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Built-in project · Arduino Uno R3

Four brightnesses from PWM: pins 5, 6, 9 and 10, on an Arduino Uno

Four LEDs on the PWM pins 5, 6, 9 and 10 glow at 2, 10, 50 and 100 percent, each set with analogWrite and map(). The whole build, an Arduino Uno and 8 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 $31. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • PWM and duty cycle
  • Using analogWrite on PWM pins
  • Converting percent to 0 to 255 with map
  • How the eye sees brightness
  • Difficulty: Beginner
  • Time: About 15 minutes
  • Board: Arduino Uno R3
  • Parts: 10

How it works

Four LEDs sit on pins 5, 6, 9 and 10, each with its own 220 ohm resistor and all sharing ground. Those are PWM pins, which switch on and off about 500 times a second.

The sketch sets each pin to a different share of the time: 2, 10, 50 and 100 percent. The eye sees the average, so you get four brightnesses. Notice that 10 percent looks much brighter than you might expect, and 50 percent does not look half as bright as 100.

Parts list

10 parts, plus the jumper wires. They retail for about $31; 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, red (1) pin CArduino Uno R3 pin GND
LED, red (2) pin CArduino Uno R3 pin GND
LED, red (3) pin CArduino Uno R3 pin GND
LED, red (4) pin CArduino Uno R3 pin GND
Resistor, 220 Ω (1) pin 1Arduino Uno R3 pin 10
Resistor, 220 Ω (2) pin 1Arduino Uno R3 pin 9
Resistor, 220 Ω (3) pin 1Arduino Uno R3 pin 6
Resistor, 220 Ω (4) pin 1Arduino Uno R3 pin 5
LED, red (1) pin AResistor, 220 Ω (1) pin 2
LED, red (2) pin AResistor, 220 Ω (2) pin 2
LED, red (3) pin AResistor, 220 Ω (3) pin 2
LED, red (4) pin AResistor, 220 Ω (4) 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
// Four LEDs, four brightnesses: PWM on pins 5, 6, 9 and 10.
//
//   pins 5, 6, 9 and 10 -> 220 ohm each -> an LED each -> GND
//
// A PWM pin switches on and off about 500 times a second. analogWrite(pin, n)
// sets how much of each cycle it is on, from 0 (never) to 255 (always), and
// the eye sees the average as brightness. map() turns a percentage into that
// 0 to 255 range.

const int PINS[4] = {5, 6, 9, 10};
const int PERCENT[4] = {2, 10, 50, 100};

void setup() {
  for (int i = 0; i < 4; i++) pinMode(PINS[i], OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: 2, 10, 50 and 100 percent on pins 5, 6, 9 and 10");
}

void loop() {
  for (int i = 0; i < 4; i++) analogWrite(PINS[i], map(PERCENT[i], 0, 100, 0, 255));
  delay(1000);
}

Code walkthrough

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

setup sketch.ino

Loops over the four pins in PINS and sets each as an output. Then it starts serial at 9600 and prints a message listing the percentages.

void setup() {
  for (int i = 0; i < 4; i++) pinMode(PINS[i], OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: 2, 10, 50 and 100 percent on pins 5, 6, 9 and 10");
}

loop sketch.ino

For each of the four pins, map turns the matching entry of PERCENT (2, 10, 50, 100) into 0 to 255, and analogWrite applies it. Then it waits a second and does it again.

void loop() {
  for (int i = 0; i < 4; i++) analogWrite(PINS[i], map(PERCENT[i], 0, 100, 0, 255));
  delay(1000);
}

Troubleshooting

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

  • An LED stays dark This project
    Use only PWM pins (5, 6, 9, 10 here), and check each LED's direction, with the short leg toward ground.
  • The 2 percent LED looks almost off This project
    That is expected. It is on only 2 percent of the time.
  • 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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