Pull-Up and Pull-Down Resistors

An unconnected (floating) input picks up noise and reads randomly. A pull-up (to $V_{CC}$) or pull-down (to GND) resistor gives it a default state.

Typical button circuit: input with a 10 kΩ pull-up and a button to GND → reads 1 when released, 0 when pressed (active-low). Most microcontrollers have internal pull-ups (pinMode(pin, INPUT_PULLUP)), typically 20–50 kΩ.

Open-drain buses such as I2C require pull-ups (typically 2.2–10 kΩ) because devices can only pull the line low.