What you'll learn

Reading a button with INPUT_PULLUP
Why a pressed button reads LOW
Driving two outputs from one input
The Pico's built-in LED

Difficulty: Beginner
Time: About 10 minutes
Board: Raspberry Pi Pico
Parts: 5

How it works

The arcade button joins GP15 to ground when pressed. The sketch turns on the pin's internal pull-up, so GP15 reads HIGH when the button is up and LOW when it is pressed. That means no outside resistor is needed.

The sketch checks the button over and over. While it is pressed, it turns on both the Pico's own LED and the green LED on GP14, which sits behind a 220 Ω resistor. Let go, and both go off. The Pico's LED is under the board, so the second LED gives you something to see on the breadboard.

Parts list

5 parts, plus the jumper wires. They retail for about $10.65; on Mokxi they're free.

1 × Raspberry Pi PicoIn Fight Stick Kit.
1 × Full-size breadboard
1 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × LED, greenIn Starter Kit, Halloween Prop Kit, Logic Lab Kit.
1 × Arcade buttonIn Claw Machine Kit, Fight Stick Kit, Pinball Table Kit.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pin	Connects to
LED, green pin C	Raspberry Pi Pico pin GND
Arcade button pin 1	Raspberry Pi Pico pin GND
Resistor, 220 Ω pin 1	Raspberry Pi Pico pin GP14
Arcade button pin 2	Raspberry Pi Pico pin GP15
LED, green pin A	Resistor, 220 Ω pin 2

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Raspberry Pi Pico

// Button: read a pin, drive a pin.
//
// Wire a pushbutton from GP15 to any GND pin. INPUT_PULLUP switches on the
// pad's 55 k pull-up, so the pin reads HIGH when the button is up and LOW when
// it is pressed: no external resistor, and no floating input.
//
// GP25, the on-board LED, is inside the module, so the sketch lights GP14 as
// well for an LED on the breadboard.

const int BUTTON = 15;
const int LED = 14;

void setup() {
  pinMode(BUTTON, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(LED, OUTPUT);
}

void loop() {
  int pressed = digitalRead(BUTTON) == LOW ? HIGH : LOW;
  digitalWrite(LED_BUILTIN, pressed);
  digitalWrite(LED, pressed);
}

Code walkthrough

The key functions of the sketch, and what each one does.

`setup` sketch.ino

Sets BUTTON (GP15) as an input with the internal pull-up. It makes both LED_BUILTIN and LED (GP14) outputs.

void setup() {
  pinMode(BUTTON, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(LED, OUTPUT);
}

`loop` sketch.ino

Reads the button. LOW means pressed, so pressed becomes HIGH. It writes that same value to both LEDs. There is no wait in the loop, so the LEDs follow the button right away.

void loop() {
  int pressed = digitalRead(BUTTON) == LOW ? HIGH : LOW;
  digitalWrite(LED_BUILTIN, pressed);
  digitalWrite(LED, pressed);
}

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

The LED is always on, or never changes This project
Check the button joins GP15 to ground, not to 3.3 V.
Only the on-board LED lights This project
Check the green LED's direction: anode (A) toward the resistor, cathode (C) to ground.
The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
The LED does not light, or it lights once and then dies. LED
LEDs have polarity. The longer leg (anode) goes toward the positive side. Always add a series resistor, about 220 ohms to 1 k ohm on a 5 V pin.
A blue or white LED stays dark on 3.3 V. LED
Blue and white LEDs drop about 3 V, so 3.3 V leaves almost nothing for the resistor. Use a 5 V supply, or a smaller resistor and a red or green LED.
The button reads pressed all the time, or never changes, when wired to a pin. Arcade button
Use the COM and NO terminals of the microswitch. Wire COM to ground, NO to the pin, and use INPUT_PULLUP. NC is the opposite and reads pressed when released.
One press counts as several presses in your code. Arcade button
Microswitch contacts bounce for a few milliseconds. Ignore changes for 10 to 20 ms after the first one, or react only to the edge and wait before checking again.