INPUT kontra INPUT_PULLUP

So far your pins pushed signals out. Now you'll make a pin listen instead, so your board can sense a button and react to it.

To read a button, you first tell a pin it should receive a signal. You do that by setting its mode to INPUT inside setup().

An INPUT pin with nothing pulling it is called floating. It picks up stray noise and flickers between HIGH and LOW all on its own.

A floating pin gives you random, false button readings. Your code thinks the button is pressed when nobody touched it. ⚡

A pull resistor gently ties the pin to a known voltage. That gives it a steady default reading until the button changes it.

A pull-up holds the pin HIGH by default; a pull-down holds it LOW. The Arduino has handy pull-ups built right into the chip.

Skip the extra resistor: set the mode to INPUT_PULLUP and the board enables its internal pull-up for you automatically.

With INPUT_PULLUP the pin reads HIGH when the button is released, and LOW when it is pressed. The logic is inverted on purpose.

Because the internal pull-up holds the pin HIGH, you wire the button between the pin and GND. Pressing it pulls the pin down to LOW.

INPUT_PULLUP means one less resistor on your breadboard and no floating pin. For most beginner buttons it is simply the easiest choice.

Reach for plain INPUT only when your circuit already supplies its own pull resistor. Otherwise INPUT_PULLUP keeps things safe and simple.

Think about how INPUT_PULLUP behaves on an idle button.

You learned that an INPUT pin can float and lie, and that INPUT_PULLUP fixes it: HIGH when released, LOW when pressed. Next you'll actually read that value. 👍