on or off using edge detection.

Why here

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

const int BUTTON_PIN = 2;
const int LED_PIN = 13;
bool ledOn = false;
bool lastReading = HIGH;

setup()

Technical

Runs one time when the board turns on.

In this project

Sets up pins and libraries for Toggle Button.

Why here

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

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
}

loop()

Technical

Runs again and again after setup() is done.

In this project

This is the main action you see in Toggle Button.

Why here

Repeating work (blink, read sensors) goes here.

void loop() {
  bool reading = digitalRead(BUTTON_PIN);
  if (lastReading == HIGH && reading == LOW) {
    ledOn = !ledOn;
    digitalWrite(LED_PIN, ledOn ? HIGH : LOW);
  }
  lastReading = reading;
  delay(10);
}

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 Toggle Button circuit ready in the simulator.

Why here

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

  pinMode(BUTTON_PIN, INPUT_PULLUP);

digitalWrite

Technical

Turns a pin ON or OFF.

In this project

Controls lights, motors, or buzzers in Toggle Button.

Why here

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

  digitalWrite(LED_PIN, LOW);

digitalRead

Technical

Checks if a pin is ON or OFF.

In this project

Reads buttons or sensors in Toggle Button.

Why here

Goes in loop() so we can react when something changes.

  bool reading = digitalRead(BUTTON_PIN);

delay

Technical

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

In this project

Controls speed so you can see Toggle Button in the simulator.

Why here

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

  delay(10);

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