Pull-up vs Pull-down Resistors

OBJLearning Objectives

• Explain how pull-up and pull-down resistors stop floating inputs.
• Wire a button using an external pull-down resistor.
• Wire a button using the ESP32 internal pull-up resistor.
• Compare INPUT and INPUT_PULLUP in real button circuits.
• Explain why pressed can mean LOW in pull-up wiring.

PARTSThings You'll Need

• ESP32ESP32 DevKit boardThe GPIO input platform
• BTNPush buttonMomentary tactile switch
• 10K10 kOhm resistorExternal pull-down resistor for the first circuit
• BOARDBreadboardKeeps the button and resistor stable
• WIREJumper wiresFor GPIO, 3.3 V, and GND connections
• USBUSB data cableUpload code and read Serial Monitor

IDEAThe Default State Problem

ENGEngineering Explanation

REALReal-Life Analogy

RECAPFloating Pin Recap

INTInternal Pull-Up Resistor

EXTExternal Pull-Up and Pull-Down Resistors

COMPAREINPUT vs INPUT_PULLUP

WIREBreadboard Wiring

Follow these steps in order. Unplug USB before you change any wires.

1. 1

   External pull-down test: place the push button across the breadboard center gap.

2. 2

   Connect one side of the button to ESP32 3.3 V.

3. 3

   Connect the other side of the button to ESP32 GPIO27.

4. 4

   Place a 10 kOhm resistor from the GPIO27 side of the button to GND.

5. 5

   Upload the INPUT example and open Serial Monitor at 115200 baud.

6. 6

   Internal pull-up test: remove the external resistor and 3.3 V button wire.

7. 7

   Connect one side of the button to GPIO27 and the other side to GND.

8. 8

   Upload the INPUT_PULLUP example and compare the released and pressed readings.

GPIOGPIO Table

⌨️Code

Copy this into Arduino IDE, then click Upload.

// Mission 04 - Example A: external pull-down resistor
// Wiring: GPIO27 -> button -> 3.3V, and GPIO27 -> 10k resistor -> GND

const int BUTTON_PIN = 27;

void setup() {
  Serial.begin(115200);
  pinMode(BUTTON_PIN, INPUT);
  Serial.println("Example A: INPUT with external pull-down");
}

void loop() {
  int reading = digitalRead(BUTTON_PIN);

  if (reading == HIGH) {
    Serial.println("Button is PRESSED");
  } else {
    Serial.println("Button is RELEASED");
  }

  delay(250);
}

/*
Mission 04 - Example B: internal pull-up resistor
Rewire first: GPIO27 -> button -> GND. No external resistor needed.
Then replace setup() and loop() with the code below.

const int BUTTON_PIN = 27;

void setup() {
  Serial.begin(115200);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  Serial.println("Example B: INPUT_PULLUP internal pull-up");
}

void loop() {
  int reading = digitalRead(BUTTON_PIN);

  if (reading == LOW) {
    Serial.println("Button is PRESSED");
  } else {
    Serial.println("Button is RELEASED");
  }

  delay(250);
}
*/

CODELine-by-Line Code Explanation

• const int BUTTON_PIN = 27; gives the button input a clear name.
• pinMode(BUTTON_PIN, INPUT); is used only when the external pull-down resistor already gives the pin a default LOW state.
• In Example A, digitalRead() returns HIGH when the button connects GPIO27 to 3.3 V.
• pinMode(BUTTON_PIN, INPUT_PULLUP); turns on the ESP32 internal pull-up resistor.
• In Example B, the released button reads HIGH because the internal pull-up holds the pin near 3.3 V.
• In Example B, the pressed button reads LOW because the button connects GPIO27 directly to GND.
• The delay(250); line slows the output so the Serial Monitor is readable. It is not button debouncing yet.

EXPExperiment: Leave the Input Floating

!Common Problems

Most ESP32 problems are wiring, power, library, or timing issues. Check these first.

• The button always reads pressed

  Likely cause: The GPIO may be permanently connected to 3.3 V or GND because the button legs are in the wrong breadboard rows.

  Fix: Rotate the button 90 degrees or move it across the breadboard center gap, then test again.

• The reading is still random

  Likely cause: The pull-down resistor is missing, in the wrong row, or not connected to GND.

  Fix: Trace GPIO27 to the resistor and make sure the other resistor leg reaches a real GND pin.

• INPUT_PULLUP prints pressed when I release the button

  Likely cause: The code may be treating HIGH as pressed even though pull-up logic is inverted.

  Fix: For INPUT_PULLUP, use digitalRead(pin) == LOW to detect a pressed button.

• The ESP32 resets when I press the button

  Likely cause: The button may be shorting 3.3 V directly to GND.

  Fix: Unplug USB and rebuild the circuit slowly. The resistor should define the GPIO, not sit as a direct short bypass.

• Serial Monitor shows old text from Example A

  Likely cause: Example B is inside a comment until you copy it into the active sketch area.

  Fix: Replace the active setup() and loop() with Example B, then upload again.

• The button works but prints many lines per press

  Likely cause: The loop prints every 250 ms while the button is held.

  Fix: That is expected in this mission. Mission 05 will teach detecting one clean press and debouncing.

?FAQs

• What is a pull-up resistor?

  A pull-up resistor connects an input pin weakly to 3.3 V so the pin reads HIGH when nothing else is pressing it LOW.

• What is a pull-down resistor?

  A pull-down resistor connects an input pin weakly to GND so the pin reads LOW when nothing else is pressing it HIGH.

• Why do buttons need pull-up or pull-down resistors?

  A button is open most of the time. Without a default path to 3.3 V or GND, the input can float and produce random readings.

• Why does INPUT_PULLUP read LOW when the button is pressed?

  With INPUT_PULLUP the pin normally rests HIGH through the internal resistor. Pressing the button connects the pin to GND, so the pressed state becomes LOW.

• Is LOW a problem in INPUT_PULLUP button logic?

  No. LOW simply means pressed in that wiring style. Your code can name it clearly, for example bool buttonPressed = digitalRead(pin) == LOW.

• When should I use INPUT?

  Use INPUT when an external circuit already drives the pin HIGH or LOW, such as a sensor output, or when you add your own external pull-up or pull-down resistor.

• When should I use INPUT_PULLUP?

  Use INPUT_PULLUP for simple buttons or switches where one side of the button connects to GND and you want the ESP32 to provide the default HIGH state.

• Does ESP32 have INPUT_PULLDOWN?

  Many ESP32 GPIOs support internal pull-down too, but beginner button examples usually use INPUT_PULLUP because it is simple, common, and works well with a button to GND.

• What resistor value should I use for an external pull-up or pull-down?

  10 kOhm is a common beginner-friendly value. It is strong enough to define the input but weak enough that only a tiny current flows when the button is pressed.

• Can I connect a button directly without a resistor?

  Only if you use an internal pull-up or pull-down mode. A plain INPUT button without a resistor will usually float when the button is open.

• Can a pull resistor damage the ESP32?

  A normal pull resistor such as 10 kOhm connected between a GPIO input and 3.3 V or GND is safe. The dangerous mistake is shorting 3.3 V directly to GND.

• Can I use 5 V with an ESP32 button input?

  No. ESP32 GPIO pins are 3.3 V logic and are not 5 V tolerant. Use 3.3 V for button circuits connected to GPIO.

• Why do many ESP32 tutorials use INPUT_PULLUP?

  It reduces parts, avoids an external resistor, and gives a stable default state with only a button wired between the GPIO and GND.

• Which is better, pull-up or pull-down?

  Neither is universally better. Pull-up is very common for buttons because internal pull-ups are available and wiring to GND is convenient. Pull-down is easier for beginners to read because pressed often equals HIGH.

• What happens if I remove the pull-up resistor?

  The input loses its default HIGH state. When the button is open, the pin can float and random HIGH/LOW readings can return.

• Why not use a very small resistor like 100 ohms?

  A small resistor wastes more current when the button is pressed. Pull resistors should be weak defaults, not strong loads.

• Do pull-up resistors debounce a button?

  No. Pull resistors stop floating. Debouncing handles the tiny rapid on/off transitions caused by the mechanical contacts inside a real button.

• Can one resistor serve multiple buttons?

  Usually each independent button input should have its own pull-up or pull-down path so every GPIO has a clear default state.

• Does a PIR sensor need INPUT_PULLUP?

  Usually no. Most PIR modules actively drive their output HIGH or LOW, so the ESP32 can read them as INPUT. Check the module documentation if the output is open-drain.

• What should I learn after pull-up and pull-down resistors?

  Learn button debouncing. Once the input has a stable default state, debouncing helps your code ignore the brief mechanical chatter of a real button press.