Push button on pin 12 lights the LED on pin 9, on an Arduino Uno
A push button on pin 12 with a 10k pull-up: hold it and the LED on pin 9 lights, let go and it goes out. The whole build, an Arduino Uno and 4 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.
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Beginner About 15 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $30. On Mokxi they're free.
What you'll learn
- Pull-up resistors
- Reading a pin with digitalRead
- A pressed button reads LOW
- Using a condition to set an output
- Difficulty: Beginner
- Time: About 15 minutes
- Board: Arduino Uno R3
- Parts: 6
How it works
A 10k resistor connects pin 12 to 5 V, so the pin normally reads HIGH. The button connects pin 12 to GND. Pressing it pulls the pin down, and the pin reads LOW.
The sketch checks pin 12 over and over. While it reads LOW, it sets pin 9 HIGH, which lights the LED through a 220 ohm resistor. Let go, and the LED goes out.
Parts list
6 parts, plus the jumper wires. They retail for about $30; on Mokxi they're free.
- 1 × Arduino Uno R3In Starter Kit, Claw Machine Kit, Vending Machine Kit.
- 1 × Half breadboard
- 1 × PushbuttonIn Starter Kit, Christmas Lights Kit, Logic Lab Kit.
- 1 × Resistor, 10k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × LED, redIn Starter Kit, Halloween Prop Kit, Logic Lab Kit.
Kits that can build it
These 12 starter kits have every part in the circuit. Each kit's page lists what else it can build.
- ELEGOO UNO R3 Super Starter Kit
- ELEGOO UNO R3 Most Complete Starter Kit
- Arduino Starter Kit
- Freenove Ultimate Starter Kit
- ELEGOO UNO R3 Basic Starter Kit
- SunFounder Starter Kit for Arduino Uno R3
- SunFounder Inventor Lab Kit
- REXQualis UNO R3 Super Starter Kit
- LAFVIN Super Starter Kit
- keyestudio Super Starter Kit
- Vilros Arduino Uno 3 Ultimate Starter Kit
- Adafruit MetroX Classic 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 |
|---|---|
| Pushbutton pin 1a | Arduino Uno R3 pin GND |
| LED, red pin C | Arduino Uno R3 pin GND |
| Pushbutton pin 2a | Arduino Uno R3 pin 12 |
| Resistor, 10k Ω pin 1 | Arduino Uno R3 pin 12 |
| Resistor, 220 Ω pin 1 | Arduino Uno R3 pin 9 |
| Resistor, 10k Ω pin 2 | Arduino Uno R3 pin 5V |
| LED, red 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.
// A push button controls an LED: hold it to light it.
// pin 9 -> 220 ohm -> LED -> GND
// 10k from 5V -> pin 12; the push button from pin 12 -> GND
//
// The 10k pull-up holds pin 12 HIGH while the button is up; pressing it
// joins the pin to ground and it reads LOW.
const int BUTTON = 12;
const int LED_PIN = 9;
void setup() {
pinMode(BUTTON, INPUT);
pinMode(LED_PIN, OUTPUT);
Serial.begin(9600);
Serial.println("Mokxi Uno: hold the button on pin 12");
}
void loop() {
digitalWrite(LED_PIN, digitalRead(BUTTON) == LOW ? HIGH : LOW);
}
Code walkthrough
The key functions of the sketch, and what each one does.
setup sketch.ino
Sets BUTTON (pin 12) as an input and LED_PIN (pin 9) as an output. It also starts the serial port at 9600 baud and prints a hello message.
void setup() {
pinMode(BUTTON, INPUT);
pinMode(LED_PIN, OUTPUT);
Serial.begin(9600);
Serial.println("Mokxi Uno: hold the button on pin 12");
}loop sketch.ino
Each pass reads the button with digitalRead. If it is LOW (pressed), digitalWrite sends HIGH to the LED pin. Otherwise it sends LOW. There is no delay, so the LED follows the button at once.
void loop() {
digitalWrite(LED_PIN, digitalRead(BUTTON) == LOW ? HIGH : LOW);
}Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The LED is always on, or always off. This project
Check the 10k resistor. One end goes to 5V and the other to pin 12, where the button also connects. - The LED lights when the button is released. This project
The pin reads LOW when pressed. Keep the test as== LOW. - The input reads random HIGH and LOW when the button is not pressed. Pushbutton
A floating pin picks up noise. Add a 10 k pull-down or pull-up resistor, or use INPUT_PULLUP and wire the button between the pin and GND. Then pressed reads LOW. - The button seems always pressed, or never changes. Pushbutton
A 4-leg tactile switch has legs joined in pairs. Place it across the breadboard center gap so the pairs sit on different rows. Wired along one side, the legs are already connected. - 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.
Change it and keep it
Open it in the editor, change the circuit or the code, and keep your version in a free account.