MotionSensor (PIR) in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when the PIR sensor detects motion?

Consider this Python code snippet running on a Raspberry Pi with a PIR motion sensor connected to GPIO pin 17. What will be printed when motion is detected?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN)

try:
    while True:
        if GPIO.input(17):
            print("Motion Detected")
        else:
            print("No Motion")
        time.sleep(1)
except KeyboardInterrupt:
    GPIO.cleanup()

A No Motion (printed once when motion is detected)

B Motion Detected (printed repeatedly every second while motion is present)

C Motion Detected (printed once and program exits)

D No output at all

💡 Hint

Think about what the while True loop does and how GPIO.input(17) behaves when motion is detected.

The code continuously checks the PIR sensor input every second. When motion is detected, GPIO.input(17) returns True, so "Motion Detected" is printed repeatedly until motion stops.

🧠 Conceptual

intermediate

What does the PIR sensor output signal represent?

When a PIR motion sensor is connected to a Raspberry Pi GPIO pin, what does the digital signal HIGH (1) from the sensor indicate?

A The sensor is in standby mode

B The sensor is powered off

C The sensor detected motion in its range

D The sensor detected no motion

💡 Hint

Think about what a motion sensor is designed to detect and how it signals that event.

A PIR sensor outputs a HIGH signal when it detects motion in its sensing area. LOW means no motion detected.

🔧 Debug

advanced

Why does this PIR sensor code raise a RuntimeError?

Examine the code below. It raises a RuntimeError: 'No access to /dev/mem. Try running as root!'. What is the cause?

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN)

if GPIO.input(17):
    print("Motion Detected")
else:
    print("No Motion")

A The PIR sensor is not connected to the correct pin

B The GPIO library is not imported correctly

C The GPIO pin number 17 is invalid on Raspberry Pi

D The program is not run with root privileges required to access GPIO

💡 Hint

Consider what permissions are needed to access hardware pins on Raspberry Pi.

Accessing GPIO pins requires root privileges. Running the script without sudo causes this RuntimeError.

📝 Syntax

advanced

Which option correctly sets up a PIR sensor input on GPIO pin 4 with pull-down resistor?

Choose the correct Python code snippet to set up GPIO pin 4 as input for a PIR sensor with an internal pull-down resistor enabled.

A GPIO.setup(4, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

B GPIO.setup(4, GPIO.IN)

C GPIO.setup(4, GPIO.IN, pull_up_down=GPIO.PUD_UP)

D GPIO.setup(4, GPIO.OUT, pull_up_down=GPIO.PUD_DOWN)

💡 Hint

Remember the direction for sensor input and the correct pull resistor for PIR sensors.

PIR sensors usually require input mode with pull-down resistor to avoid floating signals. Option A correctly sets this.

🚀 Application

expert

How many times will 'Motion Detected' print if motion lasts 3 seconds?

Given this code snippet, how many times will "Motion Detected" be printed if motion is continuously detected for exactly 3 seconds?

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN)

start = time.time()
while time.time() - start < 3:
    if GPIO.input(17):
        print("Motion Detected")
    time.sleep(0.5)

A 6 times

B 3 times

C 4 times

D 5 times

💡 Hint

Calculate how many 0.5 second intervals fit into 3 seconds.

The loop runs while time elapsed is less than 3 seconds. Each iteration waits 0.5 seconds. 3 / 0.5 = 6 iterations, so "Motion Detected" prints 6 times.