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Built-in project · Arduino Uno R3

LED module on pin 7, blinking, on an Arduino Uno

An LED and its 220 ohm resistor on pin 7, as on the kit’s LED modules: on for a second, off for a second. The whole build, an Arduino Uno and 2 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.

The circuit runs here once the page has loaded.

What you'll learn

  • Switching a pin on and off
  • A module as parts on a small board
  • The role of the series resistor
  • Timing with delay
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 4

How it works

Pin 7 feeds the white LED through a 220 ohm resistor, and the LED's other leg goes to GND. In the real kit, an LED module has the LED and its resistor on a small board. Here they sit on the breadboard.

The sketch sets pin 7 HIGH for a second, then LOW for a second, again and again. Click the LED in the simulator to change its color.

Parts list

4 parts, plus the jumper wires. They retail for about $30; on Mokxi they're free.

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.

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 pinConnects to
LED, white pin CArduino Uno R3 pin GND
Resistor, 220 Ω pin 1Arduino Uno R3 pin 7
LED, white pin AResistor, 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 · Arduino Uno R3
// An LED module on pin 7: on for a second, off for a second.
//
//   pin 7 -> 220 ohm -> LED -> GND (the module carries both on its board)
//
// The kit's white and red LED modules are an LED and its resistor on a small
// board with three pins: S for the signal, + and -. Here the same two parts
// sit on a breadboard. Click the LED to change its color.

const int LED_PIN = 7;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: LED module on pin 7");
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(1000);
  digitalWrite(LED_PIN, LOW);
  delay(1000);
}

Code walkthrough

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

setup sketch.ino

Makes LED_PIN (pin 7) an output with pinMode. It then starts the serial port at 9600 baud and prints a hello message.

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: LED module on pin 7");
}

loop sketch.ino

Sets the pin HIGH, calls delay(1000) to wait a second, sets it LOW, and waits another second. Change the 1000 values for a faster or slower blink.

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(1000);
  digitalWrite(LED_PIN, LOW);
  delay(1000);
}

Troubleshooting

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

  • The LED does not light. This project
    Check its direction. The anode (A) goes toward the resistor and pin 7, and the cathode (C) goes to GND.
  • The blink is the wrong speed. This project
    The delay values are in milliseconds. 1000 is one second.
  • 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.

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