Pull-up and pull-down resistors in Raspberry Pi

Introduction

Pull-up and pull-down resistors help your Raspberry Pi read buttons or switches correctly by setting a clear default voltage.

When connecting a button to a Raspberry Pi GPIO pin to detect presses.

When you want to avoid random signals caused by floating inputs.

When you need to ensure a GPIO pin reads a known value when a switch is open.

When building simple circuits that interact with the Raspberry Pi's input pins.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(pin_number, GPIO.IN, pull_up_down=GPIO.PUD_UP)  # For pull-up resistor
GPIO.setup(pin_number, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)  # For pull-down resistor

Use GPIO.PUD_UP to activate the internal pull-up resistor.

Use GPIO.PUD_DOWN to activate the internal pull-down resistor.

Examples

This sets GPIO pin 17 as input with a pull-up resistor, so the pin reads HIGH when the button is not pressed.

Raspberry Pi

GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP)

This sets GPIO pin 27 as input with a pull-down resistor, so the pin reads LOW when the button is not pressed.

Raspberry Pi

GPIO.setup(27, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

Sample Program

This program uses a pull-up resistor on pin 17. When the button is pressed, the pin reads LOW, and the program prints "Button pressed!". When not pressed, it prints "Button not pressed".

Raspberry Pi

import RPi.GPIO as GPIO
import time

BUTTON_PIN = 17

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

print("Press the button...")

try:
    while True:
        if GPIO.input(BUTTON_PIN) == GPIO.LOW:
            print("Button pressed!")
        else:
            print("Button not pressed")
        time.sleep(0.5)
except KeyboardInterrupt:
    print("Exiting program")
finally:
    GPIO.cleanup()

Important Notes

Pull-up means the pin is connected to HIGH voltage by default.

Pull-down means the pin is connected to LOW voltage by default.

Using these resistors prevents the pin from 'floating' and reading random values.

Summary

Pull-up and pull-down resistors set a default voltage level on input pins.

Use GPIO.PUD_UP or GPIO.PUD_DOWN in Raspberry Pi GPIO setup.

This helps your program read buttons and switches reliably.