Raspberry Pi Debouncing a Button

What Is Switch Bounce?

Inside a mechanical push button, pressing it snaps two metal contacts together. Those contacts don't touch cleanly on the first try - they physically bounce apart and together several times in rapid succession before finally settling into a stable closed connection. This happens in a tiny window, typically somewhere between one and a few tens of milliseconds, but a Raspberry Pi reads GPIO state far faster than that, so it can easily register that single bounce sequence as several separate presses and releases rather than one clean press.

How Bounce Breaks Event-Driven Code

The problem becomes obvious as soon as you attach a counter to `when_pressed`. A single, deliberate press of the button can fire the callback two, three, or more times within a few milliseconds, because gpiozero is faithfully reporting every rapid HIGH/LOW transition the bouncing contacts produce - it has no way to know the difference between rapid switch chatter and someone pressing a button several times deliberately.

from gpiozero import Button
from signal import pause

button = Button(2)  # no debounce configured
count = 0

def on_press():
    global count
    count += 1
    print(count)  # a single physical press can print 2, 3, or more times here

button.when_pressed = on_press
pause()

Fixing It Two Ways

There are two reliable ways to handle this. The manual approach records a timestamp the first time the button fires, and ignores any further presses that happen within a short window afterward - effectively treating rapid re-triggers as noise from the same physical press. The built-in approach lets gpiozero do this for you by passing a `bounce_time` value (in seconds) when creating the `Button`, which tells it to ignore any state changes that happen within that window after a detected edge.

from gpiozero import Button
from time import monotonic
from signal import pause

button = Button(2)
count = 0
last_press = 0
MIN_INTERVAL = 0.2  # seconds - ignore repeats faster than this

def on_press():
    global count, last_press
    now = monotonic()
    if now - last_press > MIN_INTERVAL:
        count += 1
        last_press = now
        print(f"Press #{count}")

button.when_pressed = on_press
pause()

from gpiozero import Button
from signal import pause

button = Button(2, bounce_time=0.1)  # ignore glitches within 0.1s of a detected edge
presses = 0

def count_press():
    global presses
    presses += 1
    print(f"Press #{presses}")

button.when_pressed = count_press
pause()