LED toggle with button in Raspberry Pi - Step-by-Step Execution

Concept Flow - LED toggle with button

Start

↓

Wait for button press

↓

Button pressed?

No→Wait

Yes↓

Toggle LED state

↓

Update LED output

↓

Back to wait

The program waits for a button press, then toggles the LED state and updates the LED output, repeating this cycle.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.setup(23, GPIO.IN, pull_up_down=GPIO.PUD_UP)

led_state = False
while True:
    button = GPIO.input(23)
    if button == GPIO.LOW:
        led_state = not led_state
        GPIO.output(18, led_state)
        time.sleep(0.3)

This code toggles an LED on pin 18 each time the button on pin 23 is pressed.

Execution Table

Step	Button Input	Condition (button pressed?)	LED State Before	LED State After	Action	Delay
1	HIGH	No	False	False	Wait	No
2	HIGH	No	False	False	Wait	No
3	LOW	Yes	False	True	Toggle LED ON	Yes (0.3s)
4	LOW	Yes	True	False	Toggle LED OFF	Yes (0.3s)
5	HIGH	No	False	False	Wait	No
6	LOW	Yes	False	True	Toggle LED ON	Yes (0.3s)
Exit	N/A	Loop continues	N/A	N/A	Program runs indefinitely	N/A

💡 The program runs forever, toggling LED each time button is pressed (button LOW).

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	Final
led_state	False	False	True	False	True	True
button	N/A	HIGH	LOW	LOW	LOW	LOW

Key Moments - 3 Insights

Why does the LED toggle only when the button input is LOW?

Because the button is set up with a pull-up resistor, pressing it connects the pin to ground, making input LOW. The execution_table rows 3,4,6 show toggling happens only when button input is LOW.

Why is there a delay after toggling the LED?

The delay (0.3s) prevents the program from toggling the LED multiple times during one press due to button bouncing, as seen in execution_table rows 3 and 4.

What happens if the button is held down?

The LED toggles once per loop iteration with button LOW, but the delay slows toggling. The program does not detect button release explicitly, so holding the button toggles LED repeatedly but slowly.

Visual Quiz - 3 Questions

Look at the execution_table, what is the LED state after step 3?

A No change

B False (LED OFF)

C True (LED ON)

D Error

💡 Hint

Check the 'LED State After' column at step 3 in execution_table.

At which step does the program detect the button is pressed for the first time?

A Step 3

B Step 1

C Step 5

D Step 6

💡 Hint

Look at the 'Condition (button pressed?)' column for the first 'Yes' in execution_table.

If the delay after toggling was removed, what would happen?

A LED would stay ON permanently

B LED would toggle very fast and flicker

C Button presses would be ignored

D Program would crash

💡 Hint

Consider the purpose of the delay in execution_table rows 3 and 4.

Concept Snapshot

LED toggle with button:
- Setup button input with pull-up resistor
- Read button state in loop
- If button pressed (input LOW), invert LED state
- Update LED output
- Add delay to avoid multiple toggles per press

Full Transcript

This program runs on a Raspberry Pi to toggle an LED on and off using a button. The button is connected with a pull-up resistor, so when pressed, the input reads LOW. The program continuously checks the button state. When it detects a press (LOW), it changes the LED state from ON to OFF or OFF to ON. It then updates the LED output pin accordingly. A short delay after toggling prevents the LED from flickering due to button bounce. The program loops forever, waiting for button presses to toggle the LED.