Alarm system with sensor and buzzer in Arduino - Time & Space Complexity

Time Complexity: Alarm system with sensor and buzzer

O(n)

Understanding Time Complexity

We want to understand how the time the alarm system takes changes as it checks the sensor repeatedly.

How does the program's work grow when it keeps watching the sensor?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const int sensorPin = 2;
const int buzzerPin = 9;

void setup() {
  pinMode(sensorPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  int sensorValue = digitalRead(sensorPin);
  if (sensorValue == HIGH) {
    digitalWrite(buzzerPin, HIGH);
  } else {
    digitalWrite(buzzerPin, LOW);
  }
}
    

This code reads a sensor input continuously and turns the buzzer on or off based on the sensor's state.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop() function runs forever, repeatedly reading the sensor and setting the buzzer.
• How many times: It runs continuously, checking the sensor once every cycle.

How Execution Grows With Input

Each cycle reads the sensor once and updates the buzzer once, no matter how many times it runs.

Input Size (n)	Approx. Operations
10	10 sensor reads and buzzer updates
100	100 sensor reads and buzzer updates
1000	1000 sensor reads and buzzer updates

Pattern observation: The work grows directly with the number of cycles; each cycle does a fixed amount of work.

Final Time Complexity

Time Complexity: O(n)

This means the time the program takes grows linearly with how many times it checks the sensor.

Common Mistake

[X] Wrong: "The program runs instantly and does not depend on how many times it loops."

[OK] Correct: The program keeps running forever, and the total work grows as it keeps checking the sensor repeatedly.

Interview Connect

Understanding how repeated checks affect time helps you explain how embedded systems handle real-time inputs efficiently.

Self-Check

"What if we added a loop inside loop() that checks multiple sensors? How would the time complexity change?"

Practice

1. What does the digitalRead(sensorPin) function do in an Arduino alarm system?

easy

A. It turns on the buzzer connected to sensorPin.

B. It sets the sensorPin to HIGH to activate the sensor.

C. It reads the current state (HIGH or LOW) of the sensor connected to sensorPin.

D. It resets the Arduino board.

Solution

1. Step 1: Understand digitalRead function

   The function digitalRead(pin) checks the electrical state of the given pin, returning HIGH or LOW.

2. Step 2: Apply to sensorPin

   When used with sensorPin, it reads if the sensor is triggered (HIGH) or not (LOW).

3. Final Answer:

   It reads the current state (HIGH or LOW) of the sensor connected to sensorPin. -> Option C

4. Quick Check:

   digitalRead(sensorPin) = sensor state [OK]

Hint: digitalRead reads input pin state, not output [OK]

Common Mistakes:

• Confusing digitalRead with digitalWrite
• Thinking digitalRead sets pin state
• Assuming digitalRead controls buzzer

2. Which of the following is the correct way to turn ON a buzzer connected to pin 9 in Arduino?

easy

A. digitalWrite(9, HIGH);

B. pinMode(9, INPUT);

C. digitalRead(9);

D. analogWrite(9, LOW);

Solution

1. Step 1: Identify function to set pin output

   digitalWrite(pin, HIGH) sets the pin to HIGH voltage, turning ON devices like buzzers.

2. Step 2: Check pin number and value

   Pin 9 is correct, and HIGH turns the buzzer ON.

3. Final Answer:

   digitalWrite(9, HIGH); -> Option A

4. Quick Check:

   digitalWrite(pin, HIGH) = turn ON output [OK]

Hint: Use digitalWrite(pin, HIGH) to turn ON output devices [OK]

Common Mistakes:

• Using digitalRead instead of digitalWrite
• Setting pinMode to INPUT to turn ON buzzer
• Using analogWrite with LOW to turn ON buzzer

3. What will be the output behavior of this Arduino code snippet?

const int sensorPin = 2;
const int buzzerPin = 9;

void setup() {
  pinMode(sensorPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  int sensorState = digitalRead(sensorPin);
  if (sensorState == HIGH) {
    digitalWrite(buzzerPin, HIGH);
  } else {
    digitalWrite(buzzerPin, LOW);
  }
}

medium

A. The buzzer will sound only when the sensor detects HIGH signal.

B. The buzzer will always be ON regardless of sensor state.

C. The buzzer will never turn ON.

D. The code will cause a compilation error.

Solution

1. Step 1: Analyze sensor reading

   The code reads sensorPin state; if HIGH, it triggers the buzzer.

2. Step 2: Check buzzer control logic

   If sensorState is HIGH, buzzerPin is set HIGH (ON), else LOW (OFF).

3. Final Answer:

   The buzzer will sound only when the sensor detects HIGH signal. -> Option A

4. Quick Check:

   sensor HIGH -> buzzer ON, else OFF [OK]

Hint: If sensor reads HIGH, buzzer turns ON [OK]

Common Mistakes:

• Assuming buzzer is always ON
• Thinking code has syntax errors
• Confusing sensor input with buzzer output

4. Identify the error in this Arduino alarm code snippet:

const int sensorPin = 3;
const int buzzerPin = 8;

void setup() {
  pinMode(sensorPin, OUTPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  if (digitalRead(sensorPin) == HIGH) {
    digitalWrite(buzzerPin, HIGH);
  } else {
    digitalWrite(buzzerPin, LOW);
  }
}

medium

A. digitalWrite should use LOW to turn buzzer ON.

B. sensorPin should be set as INPUT, not OUTPUT.

C. digitalRead cannot be used on sensorPin.

D. buzzerPin should be set as INPUT, not OUTPUT.

Solution

1. Step 1: Check pinMode for sensorPin

   sensorPin reads sensor input, so it must be INPUT, not OUTPUT.

2. Step 2: Verify buzzerPin mode

   buzzerPin controls buzzer output, so OUTPUT is correct.

3. Final Answer:

   sensorPin should be set as INPUT, not OUTPUT. -> Option B

4. Quick Check:

   Input pins need pinMode(INPUT) [OK]

Hint: Set sensor pins as INPUT, buzzer pins as OUTPUT [OK]

Common Mistakes:

• Setting sensorPin as OUTPUT
• Confusing input and output pin modes
• Misusing digitalWrite for sensor reading

5. You want to modify an Arduino alarm system so the buzzer sounds only if the sensor detects HIGH for 3 seconds continuously. Which code change correctly implements this?

hard

A. Add a delay(3000) after detecting HIGH before turning buzzer ON.

B. Set sensorPin as OUTPUT and buzzerPin as INPUT to control timing.

C. Turn buzzer ON immediately when sensorPin is HIGH, then delay 3000 ms.

D. Use a timer to check if sensorPin stays HIGH for 3000 ms before activating buzzer.

Solution

1. Step 1: Understand requirement for continuous HIGH detection

   We need to confirm sensorPin stays HIGH for 3 seconds before buzzer sounds.

2. Step 2: Evaluate options for timing logic

   Using a timer to measure continuous HIGH duration is correct; delay alone pauses code and is insufficient.

3. Final Answer:

   Use a timer to check if sensorPin stays HIGH for 3000 ms before activating buzzer. -> Option D

4. Quick Check:

   Continuous HIGH detection needs timer logic [OK]

Hint: Use timer logic, not delay, for continuous sensor check [OK]

Common Mistakes:

• Using delay which blocks code execution
• Switching pin modes incorrectly
• Turning buzzer ON immediately without timing