Two LEDs take turns on pins 4 and 5, on an Arduino Uno
Two LEDs on pins 4 and 5 blink in turn, one on while the other is off, switched by one function that sets both at once. 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 10 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
- Controlling two outputs at once
- Writing your own function with parameters
- Sharing a ground between parts
- Making code say what it means
- Difficulty: Beginner
- Time: About 10 minutes
- Board: Arduino Uno R3
- Parts: 6
How it works
Two LEDs each have their own 220 Ω resistor. The red one is on pin 4 and the green one is on pin 5, and both cathodes share GND. Each pin controls its own LED.
The sketch sets the pins in two states. First pin 4 is HIGH and pin 5 is LOW, so the red LED is on and the green is off. A second later it flips them. That repeats forever, so the two LEDs take turns.
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
- 2 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × LED, greenIn Starter Kit, Halloween Prop Kit, Logic Lab 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 |
|---|---|
| LED, green pin C | Arduino Uno R3 pin GND |
| LED, red pin C | Arduino Uno R3 pin GND |
| Resistor, 220 Ω (1) pin 1 | Arduino Uno R3 pin 5 |
| Resistor, 220 Ω (2) pin 1 | Arduino Uno R3 pin 4 |
| LED, green pin A | Resistor, 220 Ω (1) pin 2 |
| LED, red pin A | Resistor, 220 Ω (2) pin 2 |
The code
The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.
// Two LEDs take turns: pins 4 and 5.
//
// pin 4 -> 220 ohm -> LED 1 -> GND
// pin 5 -> 220 ohm -> LED 2 -> GND
//
// setLeds() takes the state of both LEDs as its two parameters, so the loop
// says what it means: first LED 1 on and LED 2 off, then the other way round.
const int LED1 = 4;
const int LED2 = 5;
void setLeds(int led1, int led2) {
digitalWrite(LED1, led1);
digitalWrite(LED2, led2);
delay(1000);
}
void setup() {
pinMode(LED1, OUTPUT);
pinMode(LED2, OUTPUT);
Serial.begin(9600);
Serial.println("Mokxi Uno: two LEDs taking turns on pins 4 and 5");
}
void loop() {
setLeds(HIGH, LOW);
setLeds(LOW, HIGH);
}
Code walkthrough
The key functions of the sketch, and what each one does.
setLeds sketch.ino
Takes the state for each LED as two parameters. It writes led1 to LED1 and led2 to LED2, then waits 1000 ms. The wait is inside the function, so each call holds one state for a second.
void setLeds(int led1, int led2) {
digitalWrite(LED1, led1);
digitalWrite(LED2, led2);
delay(1000);
}setup sketch.ino
Makes both LED pins outputs, starts serial at 9600 baud, and prints a hello line.
void setup() {
pinMode(LED1, OUTPUT);
pinMode(LED2, OUTPUT);
Serial.begin(9600);
Serial.println("Mokxi Uno: two LEDs taking turns on pins 4 and 5");
}loop sketch.ino
Calls setLeds(HIGH, LOW) and then setLeds(LOW, HIGH). Reading it tells you what happens: first one LED is on, then the other.
void loop() {
setLeds(HIGH, LOW);
setLeds(LOW, HIGH);
}Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- Both LEDs look on at once This project
Check each LED has its own resistor and pin: pin 4 for red, pin 5 for green. - One LED never lights This project
Check its direction: anode (A) toward the resistor, cathode (C) to GND. - 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. - Upload fails or nothing runs after wiring pins 0 and 1 to a module. Arduino Uno R3
Pins 0 and 1 are the USB serial port. Move the module to other pins, or unplug it while uploading. Also check the right board is selected. - Pins misbehave or the board resets when you power motors or servos from it. Arduino Uno R3
Each I/O pin gives about 20 mA, 40 mA max. Power motors and servos from their own supply and join its ground to the Uno's GND.
Change it and keep it
Open it in the editor, change the circuit or the code, and keep your version in a free account.