MotionSensor (PIR) in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a PIR Motion Sensor detect?

A PIR (Passive Infrared) Motion Sensor detects changes in infrared radiation, which usually means it senses movement of warm objects like people or animals.

beginner

How do you connect a PIR sensor to a Raspberry Pi?

You connect the PIR sensor's power pin to 5V or 3.3V, ground pin to GND, and the output pin to a GPIO pin on the Raspberry Pi to read motion signals.

beginner

What Python library is commonly used to read GPIO pins on a Raspberry Pi?

The RPi.GPIO library is commonly used to control and read GPIO pins on a Raspberry Pi in Python programs.

intermediate

What is the basic Python code structure to detect motion using a PIR sensor?

Set up the GPIO pin as input, then use a loop to check if the pin reads HIGH (motion detected) or LOW (no motion), and respond accordingly.

intermediate

Why is it important to add a delay or debounce when reading PIR sensor signals?

Adding a delay helps avoid false triggers caused by sensor noise or rapid changes, ensuring the program reacts only to real motion events.

What type of signal does a PIR sensor output when motion is detected?

A Digital HIGH signal

B Analog voltage

C PWM signal

D No signal

A PIR sensor outputs a digital HIGH signal on its output pin when it detects motion.

Which Raspberry Pi pin type should you use to read a PIR sensor output?

A GPIO output pin

B GPIO input pin

C Power pin

D Ground pin

The PIR sensor output connects to a GPIO input pin to read motion signals.

What Python command sets a GPIO pin as input using RPi.GPIO?

A GPIO.output(pin, True)

B GPIO.setup(pin, GPIO.OUT)

C GPIO.input(pin)

D GPIO.setup(pin, GPIO.IN)

GPIO.setup(pin, GPIO.IN) configures the pin as an input to read signals.

Why should you use a try-except or cleanup in a PIR sensor Python script?

A To safely release GPIO resources on exit

B To speed up the sensor

C To increase sensor sensitivity

D To change sensor voltage

Using try-except or GPIO.cleanup() ensures GPIO pins are properly reset when the program ends.

What does a PIR sensor NOT detect?

A Motion of warm objects

B Changes in infrared radiation

C Sound waves

D Presence of people

PIR sensors do not detect sound waves; they detect infrared radiation changes.

Explain how a PIR sensor works with a Raspberry Pi to detect motion.

Think about how the sensor senses heat changes and how the Pi reads that signal.

Describe the steps to write a Python program that alerts you when motion is detected by a PIR sensor.

Start from setting up the pin to cleaning up after the program ends.

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