Pico Button Controls an LED, Step by Step

A button that turns a light on and off is the "hello world" of input. In this post you will make one work on a Raspberry Pi Pico, in your browser, with no resistor for the button. Along the way you will learn what INPUT_PULLUP does, why a button needs "debouncing", and the one thing that trips up almost every Pico beginner: the difference between a GPIO number and a pin number.

GPIO numbers and pin numbers

The Pico has 40 pins in two rows. Each pin has a position on the board, from 1 to 40. Some of those pins connect to the chip's general-purpose input and output lines, which are named GP0, GP1, GP2 and so on. Those are the GPIO numbers.

In Arduino code you always use the GPIO number. pinMode(15, INPUT_PULLUP) means GP15, which is physical pin 20 on the board. It does not mean pin 15. On Mokxi's Raspberry Pi Pico the left column runs from pin 1 (GP0) down to pin 20 (GP15), and the board's help page lists every pin in order. Our two data pins today sit next to each other at the bottom of that column: GP14 is pin 19 and GP15 is pin 20.

One more detail. The Pico has seven ground pins. The real board prints GND on all of them, and in Mokxi they carry letters (GND, GNDb, GNDc and so on) so each name is unique. Any of them works as ground.

What you need

A Raspberry Pi Pico
A pushbutton
One LED and one 220 ohm resistor
Jumper wires

Wire it up

From	To	What it does
Button, one side	GP15 (pin 20)	The input.
Button, other side	Any GND pin	Pulls the input low when pressed.
GP14 (pin 19)	220 ohm resistor	Limits the LED current.
Resistor, other end	LED anode (long leg)	Positive side of the LED.
LED cathode (short leg)	Any GND pin	Completes the loop.

Notice there is no resistor on the button. That is on purpose, and the next section explains why.

What INPUT_PULLUP does

A pin that is set as an input and connected to nothing "floats". It can read high or low at random. To fix that you want a resistor that gently pulls the pin to a known level when the button is not pressed. A pull-up resistor pulls the pin up to 3.3 volts. A pull-down pulls it to ground.

The Pico has pull-up resistors built into its pins. pinMode(15, INPUT_PULLUP) switches the one on GP15 on. Now the pin reads high when the button is up. When you press the button it connects the pin to ground, and the pin reads low. So with a pull-up, pressed means LOW. That feels backwards at first, and every sketch below accounts for it. Our pull-up resistor guide shows the same idea running in a circuit.

The complete sketch

const int BUTTON_PIN = 15;  // GP15, pin 20 on the board
const int LED_PIN = 14;     // GP14, pin 19 on the board
const unsigned long DEBOUNCE_MS = 30;

bool ledOn = false;
int lastReading = HIGH;     // HIGH means "not pressed" with INPUT_PULLUP
int stableState = HIGH;
unsigned long lastChange = 0;

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
  Serial.begin(115200);
  Serial.println("Press the button on GP15");
}

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

  if (reading != lastReading) {
    lastChange = millis();      // the pin moved: start the timer again
    lastReading = reading;
  }

  if (millis() - lastChange >= DEBOUNCE_MS && reading != stableState) {
    stableState = reading;
    if (stableState == LOW) {   // LOW is a real press
      ledOn = !ledOn;
      digitalWrite(LED_PIN, ledOn ? HIGH : LOW);
      Serial.println(ledOn ? "LED on" : "LED off");
    }
  }
}

I compiled this sketch for the Pico with Mokxi's own build path and ran it in the simulator. Pressing the button three times printed LED on, LED off, LED on, and the LED followed.

Line by line

BUTTON_PIN = 15 and LED_PIN = 14 are GPIO numbers. The comments give the physical pins as a reminder.
DEBOUNCE_MS = 30 is how long a reading must stay the same before we believe it.
lastReading is what the pin said the last time through loop(). stableState is the last value we trusted. Both start HIGH, because the button starts up.
pinMode(BUTTON_PIN, INPUT_PULLUP) makes the button pin an input with its pull-up on.
Serial.begin(115200) opens the serial monitor.
In loop(), digitalRead takes a snapshot of the button.
if (reading != lastReading) notices that the pin moved. It starts the timer again by saving millis(), the number of milliseconds since the board started.
The next if asks two questions at once: has the reading been steady for at least 30 milliseconds, and is it different from the state we last trusted? Only then do we act.
if (stableState == LOW) means the button was really pressed, not released. We ignore the release.
ledOn = !ledOn; flips true to false and back. The ! means "not".
digitalWrite(LED_PIN, ledOn ? HIGH : LOW) sets the light. The ? : is a short if-else.

Why debounce

A button is a small piece of metal that lands on a contact. It does not land and stay still. It chatters for a few milliseconds, closing and opening several times before it settles. Your Pico is fast enough to see every one of those flickers as a separate press.

Mokxi's pushbutton models this on purpose. It has a bounce property with three settings: none, typical (the default) and worst. To see why it matters, I ran a naive version of this sketch that toggles on every falling edge, with no debounce. With the default setting, three quick presses printed 16 edges. On worst, they printed 50. The debounced sketch above toggled exactly three times on both settings.

If you want to try it yourself, delete the timing check and watch the monitor. Our debounce guide walks through the same effect.

Try next

Make the LED light only while you hold the button. You need no debounce for that, because the LED follows the pin directly.
Change DEBOUNCE_MS to 1 and test with the bounce property on worst. See where it starts to fail.
Add a second LED on GP13 and make the two take turns on each press.
Add a press counter and print it on the serial monitor.
Open the built-in Pico button project, which puts a button on GP15 with the internal pull-up and lights the board's own LED on GP25, and compare it with your sketch.

Try it on Mokxi

Open the Raspberry Pi Pico simulator to build this circuit with the Pico and parts above. The Pico board page lists every pin and what the model includes.

Ready to build it yourself? Create your free Mokxi account and open the editor in your browser. No installs, no parts to buy.