Button with interrupt (GPIO.add_event_detect) in Raspberry Pi - Step-by-Step Execution

Concept Flow - Button with interrupt (GPIO.add_event_detect)

Setup GPIO pin as input

↓

Attach event detect on pin

↓

Wait for button press

↓

Interrupt triggers callback

↓

Callback runs user code

↓

Return to waiting for next press

The program sets a pin as input, attaches an event detect to watch for button presses, and runs a callback function automatically when the button is pressed.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def button_pressed(channel):
    print("Button was pressed!")

GPIO.add_event_detect(18, GPIO.FALLING, callback=button_pressed, bouncetime=200)

while True:
    pass

This code waits for a button press on pin 18 and prints a message when pressed, using an interrupt callback.

Execution Table

Step	Action	Pin State	Event Detected?	Callback Called	Output
1	Setup pin 18 as input with pull-up	HIGH (not pressed)	No	No
2	Attach event detect on FALLING edge	HIGH	No	No
3	Wait for button press	HIGH	No	No
4	Button pressed, pin goes LOW	LOW (pressed)	Yes	Yes	Button was pressed!
5	Callback runs and returns	LOW	No	No
6	Button released, pin goes HIGH	HIGH (not pressed)	No	No
7	Wait for next press	HIGH	No	No

💡 Program runs indefinitely, waiting for button presses and calling callback on each press.

Variable Tracker

Variable	Start	After Setup	After Press	After Callback	Final
Pin 18 State	Undefined	HIGH (not pressed)	LOW (pressed)	LOW (pressed)	HIGH (not pressed)
Event Detected	False	False	True	False	False
Callback Called	False	False	True	True	False

Key Moments - 3 Insights

Why does the callback run only when the button is pressed and not all the time?

Because the event detect is set to trigger only on the FALLING edge (pin going from HIGH to LOW), as shown in execution_table rows 4 and 5.

What is the purpose of the 'bouncetime' parameter in add_event_detect?

It prevents multiple triggers caused by the button physically bouncing, ensuring the callback runs only once per press, as implied in the stable callback calls in the execution_table.

Why do we use pull_up_down=GPIO.PUD_UP when setting up the pin?

It keeps the pin normally HIGH when the button is not pressed, so pressing the button pulls it LOW, which the event detect watches for, as seen in the Pin State changes in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the callback function first run?

A Step 6

B Step 3

C Step 4

D Step 7

💡 Hint

Check the 'Callback Called' column in execution_table rows.

According to variable_tracker, what is the pin state after the button is pressed?

A HIGH (not pressed)

B LOW (pressed)

C Undefined

D Floating

💡 Hint

Look at 'Pin 18 State' values after press in variable_tracker.

If we remove the 'bouncetime' parameter, what might happen?

A Callback might run multiple times per press

B Callback will never run

C Pin state will stay HIGH

D Program will crash

💡 Hint

Think about what 'bouncetime' prevents as explained in key_moments.

Concept Snapshot

Setup GPIO pin as input with pull-up resistor
Attach event detect on FALLING edge with callback
When button pressed, pin goes LOW triggering callback
Callback runs user code (e.g., print message)
Use bouncetime to avoid multiple triggers
Program waits indefinitely for button presses

Full Transcript

This example shows how to use GPIO.add_event_detect on a Raspberry Pi to detect button presses. First, the GPIO pin 18 is set as input with a pull-up resistor, so it reads HIGH normally. Then, an event detect is attached to watch for the FALLING edge, which happens when the button is pressed and the pin goes LOW. When this event occurs, the callback function button_pressed runs and prints a message. The program waits in an infinite loop doing nothing else, relying on interrupts to respond to button presses. The bouncetime parameter helps prevent multiple triggers caused by button bounce. The execution table traces the pin state and callback calls step-by-step, and the variable tracker shows how pin state and event flags change over time.

Practice

1.

What is the main purpose of using GPIO.add_event_detect with a button on a Raspberry Pi?

easy

A. To increase the voltage on the button pin

B. To set the button pin as an output

C. To disable the button input temporarily

D. To detect button presses instantly without constantly checking in a loop

Solution

1. Step 1: Understand what GPIO.add_event_detect does

   This function listens for changes on a GPIO pin and triggers a callback or event without needing to check the pin repeatedly.

2. Step 2: Compare options to the function's purpose

   Only To detect button presses instantly without constantly checking in a loop correctly describes the purpose: detecting button presses instantly without polling.

3. Final Answer:

   To detect button presses instantly without constantly checking in a loop -> Option D

4. Quick Check:

   Event detection = instant button press response [OK]

Hint: Think: event means reacting immediately, no waiting loop needed [OK]

Common Mistakes:

• Confusing input and output pin setup
• Thinking it disables the button
• Assuming it changes voltage levels

2.

Which of the following is the correct syntax to add an event detect for a button press on GPIO pin 17 for a falling edge?

GPIO.add_event_detect(____, GPIO.FALLING, callback=my_callback)

easy

A. 17

B. '17'

C. GPIO.FALLING

D. my_callback

Solution

1. Step 1: Identify the pin number argument

   The first argument to GPIO.add_event_detect is the pin number as an integer, here 17.

2. Step 2: Check the options for correct type

   17 uses integer 17, which is correct. '17' uses a string which is invalid. Options C and D are incorrect because they are not pin numbers.

3. Final Answer:

   17 -> Option A

4. Quick Check:

   Pin number must be integer 17 [OK]

Hint: Pin numbers are integers, not strings or functions [OK]

Common Mistakes:

• Using pin number as a string
• Passing edge type or callback as pin number
• Missing required arguments

3.

Consider this code snippet:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.IN, pull_up_down=GPIO.PUD_UP)

button_pressed = False

def button_callback(channel):
    global button_pressed
    button_pressed = True

GPIO.add_event_detect(18, GPIO.FALLING, callback=button_callback, bouncetime=200)

print("Waiting for button press...")

while not button_pressed:
    time.sleep(0.1)

print("Button was pressed!")
GPIO.cleanup()

What will be printed when the button connected to pin 18 is pressed?

medium

A. Waiting for button press...

B. Button was pressed!\nWaiting for button press...

C. Waiting for button press...\nButton was pressed!

D. No output

Solution

1. Step 1: Understand program flow before button press

   The program prints "Waiting for button press..." and then waits in a loop until button_pressed becomes True.

2. Step 2: Effect of button press and callback

   When the button is pressed, the callback sets button_pressed = True, breaking the loop and printing "Button was pressed!" before cleanup.

3. Final Answer:

   Waiting for button press... Button was pressed! -> Option C

4. Quick Check:

   Prints before and after button press [OK]

Hint: Callback sets flag; loop waits; prints before and after press [OK]

Common Mistakes:

• Thinking callback prints directly
• Ignoring the loop waiting for flag
• Assuming no output before press

4.

What is wrong with this code snippet that tries to detect a button press on GPIO pin 22?

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(22, GPIO.IN)

def on_press(channel):
    print("Button pressed")

GPIO.add_event_detect(22, GPIO.RISING, callback=on_press)

while True:
    pass

GPIO.cleanup()

medium

A. Missing pull-up or pull-down resistor setup on pin 22

B. Callback function should return a value

C. GPIO.cleanup() is called before the loop ends

D. Using GPIO.RISING instead of GPIO.FALLING is invalid

Solution

1. Step 1: Check pin setup for input

   The code sets pin 22 as input but does not specify pull-up or pull-down resistors, which can cause floating input and unreliable detection.

2. Step 2: Review other options

   Callback functions do not need to return values. The cleanup call is unreachable but not an error. Both RISING and FALLING edges are valid depending on wiring.

3. Final Answer:

   Missing pull-up or pull-down resistor setup on pin 22 -> Option A

4. Quick Check:

   Input pins need pull resistors to avoid floating [OK]

Hint: Always set pull-up/down resistors for input pins [OK]

Common Mistakes:

• Ignoring pull-up/down resistor setup
• Expecting callback to return a value
• Misunderstanding edge types

5.

You want to detect both button press and release events on GPIO pin 5 using interrupts. Which code snippet correctly sets up event detection for both edges and avoids multiple triggers due to button bounce?

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(5, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def button_event(channel):
    if GPIO.input(channel) == GPIO.LOW:
        print("Button pressed")
    else:
        print("Button released")

# Which line correctly adds event detection?

hard

A. GPIO.add_event_detect(5, GPIO.RISING, callback=button_event)

B. GPIO.add_event_detect(5, GPIO.BOTH, callback=button_event, bouncetime=300)

C. GPIO.add_event_detect(5, GPIO.FALLING, callback=button_event, bouncetime=50)

D. GPIO.add_event_detect(5, GPIO.BOTH, callback=button_event)

Solution

1. Step 1: Identify edge detection for press and release

   To detect both press and release, use GPIO.BOTH to catch rising and falling edges.

2. Step 2: Add debounce to avoid multiple triggers

   Button bounce causes multiple events; setting bouncetime=300 ms helps debounce effectively.

3. Final Answer:

   GPIO.add_event_detect(5, GPIO.BOTH, callback=button_event, bouncetime=300) -> Option B

4. Quick Check:

   Both edges + debounce = correct setup [OK]

Hint: Use BOTH edges with bouncetime to catch press and release cleanly [OK]

Common Mistakes:

• Using only RISING or FALLING edge misses events
• Forgetting debounce causes multiple prints
• Not setting pull-up resistor on input pin