MotionSensor (PIR) in Raspberry Pi

Introduction

A MotionSensor (PIR) detects movement by sensing changes in infrared light. It helps your Raspberry Pi know when someone or something moves nearby.

To turn on a light automatically when someone enters a room.

To start recording video only when motion is detected.

To alert you if someone moves near your Raspberry Pi setup.

To save energy by activating devices only when needed.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO
import time

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

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

Use GPIO.setmode(GPIO.BCM) to refer to pins by their Broadcom number.

Always clean up GPIO pins with GPIO.cleanup() to avoid warnings.

Examples

Check if motion is detected on pin 7 and print a message.

Raspberry Pi

GPIO.setup(7, GPIO.IN)
if GPIO.input(7):
    print("Motion detected!")

Check motion every half second and print "Motion!" if detected.

Raspberry Pi

import time
while True:
    if GPIO.input(7):
        print("Motion!")
    time.sleep(0.5)

Sample Program

This program continuously checks the PIR sensor on pin 7. It prints "Motion Detected!" when it senses movement, otherwise "No Motion". Press Ctrl+C to stop safely.

Raspberry Pi

import RPi.GPIO as GPIO
import time

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

print("Starting motion sensor. Press Ctrl+C to stop.")

try:
    while True:
        if GPIO.input(PIR_PIN):
            print("Motion Detected!")
        else:
            print("No Motion")
        time.sleep(1)
except KeyboardInterrupt:
    print("Stopping motion sensor.")
    GPIO.cleanup()

Important Notes

Make sure your PIR sensor is connected correctly: power, ground, and signal to the correct GPIO pin.

Sometimes PIR sensors need a few seconds to warm up before detecting motion.

Use try-except to handle stopping the program safely and cleaning GPIO pins.

Summary

A PIR MotionSensor detects movement by sensing infrared changes.

Use GPIO input to read the sensor state on your Raspberry Pi.

Always clean up GPIO pins when your 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