MotionSensor (PIR) in Raspberry Pi - Time & Space Complexity

Time Complexity: MotionSensor (PIR)

O(n)

Understanding Time Complexity

When working with a MotionSensor (PIR) on a Raspberry Pi, it is important to understand how the program's running time changes as it checks for motion repeatedly.

We want to know how the time needed grows as the sensor keeps checking for motion over time.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
PIR_PIN = 7
GPIO.setup(PIR_PIN, GPIO.IN)

while True:
    if GPIO.input(PIR_PIN):
        print("Motion Detected!")
    time.sleep(1)

This code continuously checks the PIR sensor every second and prints a message if motion is detected.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that repeatedly reads the sensor input.
• How many times: It runs indefinitely, checking once every second.

How Execution Grows With Input

As time passes, the program keeps checking the sensor once every second, so the number of checks grows directly with time.

Input Size (seconds)	Approx. Operations (checks)
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of operations grows linearly with the time the program runs.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows directly in proportion to how long the program keeps checking the sensor.

Common Mistake

[X] Wrong: "The program only runs once, so time complexity is constant."

[OK] Correct: The program runs an endless loop checking the sensor repeatedly, so the time grows as it keeps running.

Interview Connect

Understanding how loops that run continuously affect time helps you reason about programs that monitor sensors or devices in real time.

Self-Check

"What if we changed the sleep time from 1 second to 0.1 seconds? How would the time complexity change?"

Practice

1.

What does a PIR MotionSensor detect when connected to a Raspberry Pi?

easy

A. Temperature changes in the room

B. Sound levels around the sensor

C. Movement by sensing infrared changes

D. Light intensity variations

Solution

1. Step 1: Understand PIR sensor function

   A PIR sensor detects motion by sensing changes in infrared radiation emitted by objects.

2. Step 2: Compare options with sensor purpose

   Only Movement by sensing infrared changes matches the PIR sensor's function of detecting movement via infrared changes.

3. Final Answer:

   Movement by sensing infrared changes -> Option C

4. Quick Check:

   PIR sensor = Movement detection [OK]

Hint: PIR sensors detect motion, not temperature or sound [OK]

Common Mistakes:

• Confusing PIR with temperature sensors
• Thinking PIR detects sound or light
• Assuming PIR measures humidity

2.

Which of the following is the correct way to set up a GPIO pin as input for a PIR sensor in Python using RPi.GPIO?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
# What goes here?

easy

A. GPIO.setup(17, GPIO.IN)

B. GPIO.setup(17, GPIO.OUT)

C. GPIO.input(17)

D. GPIO.output(17, GPIO.HIGH)

Solution

1. Step 1: Identify pin mode for PIR sensor input

   PIR sensor output should be read as input on the Raspberry Pi GPIO pin.

2. Step 2: Match correct GPIO setup syntax

   GPIO.setup(pin_number, GPIO.IN) sets the pin as input, which is needed to read sensor data.

3. Final Answer:

   GPIO.setup(17, GPIO.IN) -> Option A

4. Quick Check:

   GPIO input setup = GPIO.setup(pin, GPIO.IN) [OK]

Hint: Use GPIO.IN to read sensor signals, not GPIO.OUT [OK]

Common Mistakes:

• Using GPIO.OUT instead of GPIO.IN
• Calling GPIO.input() instead of GPIO.setup() to configure pin
• Trying to write output to sensor pin

3.

What will be the output of this Python code snippet when the PIR sensor detects motion?

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
PIR_PIN = 4
GPIO.setup(PIR_PIN, GPIO.IN)

if GPIO.input(PIR_PIN):
    print("Motion Detected")
else:
    print("No Motion")

medium

A. Motion Detected

B. No Motion

C. SyntaxError

D. GPIO pin not set

Solution

1. Step 1: Understand GPIO.input behavior

   GPIO.input(PIR_PIN) returns True (1) if motion is detected, False (0) otherwise.

2. Step 2: Analyze if condition with PIR sensor detecting motion

   If motion is detected, GPIO.input returns True, so the code prints "Motion Detected".

3. Final Answer:

   Motion Detected -> Option A

4. Quick Check:

   GPIO.input True = Motion Detected [OK]

Hint: GPIO.input True means motion detected, prints accordingly [OK]

Common Mistakes:

• Assuming GPIO.input returns string
• Confusing True/False logic in if statement
• Not setting up GPIO pin before reading

4.

Identify the error in this PIR sensor code snippet and choose the fix:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
PIR_PIN = 18
GPIO.setup(PIR_PIN, GPIO.IN)

try:
    while True:
        if GPIO.input(PIR_PIN):
            print("Motion!")
        time.sleep(1)
except KeyboardInterrupt:
    print("Exit")

medium

A. Use GPIO.input() outside the loop

B. Change GPIO.setup to GPIO.OUT

C. Remove time.sleep(1)

D. Add GPIO.cleanup() in except block

Solution

1. Step 1: Check for GPIO cleanup after program ends

   GPIO pins should be cleaned up to reset their state when the program exits.

2. Step 2: Add GPIO.cleanup() in the except block

   Adding GPIO.cleanup() ensures pins are properly reset on KeyboardInterrupt.

3. Final Answer:

   Add GPIO.cleanup() in except block -> Option D

4. Quick Check:

   Always cleanup GPIO pins on exit [OK]

Hint: Always call GPIO.cleanup() when program ends [OK]

Common Mistakes:

• Forgetting GPIO.cleanup() causes warnings on rerun
• Setting input pin as output
• Removing sleep causes high CPU usage

5.

You want to count how many times motion is detected in 10 seconds using a PIR sensor on GPIO pin 23. Which code snippet correctly implements this?

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
PIR_PIN = 23
GPIO.setup(PIR_PIN, GPIO.IN)

count = 0
start_time = time.time()
while time.time() - start_time < 10:
    if GPIO.input(PIR_PIN):
        count += 1
        time.sleep(0.5)  # debounce delay
print(count)
GPIO.cleanup()

hard

A. Does not count because GPIO pin is not set as output

B. Counts motion detections with debounce, prints count after 10 seconds

C. Counts continuously without delay, causing multiple counts per motion

D. Infinite loop without exit condition

Solution

1. Step 1: Analyze loop timing and counting logic

   The loop runs for 10 seconds, checking PIR input and incrementing count on detection.

2. Step 2: Understand debounce with time.sleep(0.5)

   The 0.5 second sleep prevents counting the same motion multiple times quickly.

3. Step 3: Confirm GPIO cleanup and print

   After loop ends, count is printed and GPIO.cleanup() resets pins properly.

4. Final Answer:

   Counts motion detections with debounce, prints count after 10 seconds -> Option B

5. Quick Check:

   Debounce delay + timed loop = correct count [OK]

Hint: Use sleep to debounce and time.time() for timed loops [OK]

Common Mistakes:

• Missing debounce causes inflated counts
• Setting pin as output instead of input
• No exit condition causes infinite loop