Mission 20 · Stage 2

Mini Alarm

armed and disarmed if tree

Arm or disarm an alarm with a button; motion triggers the buzzer when armed.

Pin connections

Part 1		Part 2
Arduino pin 2	→	Arm button pin 1
Arm button pin 2	→	Arduino GND
Motion sensor VCC	→	Arduino 5V
Motion sensor GND	→	Arduino GND
Motion sensor OUT	→	Arduino pin 7
Arduino pin 8	→	Buzzer pin 1 (+)
Buzzer pin 2 (-)	→	Arduino GND
Arduino pin 13	→	Resistor pin 1
Resistor pin 2	→	Status LED anode (+)
Status LED cathode (-)	→	Arduino GND

See it

Arm it — motion sounds the alarm!

Toggle armed with a button; if armed AND motion, buzzer rings.

Home alarms combine mode switches with sensor if conditions.

The story

The problem

Real systems nest if tests — armed mode changes what motion means.

Think of it like

Like turning on a house alarm before leaving — only then does motion matter.

Meet the parts

Runs your sketch

Arduino

Brain

Loading part…

Toggles armed/disarmed

Arm button

Input

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Detects movement

Motion sensor

Input

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Sounds when armed and motion

Buzzer

Alarm

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Protects status LED

Resistor

Safety

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On when system is armed

Status LED

Output

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How it works

Toggle armed

Button edge flips armed and the status LED shows the mode.

armed = !armed;

Check motion

Both must be true — armed AND motion — before alarming.

if (armed && digitalRead(PIR_PIN) == HIGH)

Sound or silence

Buzzer on only in armed+motion; disarmed always silent.

tone(BUZZER, 880) / noTone(BUZZER)

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 Arm button
Place the Arm button (btn1) on the breadboard.
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3
Place Motion sensor
Place the Motion sensor (pir1) on the breadboard.
Loading part…
4
Place Buzzer
Place the Buzzer (bz1) on the breadboard.
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5
Place Resistor
Place the Resistor (r1) on the breadboard.
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6
Place Status LED
Place the Status LED (led1) on the breadboard.
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7
Connect Arduino pin 2 to Arm button (btn1) 1.l
Connect Arduino pin 2 to Arm button (btn1) 1.l.
8
Connect Arm button (btn1) 2.l to Arduino GND
Connect Arm button (btn1) 2.l to Arduino GND.
9
Connect Motion sensor (pir1) VCC to Arduino pin 5
Connect Motion sensor (pir1) VCC to Arduino pin 5.
10
Connect Motion sensor (pir1) GND to Arduino GND
Connect Motion sensor (pir1) GND to Arduino GND.
11
Connect Motion sensor (pir1) OUT to Arduino pin 7
Connect Motion sensor (pir1) OUT to Arduino pin 7.
12
Connect Arduino pin 8 to Buzzer (bz1) 1
Connect Arduino pin 8 to Buzzer (bz1) 1.
13
Connect Buzzer (bz1) 2 to Arduino GND
Connect Buzzer (bz1) 2 to Arduino GND.
14
Connect Arduino pin 13 to Resistor (r1) 1
Connect Arduino pin 13 to Resistor (r1) 1.
15
Connect Resistor (r1) 2 to Status LED (led1) anode (+)
Connect Resistor (r1) 2 to Status LED (led1) anode (+).
16
Connect Status LED (led1) cathode (-) to Arduino GND
Connect Status LED (led1) cathode (-) to Arduino GND.

Try it

Press button to arm — status LED on.
Trigger motion while armed — buzzer should sound. Disarm to stop.

Peek at code

Arm toggle

void loop() {
  bool btn = digitalRead(ARM_BTN);
  if (lastBtn == HIGH && btn == LOW) {
    armed = !armed;
    digitalWrite(STATUS_LED, armed ? HIGH : LOW);
    Serial.println(armed ? "ARMED" : "DISARMED");
    noTone(BUZZER);
  }
  lastBtn = btn;

Edge detection on the arm button flips armed and prints ARMED/DISARMED.

Nested if alarm

  if (armed && digitalRead(PIR_PIN) == HIGH) {
    tone(BUZZER, 880);
  } else {
    noTone(BUZZER);
  }
  delay(20);

armed && motion is an AND condition — classic security logic.

Show full sketch (mini-alarm.ino)

const int ARM_BTN = 2;
const int PIR_PIN = 7;
const int BUZZER = 8;
const int STATUS_LED = 13;
bool armed = false;
bool lastBtn = HIGH;
void setup() {
  pinMode(ARM_BTN, INPUT_PULLUP);
  pinMode(PIR_PIN, INPUT);
  pinMode(BUZZER, OUTPUT);
  pinMode(STATUS_LED, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mini Alarm — press button to arm/disarm");
}
void loop() {
  bool btn = digitalRead(ARM_BTN);
  if (lastBtn == HIGH && btn == LOW) {
    armed = !armed;
    digitalWrite(STATUS_LED, armed ? HIGH : LOW);
    Serial.println(armed ? "ARMED" : "DISARMED");
    noTone(BUZZER);
  }
  lastBtn = btn;
  if (armed && digitalRead(PIR_PIN) == HIGH) {
    tone(BUZZER, 880);
  } else {
    noTone(BUZZER);
  }
  delay(20);
}

Quick quiz

Q1. Where does repeating work belong?

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

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

Q2. When does the buzzer sound?

A. When armed AND motion detected ✓
B. Whenever motion happens
C. Only in setup()

Why: Correct — armed && PIR HIGH triggers tone().

Code lab — try on your own

Change alarm pitch — use tone(BUZZER, 660) instead of 880.
Hint: Line 25.
Comment line 24 explaining armed AND motion.
Hint: The if (armed && ...) line.

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

Arm or disarm an alarm with a button; motion triggers the buzzer when armed.

Why here

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

const int ARM_BTN = 2;
const int PIR_PIN = 7;
const int BUZZER = 8;
const int STATUS_LED = 13;
bool armed = false;
bool lastBtn = HIGH;

setup()

Technical

Runs one time when the board turns on.

In this project

Sets up pins and libraries for Mini Alarm.

Why here

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

void setup() {
  pinMode(ARM_BTN, INPUT_PULLUP);
  pinMode(PIR_PIN, INPUT);
  pinMode(BUZZER, OUTPUT);
  pinMode(STATUS_LED, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mini Alarm — press button to arm/disarm");
}

loop()

Technical

Runs again and again after setup() is done.

In this project

This is the main action you see in Mini Alarm.

Why here

Repeating work (blink, read sensors) goes here.

void loop() {
  bool btn = digitalRead(ARM_BTN);
  if (lastBtn == HIGH && btn == LOW) {
    armed = !armed;
    digitalWrite(STATUS_LED, armed ? HIGH : LOW);
    Serial.println(armed ? "ARMED" : "DISARMED");
    noTone(BUZZER);
  }
  lastBtn = btn;
  if (armed && digitalRead(PIR_PIN) == HIGH) {
    tone(BUZZER, 880);
  } else {
    noTone(BUZZER);
  }
  delay(20);
}

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 Mini Alarm circuit ready in the simulator.

Why here

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

  pinMode(ARM_BTN, INPUT_PULLUP);

digitalWrite

Technical

Turns a pin ON or OFF.

In this project

Controls lights, motors, or buzzers in Mini Alarm.

Why here

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

    digitalWrite(STATUS_LED, armed ? HIGH : LOW);

digitalRead

Technical

Checks if a pin is ON or OFF.

In this project

Reads buttons or sensors in Mini Alarm.

Why here

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

  bool btn = digitalRead(ARM_BTN);

tone

Technical

Plays a beep on a buzzer pin at a chosen pitch.

In this project

Makes sounds in Mini Alarm.

Why here

Goes in loop() when you want notes or alarms.

    tone(BUZZER, 880);

noTone

Technical

Stops the buzzer sound on that pin.

In this project

Turns off the sound in Mini Alarm.

Why here

After tone() or before a pause so the buzzer is silent.

    noTone(BUZZER);

delay

Technical

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

In this project

Controls speed so you can see Mini Alarm in the simulator.

Why here

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

  delay(20);

begin

Technical

Starts talking to the computer screen (serial monitor).

In this project

Lets Mini Alarm print debug messages.

Why here

Goes in setup() once before any Serial.print.

  Serial.begin(9600);

println

Technical

Sends text to the serial monitor and starts a new line.

In this project

Prints one line of output for Mini Alarm.

Why here

In loop() when each reading should appear on its own line.

  Serial.println("Mini Alarm — press button to arm/disarm");

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