Motor direction with H-Bridge (L293D/L298N) in Arduino - Time & Space Complexity

Time Complexity: Motor direction with H-Bridge (L293D/L298N)

O(n)

Understanding Time Complexity

When controlling a motor with an H-Bridge, the code runs commands to set motor direction and speed.

We want to see how the time to run this code changes as we repeat motor commands more times.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const int motorPin1 = 3;
const int motorPin2 = 4;

void setup() {
  pinMode(motorPin1, OUTPUT);
  pinMode(motorPin2, OUTPUT);
}

void loop() {
  for (int i = 0; i < 100; i++) {
    digitalWrite(motorPin1, HIGH);
    digitalWrite(motorPin2, LOW);
    delay(1000);
  }
}
    

This code sets motor direction by turning one pin on and the other off repeatedly 100 times, each with a delay.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that runs 100 times.
• How many times: The motor direction commands inside the loop run exactly 100 times.

How Execution Grows With Input

Each time we increase the loop count, the number of motor commands grows the same amount.

Input Size (n)	Approx. Operations
10	10 motor direction sets
100	100 motor direction sets
1000	1000 motor direction sets

Pattern observation: The number of operations grows directly with the number of loop repetitions.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the motor commands grows in a straight line as we increase the number of repeats.

Common Mistake

[X] Wrong: "The motor commands inside the loop run instantly and don't add to time."

[OK] Correct: Each command takes some time to execute, and repeating them more times adds up linearly.

Interview Connect

Understanding how loops affect execution time helps you write efficient code for hardware control, a useful skill in many projects.

Self-Check

"What if we added a nested loop inside the existing loop to run motor commands multiple times per iteration? How would the time complexity change?"

Practice

1. Which of the following correctly describes how to make a motor spin forward using an H-Bridge (L293D/L298N) with two Arduino pins pinA and pinB?

easy

A. Set both pinA and pinB HIGH

B. Set pinA LOW and pinB HIGH

C. Set both pinA and pinB LOW

D. Set pinA HIGH and pinB LOW

Solution

1. Step 1: Understand motor direction control

   To spin the motor forward, one pin must be HIGH and the other LOW to create current flow in one direction.

2. Step 2: Apply pin states for forward spin

   Setting pinA HIGH and pinB LOW makes the motor spin forward.

3. Final Answer:

   Set pinA HIGH and pinB LOW -> Option D

4. Quick Check:

   Forward spin = pinA HIGH, pinB LOW [OK]

Hint: Forward spin means one pin HIGH, other LOW [OK]

Common Mistakes:

• Setting both pins HIGH or LOW thinking motor spins
• Confusing which pin should be HIGH for forward
• Assuming both pins LOW spins motor forward

2. Which Arduino code snippet correctly sets pin1 HIGH and pin2 LOW to spin a motor forward using an H-Bridge?

easy

A. digitalWrite(pin1, LOW); digitalWrite(pin2, HIGH);

B. digitalWrite(pin1, HIGH); digitalWrite(pin2, LOW);

C. digitalWrite(pin1, HIGH); digitalWrite(pin2, HIGH);

D. digitalWrite(pin1, LOW); digitalWrite(pin2, LOW);

Solution

1. Step 1: Recall correct pin states for forward

   Forward spin requires first pin HIGH and second pin LOW.

2. Step 2: Match code to pin states

   Code in digitalWrite(pin1, HIGH); digitalWrite(pin2, LOW); sets pin1 HIGH and pin2 LOW correctly.

3. Final Answer:

   digitalWrite(pin1, HIGH); digitalWrite(pin2, LOW); -> Option B

4. Quick Check:

   Pin1 HIGH, Pin2 LOW = forward spin [OK]

Hint: HIGH then LOW means forward spin [OK]

Common Mistakes:

• Swapping HIGH and LOW values
• Setting both pins HIGH or LOW
• Using analogWrite instead of digitalWrite

3. What will be the motor behavior when the following Arduino code runs?

digitalWrite(3, LOW);
digitalWrite(4, HIGH);

medium

A. Motor spins backward

B. Motor spins forward

C. Motor stops

D. Motor speed changes but direction stays same

Solution

1. Step 1: Identify pin states and their meaning

   Pin 3 is LOW and pin 4 is HIGH, which reverses the current flow compared to forward spin.

2. Step 2: Determine motor direction

   This pin combination makes the motor spin backward.

3. Final Answer:

   Motor spins backward -> Option A

4. Quick Check:

   LOW then HIGH = backward spin [OK]

Hint: LOW then HIGH means backward spin [OK]

Common Mistakes:

• Assuming LOW then HIGH is forward spin
• Thinking motor stops when pins differ
• Confusing speed control with direction

4. The following code is intended to stop the motor, but it does not. What is the error?

digitalWrite(5, HIGH);
digitalWrite(6, HIGH);

medium

A. Pins must be set to LOW and HIGH to stop

B. Pins should be set to INPUT to stop motor

C. Both pins HIGH causes motor to brake, not stop

D. digitalWrite cannot be used to stop motor

Solution

1. Step 1: Understand stopping motor with H-Bridge

   To stop the motor, both pins should be LOW to cut power and stop current flow.

2. Step 2: Analyze given pin states

   Both pins are set HIGH, which causes the motor to brake but not coast to a stop.

3. Final Answer:

   Both pins HIGH causes motor to brake, not stop -> Option C

4. Quick Check:

   Stop motor = both pins LOW, not HIGH [OK]

Hint: Stop motor by setting both pins LOW [OK]

Common Mistakes:

• Setting both pins HIGH thinking motor stops
• Using INPUT mode instead of LOW
• Confusing brake with stop

5. You want to write Arduino code to spin a motor forward for 3 seconds, then stop for 2 seconds, then spin backward for 3 seconds using pins 7 and 8 with an L298N. Which code snippet correctly implements this?

hard

A. digitalWrite(7, HIGH); digitalWrite(8, LOW); delay(3000); digitalWrite(7, LOW); digitalWrite(8, LOW); delay(2000); digitalWrite(7, LOW); digitalWrite(8, HIGH); delay(3000);

B. digitalWrite(7, LOW); digitalWrite(8, HIGH); delay(3000); digitalWrite(7, LOW); digitalWrite(8, LOW); delay(2000); digitalWrite(7, HIGH); digitalWrite(8, LOW); delay(3000);

C. digitalWrite(7, HIGH); digitalWrite(8, HIGH); delay(3000); digitalWrite(7, LOW); digitalWrite(8, LOW); delay(2000); digitalWrite(7, LOW); digitalWrite(8, LOW); delay(3000);

D. digitalWrite(7, LOW); digitalWrite(8, LOW); delay(3000); digitalWrite(7, HIGH); digitalWrite(8, LOW); delay(2000); digitalWrite(7, LOW); digitalWrite(8, HIGH); delay(3000);

Solution

1. Step 1: Set pins for forward spin and delay

   Forward spin requires pin 7 HIGH and pin 8 LOW, then delay 3000 ms (3 seconds).

2. Step 2: Stop motor by setting both pins LOW and delay

   Both pins LOW stops motor, delay 2000 ms (2 seconds).

3. Step 3: Set pins for backward spin and delay

   Backward spin requires pin 7 LOW and pin 8 HIGH, delay 3000 ms.

4. Final Answer:

   digitalWrite(7, HIGH); digitalWrite(8, LOW); delay(3000); digitalWrite(7, LOW); digitalWrite(8, LOW); delay(2000); digitalWrite(7, LOW); digitalWrite(8, HIGH); delay(3000); -> Option A

5. Quick Check:

   Forward, stop, backward with correct pins and delays [OK]

Hint: Forward=HIGH-LOW, Stop=LOW-LOW, Backward=LOW-HIGH [OK]

Common Mistakes:

• Swapping forward and backward pin states
• Not stopping motor between directions
• Using HIGH-HIGH which brakes but doesn't stop