Introduction to Python and Microcontrollers

Button and LED On and Off Switch

Toggle an LED on and off with a push button — press once to turn ON, press again to turn OFF. Unlike Project 2.1, the LED state persists after releasing the button.

Uses the same components and circuit as Project 3

New Concepts

• debouncing
• functions

Concept: Debouncing

When a push button is pressed or released, the mechanical contacts briefly bounce — rapidly switching between connected and disconnected many times before settling. This is invisible to humans but causes a microcontroller to register multiple presses in one action.

Solution: After detecting a press, wait 20ms and check again. If still pressed, treat it as a genuine button press. This filters out the noise from bouncing.

Concept: Functions

A function encapsulates code so it can be used multiple times using just the function name. It also makes code easier to read and understand.

Defining a function

def reverseGPIO():
    if led.value():
        led.value(0)
    else:
        led.value(1)

Calling a function

reverseGPIO()

Component List & Circuit Diagram

Circuit and components are identical to Project 1.3

Code

File: 01_first_examples/code/ButtonAndLed_OnOffSwitch.py

import time
from machine import Pin

led = Pin(2, Pin.OUT)
button = Pin(13, Pin.IN, Pin.PULL_UP)

def reverseGPIO():
    if led.value():
        led.value(0)
    else:
        led.value(1)

while True:
    if not button.value():          # button pressed?
        time.sleep_ms(20)           # wait 20ms (debounce)
        if not button.value():      # still pressed? (genuine press)
            reverseGPIO()           # toggle LED
            while not button.value():   # wait for release
                time.sleep_ms(20)

How to Run

Online

1. Open Thonny → 01_first_examples/code/.
2. Double-click ButtonAndLed_OnOffSwitch.py.
3. Click Run current script.
4. Press the button — LED toggles ON/OFF with each press.

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Code Explanation

Toggle function

def reverseGPIO():
    if led.value():
        led.value(0)
    else:
        led.value(1)

Reads the current LED state and sets it to the opposite — ON becomes OFF, OFF becomes ON.

Debounce logic

if not button.value():        # 1. initial press detected
    time.sleep_ms(20)         # 2. wait 20ms
    if not button.value():    # 3. confirm still pressed
        reverseGPIO()         # 4. toggle
        while not button.value():  # 5. wait until released
            time.sleep_ms(20)

Step 5 prevents the LED from toggling again while the button is held down — the action fires once per press, not continuously.

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Key Concepts

• Debouncing: ignoring the brief noise when a button transitions state
• Toggle pattern: reading current state and setting its inverse
• Defining functions with def in MicroPython
• Waiting for release: important to avoid registering a single press as multiple events

Further Exploration

• Rename the reverseGPIO button to something more descriptive. For example toggleLED.

Back to Module

Adapted from Python_Tutorial.pdf Project 2.2

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