Button press detection in Raspberry Pi - Step-by-Step Execution

Concept Flow - Button press detection

Start program

↓

Setup GPIO pin as input

↓

Wait for button press event

↓

Detect button state change

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If pressed, execute action

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Loop back to wait for next press

↓

Program ends on exit

The program sets up a GPIO pin to read button presses, waits for the button to be pressed, then runs code when pressed, repeating until stopped.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
import time

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

while True:
    if GPIO.input(18) == GPIO.LOW:
        print("Button pressed!")
        time.sleep(0.3)

This code waits for a button connected to pin 18 to be pressed and prints a message each time.

Execution Table

Step	GPIO Pin 18 State	Condition (GPIO.input(18) == LOW)	Action	Output
1	HIGH (not pressed)	False	Wait
2	HIGH (not pressed)	False	Wait
3	LOW (pressed)	True	Print message	Button pressed!
4	LOW (pressed)	True	Print message	Button pressed!
5	HIGH (released)	False	Wait
6	LOW (pressed)	True	Print message	Button pressed!
7	HIGH (released)	False	Wait
...	...	...	...	...
Exit	Program stopped by user	N/A	Exit loop

💡 Program runs until user stops it manually (e.g., Ctrl+C).

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	Final
GPIO Pin 18 State	HIGH	LOW	HIGH	LOW	Depends on button
Condition (GPIO.input(18) == LOW)	False	True	False	True	Depends on button

Key Moments - 3 Insights

Why does the program print multiple times when the button is held down?

Because the loop keeps checking the button state and prints each time it sees LOW (pressed), as shown in execution_table rows 3 and 4.

Why do we use pull_up_down=GPIO.PUD_UP in setup?

It sets the pin to HIGH by default so the button press pulls it LOW, making detection reliable, as seen in the initial HIGH state in the execution_table.

What stops the program from running forever?

The program runs in an infinite loop until the user manually stops it, as noted in the exit_note and final execution_table row.

Visual Quiz - 3 Questions

Look at the execution_table, what is the GPIO Pin 18 state at step 5?

A LOW (pressed)

B Floating (undefined)

C HIGH (released)

D Not connected

💡 Hint

Check the 'GPIO Pin 18 State' column at step 5 in the execution_table.

At which step does the condition GPIO.input(18) == LOW become true for the first time?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Condition' column in the execution_table to find when it first becomes True.

If the button is never pressed, what will the output be?

A No output, program waits

B Button pressed! printed repeatedly

C Error message

D Program crashes

💡 Hint

Refer to the execution_table rows where GPIO Pin 18 state is HIGH and condition is False.

Concept Snapshot

Button press detection on Raspberry Pi:
- Setup GPIO pin as input with pull-up resistor
- Loop: read pin state
- If LOW (button pressed), run action
- Use time.sleep to debounce
- Program runs until stopped manually

Full Transcript

This program sets up a Raspberry Pi GPIO pin 18 as an input with a pull-up resistor. It then enters an infinite loop checking the pin state. When the button connected to pin 18 is pressed, the pin reads LOW, triggering the program to print 'Button pressed!'. The program waits briefly to avoid multiple prints from one press. It continues checking until the user stops it. The execution table shows the pin state and actions step by step, helping beginners see how the program reacts to button presses.