Reading a potentiometer in Arduino - Time & Space Complexity

Time Complexity: Reading a potentiometer

O(n)

Understanding Time Complexity

When reading a potentiometer with Arduino, we want to know how the time to read changes as we do more readings.

We ask: How does the number of readings affect the total time the program takes?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int sensorPin = A0;
int sensorValue = 0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  sensorValue = analogRead(sensorPin);
  Serial.println(sensorValue);
  delay(100);
}
    

This code reads the potentiometer value once every loop and prints it, pausing briefly each time.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop() function runs repeatedly, reading the sensor once each time.
• How many times: It runs indefinitely, but if we consider n readings, it runs n times.

How Execution Grows With Input

Each reading takes about the same time, so total time grows directly with number of readings.

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

Pattern observation: Doubling the number of readings doubles the total work.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows in direct proportion to how many times we read the potentiometer.

Common Mistake

[X] Wrong: "Reading the potentiometer once or many times takes the same total time."

[OK] Correct: Each reading takes time, so more readings add up and increase total time.

Interview Connect

Understanding how repeated sensor readings affect program time helps you write efficient Arduino code and shows you can think about how programs grow.

Self-Check

"What if we read the potentiometer inside a nested loop that runs m times inside the main loop? How would the time complexity change?"

Practice

1. What does the analogRead(pin) function do when reading a potentiometer on Arduino?

easy

A. It reads the voltage level on the analog pin and returns a value from 0 to 1023.

B. It sets the output voltage of the pin to control the potentiometer.

C. It converts a digital signal to an analog voltage.

D. It resets the potentiometer to zero position.

Solution

1. Step 1: Understand analogRead function

   The analogRead(pin) reads the voltage on the specified analog pin and converts it to a number between 0 and 1023.

2. Step 2: Relate to potentiometer reading

   Since a potentiometer outputs a variable voltage depending on its position, analogRead returns a value representing that voltage level.

3. Final Answer:

   It reads the voltage level on the analog pin and returns a value from 0 to 1023. -> Option A

4. Quick Check:

   analogRead() returns 0-1023 value [OK]

Hint: Remember analogRead returns 0-1023 for voltage levels [OK]

Common Mistakes:

• Thinking analogRead sets voltage instead of reading it
• Confusing analogRead with digitalRead
• Assuming analogRead returns voltage in volts

2. Which of the following is the correct syntax to read a potentiometer connected to analog pin A0 and store the value in a variable named sensorValue?

easy

A. sensorValue = analogWrite(A0);

B. sensorValue = digitalRead(A0);

C. sensorValue = analogRead(A0);

D. sensorValue = readAnalog(A0);

Solution

1. Step 1: Identify correct function for analog input

   The function to read analog input is analogRead(pin), not digitalRead or analogWrite.

2. Step 2: Check variable assignment syntax

   Assigning the result of analogRead(A0) to sensorValue uses the syntax: sensorValue = analogRead(A0);

3. Final Answer:

   sensorValue = analogRead(A0); -> Option C

4. Quick Check:

   Use analogRead() to read analog pin [OK]

Hint: Use analogRead(pin) to read analog sensors [OK]

Common Mistakes:

• Using digitalRead instead of analogRead
• Using analogWrite which is for output
• Using a non-existent function readAnalog

3. What will be printed on the Serial Monitor when the following Arduino code runs and the potentiometer is turned to mid position?

void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensorValue = analogRead(A0);
  Serial.println(sensorValue);
  delay(1000);
}

medium

A. A value close to 0 printed every second

B. A value close to 1023 printed every second

C. No output because Serial.begin is missing

D. A value close to 512 printed every second

Solution

1. Step 1: Understand analogRead output range

   The potentiometer at mid position outputs about half the voltage, so analogRead(A0) returns around 512 (half of 1023).

2. Step 2: Analyze Serial output

   The code prints the sensorValue every 1000 milliseconds (1 second), so the Serial Monitor shows a value near 512 each second.

3. Final Answer:

   A value close to 512 printed every second -> Option D

4. Quick Check:

   Mid potentiometer = ~512 output [OK]

Hint: Mid potentiometer gives about half max value 512 [OK]

Common Mistakes:

• Expecting 0 or 1023 at mid position
• Forgetting Serial.begin causes no output
• Confusing analogRead with digitalRead values

4. The following code is intended to read a potentiometer and print its value, but it does not work correctly. What is the error?

void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensorValue;
  sensorValue = analogRead(0);
  Serial.print(sensorValue);
  delay(500);
}

medium

A. Missing Serial.println instead of Serial.print

B. Serial.begin baud rate is too low

C. No delay after Serial.print

D. Using analogRead(0) instead of analogRead(A0)

Solution

1. Step 1: Check Serial output method

   The code uses Serial.print(sensorValue) without a newline, causing consecutive values to print on the same line and appear garbled or unreadable.

2. Step 2: Use println for readability

   Serial.println(sensorValue) adds a newline after each value, making the output clear on the Serial Monitor.

3. Final Answer:

   Missing Serial.println instead of Serial.print -> Option A

4. Quick Check:

   Serial.print vs println for newlines [OK]

Hint: Use Serial.println() not print() for readable output [OK]

Common Mistakes:

• Using numeric 0 instead of A0 for analogRead
• Confusing Serial.print and Serial.println
• Ignoring delay causing fast output

5. You want to read a potentiometer and map its 0-1023 value to a 0-255 range to control LED brightness using PWM on pin 9. Which code snippet correctly does this?

hard

A. int sensorValue = analogRead(9);
int brightness = map(sensorValue, 0, 255, 0, 1023);
analogWrite(A0, brightness);

B. int sensorValue = analogRead(A0);
int brightness = map(sensorValue, 0, 1023, 0, 255);
analogWrite(9, brightness);

C. int sensorValue = digitalRead(A0);
int brightness = sensorValue * 255;
analogWrite(9, brightness);

D. int sensorValue = analogRead(A0);
int brightness = sensorValue / 4;
digitalWrite(9, brightness);

Solution

1. Step 1: Read potentiometer value correctly

   Use analogRead(A0) to get a value from 0 to 1023.

2. Step 2: Map value to 0-255 for PWM

   Use map(sensorValue, 0, 1023, 0, 255) to convert the range for LED brightness.

3. Step 3: Write PWM value to LED pin

   Use analogWrite(9, brightness) to set LED brightness on pin 9.

4. Final Answer:

   int sensorValue = analogRead(A0); int brightness = map(sensorValue, 0, 1023, 0, 255); analogWrite(9, brightness); -> Option B

5. Quick Check:

   Read analog, map range, write PWM [OK]

Hint: Use map() to convert 0-1023 to 0-255 for PWM [OK]

Common Mistakes:

• Using digitalRead instead of analogRead
• Writing PWM to analog pin or wrong pin
• Using digitalWrite for PWM control