Pull-up vs Pull-down Resistors

TimeEstimated Time

LevelDifficulty

BeforePrerequisites

• Understanding Digital Inputs and Floating Pins You should understand why a GPIO input must not be left undecided.

Skills You'll Learn

• Explain why floating pins are unreliable
• Wire a button with an external pull-down resistor
• Use the ESP32 internal pull-up resistor with INPUT_PULLUP
• Predict why a pull-up button reads LOW when pressed
• Choose between INPUT and INPUT_PULLUP for beginner button circuits

PartsComponents Required

• 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

Engineering Explanation

Real WorldReal-World Analogy

RecapFloating Pin Recap

InternalESP32 Internal Pull-up and Pull-down Resistors

External Pull-Up and Pull-Down Resistors

ValueWhat Value Pull-up Resistor Should I Use with ESP32?

ComparePull-up vs Pull-down Comparison

Wiring Diagram

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.

GPIO Table

⌨️Arduino 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 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.

Experiment: Leave the Input Floating

MistakesCommon Mistakes

• The button always reads pressed

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

• The reading is still random

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

• INPUT_PULLUP prints pressed when I release the button

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

• The ESP32 resets when I press the button

  The button may be shorting 3.3 V directly to GND.

• Serial Monitor shows old text from Example A

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

• The button works but prints many lines per press

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

!Troubleshooting

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.

TipEngineer Tip

Remember This Forever

?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?

  Arduino ESP32 supports INPUT_PULLDOWN on GPIOs that provide an internal pull-down resistor. Pin capabilities vary by ESP32 family and board, and classic ESP32 GPIO34-GPIO39 do not have integrated pull-up or pull-down resistors.

• 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.