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JLed - Advanced LED Library

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An embedded C++ library to control LEDs. It uses a non-blocking approach and can control LEDs in simple (on/off) and complex (blinking, breathing and more) ways in a time-driven manner.

JLed got some coverage on Hackaday and someone did a video tutorial for JLed - Thanks!

JLed in action Interactive JLed playground
JLed in action jled running in the browser


// breathe LED (on gpio 9) 6 times for 1500ms, waiting for 500ms after each run
#include <jled.h>

auto led_breathe = JLed(9).Breathe(1500).Repeat(6).DelayAfter(500);

void setup() { }

void loop() {



  • non-blocking
  • effects: simple on/off, breathe, blink, candle, fade-on, fade-off, user-defined (e.g. morse)
  • supports inverted polarity of LED
  • easy configuration using fluent interface
  • can control groups of LEDs sequentially or in parallel
  • Portable: Arduino, ESP8266, ESP32, Mbed, Raspberry Pi Pico and more platforms compatible, runs even in the browser
  • supports Arduino, mbed, Raspberry Pi Pico and ESP32 ESP-IDF SDK's
  • well tested

Cheat Sheet

JLed Cheat Sheet


Arduino IDE

In the main menu of the Arduino IDE, select Sketch > Include Library > Manage Libraries... and search for jled, then press install.


Add jled to your library dependencies in your platformio.ini project file, e.g.

platform = atmelavr
board = nanoatmega328
framework = arduino


First, the LED object is constructed and configured, then the state is updated with subsequent calls to the Update() method, typically from the loop() function. While the effect is active, Update returns true, otherwise false.

The constructor takes the pin, to which the LED is connected to as the only argument. Further configuration of the LED object is done using a fluent interface, e.g. JLed led = JLed(13).Breathe(2000).DelayAfter(1000).Repeat(5). See the examples section below for further details.


Static on and off

Calling On(uint16_t period=1) turns the LED on. To immediately turn a LED on, make a call like JLed(LED_BUILTIN).On().Update(). The period is optional and defaults to 1ms.

Off() works like On(), except that it turns the LED off, i.e. it sets the brightness to 0.

Use the Set(uint8_t brightness, uint16_t period=1) method to set the brightness to the given value, i.e. Set(255) is equivalent to calling On() and Set(0) is equivalent to calling Off().

Technically, Set, On and Off are effects with a default period of 1ms, that set the brightness to a constant value. Specifiying a different period has an effect on when the Update() method will be done updating the effect and return false (like for any other effects). This is important when for example in a JLedSequence the LED should stay on for a given amount of time.

Static on example
#include <jled.h>

// turn builtin LED on after 1 second.
auto led = JLed(LED_BUILTIN).On().DelayBefore(1000);

void setup() { }

void loop() {


In blinking mode, the LED cycles through a given number of on-off cycles, on- and off-cycle durations are specified independently. The Blink() method takes the duration for the on- and off cycle as arguments.

Blinking example
#include <jled.h>

// blink internal LED every second; 1 second on, 0.5 second off.
auto led = JLed(LED_BUILTIN).Blink(1000, 500).Forever();

void setup() { }

void loop() {


In breathing mode, the LED smoothly changes the brightness using PWM. The Breathe() method takes the period of the effect as an argument.

Breathing example
#include <jled.h>

// connect LED to pin 13 (PWM capable). LED will breathe with period of
// 2000ms and a delay of 1000ms after each period.
auto led = JLed(13).Breathe(2000).DelayAfter(1000).Forever();

void setup() { }

void loop() {

It is also possible to specify fade-on, on and fade-off durations for the breathing mode to customize the effect.

// LED will fade-on in 500ms, stay on for 1000ms, and fade-off in 500ms.
// It will delay for 1000ms afterwards and continue the pattern.
auto led = JLed(13).Breathe(500, 1000, 500).DelayAfter(1000).Forever();


In candle mode, the random flickering of a candle or fire is simulated. The builder method has the following signature: Candle(uint8_t speed, uint8_t jitter, uin16_t period)

  • speed - controls the speed of the effect. 0 for fastest, increasing speed divides into halve per increment. The default value is 7.
  • jitter - the amount of jittering. 0 none (constant on), 255 maximum. Default value is 15.
  • period - Period of effect in ms. The default value is 65535 ms.

The default settings simulate a candle. For a fire effect for example use call the method with Candle(5 /*speed*/, 100 /* jitter*/).

Candle example
#include <jled.h>

// Candle on LED pin 13 (PWM capable). 
auto led = JLed(13).Candle();

void setup() { }

void loop() {


In FadeOn mode, the LED is smoothly faded on to 100% brightness using PWM. The FadeOn() method takes the period of the effect as an argument.

The brightness function uses an approximation of this function (example with period 1000):

fadeon function

FadeOn example
#include <jled.h>

// LED is connected to pin 9 (PWM capable) gpio
auto led = JLed(9).FadeOn(1000).DelayBefore(2000);

void setup() { }

void loop() {


In FadeOff mode, the LED is smoothly faded off using PWM. The fade starts at 100% brightness. Internally it is implemented as a mirrored version of the FadeOn function, i.e. FadeOn(t) = FadeOff(period-t). The FadeOff() method takes the period of the effect as argument.

User provided brightness function

It is also possible to provide a user defined brightness evaluator. The class must be derived from the jled::BrightnessEvaluator class and implement two methods:

  • uint8_t Eval(uint32_t t) const - the brightness evaluation function that calculates a brightness for the given time t. The brightness must be returned as an unsigned byte, where 0 means LED off and 255 means full brightness.
  • uint16_t Period() const - period of the effect.

All time values are specified in milliseconds.

The user_func example demonstrates a simple user provided brightness function, while the morse example shows how a more complex application, allowing you to send morse codes (not necessarily with an LED), can be realized.

User provided brightness function example

The example uses a user provided function to calculate the brightness.

class UserEffect : public jled::BrightnessEvaluator {
    uint8_t Eval(uint32_t t) const override {
        // this function changes between 0 and 255 and
        // vice versa every 250 ms.
        return 255*((t/250)%2);
    // duration of effect: 5 seconds.
    uint16_t Period() const override { return 5000; }

Delays and repetitions

Initial delay before effect starts

Use the DelayBefore() method to specify a delay before the first effect starts. The default value is 0 ms.

Delay after effect finished

Use the DelayAfter() method to specify a delay after each repetition of an effect. The default value is 0 ms.


Use the Repeat() method to specify the number of repetitions. The default value is 1 repetition. The Forever() methods sets to repeat the effect forever. Each repetition includes a full period of the effect and the time specified by DelayAfter() method.

State functions


Call Update() periodically to update the state of the LED. Update returns true if the effect is active, and false when it finished.


IsRunning() returns true if the current effect is running, else false.


A call to Reset() brings the JLed object to its initial state. Use it when you want to start-over an effect.

Immediate Stop

Call Stop() to immediately turn the LED off and stop any running effects. Further calls to Update() will have no effect unless the Led is reset (using Reset()) or a new effect activated.

Misc functions

Low active for inverted output

Use the LowActive() method when the connected LED is low active. All output will be inverted by JLed (i.e. instead of x, the value of 255-x will be set).

Maximum brightness level

The MaxBrightness(uint8_t level) method is used to set the maximum brightness level of the LED. A level of 255 (the default) is full brightness, while 0 effectively turns the LED off.

The uint_8 MaxBrightness() const method returns the current maximum brightness level. Since currently only the upper 5 bits of the provided brighness value are used, the lower 3 bits returned are always 0.

If you want to programmatically increment or decrement the maximum brightness level, use the JLed::kBrightnessStep constant (which is defined as 1 << (8-JLed::kBitsBrightness) as the increment (instead of the hard wired value 8) to be independent of the current JLed implementation using 5 bits.

Controlling a group of LEDs

The JLedSequence class allows controlling a group of JLed objects simultaneously, either in parallel or sequentially, starting the next JLed effect when the previous finished. The constructor takes the mode (PARALLEL, SEQUENCE), an array of JLed objects and the size of the array, e.g.

JLed leds[] = {
    JLed(4).Blink(750, 250).Repeat(10),

auto sequence = JLedSequence(JLedSequence::eMode::PARALLEL, leds).Repeat(2);

void setup() {

void loop() {

Because the size of the array is known at compile time in this example, it is not necessary to pass the array size to the constructor. A second constructor is available in case the JLed array is created dynamically at runtime: JLed(eMode mode, JLed* leds, size_t n).

The JLedSequence provides the following methods:

  • Update() - updates the active JLed objects controlled by the sequence. Like the JLed::Update() method, it returns true if an effect is running, else false.
  • Use the Repeat(n) method to specify the number of repetitions. The default value is 1 repetition. The Forever() methods sets to repeat the sequence forever.
  • Stop() - turns off all JLed objects controlled by the sequence and stops the sequence. Further calls to Update() will have no effect.
  • Reset() - Resets all JLed objects controlled by the sequence and the sequence, resulting in a start-over.

Framework notes

JLed supports the Arduino and mbed frameworks. When using platformio, the framework to be used is configured in the platform.ini file, as shown in the following example, which for example selects the mbed framework:

board = nucleo_f401re
framework = mbed
build_flags = -Isrc 
src_filter = +<../../src/>  +<./>

An mbed example is provided here. To compile it for the F401RE, make your plaform.ini look like:

default_envs = nucleo_f401re_mbed
src_dir = examples/multiled_mbed

Platform notes


The DAC of the ESP8266 operates with 10 bits, every value JLed writes out gets automatically scaled to 10 bits, since JLed internally only uses 8 bits. The scaling methods make sure that min/max relationships are preserved, i.e. 0 is mapped to 0 and 255 is mapped to 1023. When using a user-defined brightness function on the ESP8266, 8-bit values must be returned, all scaling is done by JLed transparently for the application, yielding platform-independent code.


When compiling for the ESP32, JLed uses ledc functions provided by the ESP32 ESP-IDF SDK. (See esspressif documentation for details).

The ledc API connects so-called channels to GPIO pins, enabling them to use PWM. There are 16 channels available. Unless otherwise specified, JLed automatically picks the next free channel, starting with channel 0 and wrapping over after channel 15. To manually specify a channel, the JLed object must be constructed this way:

auto esp32Led = JLed(jled::Esp32Hal(2, 7)).Blink(1000, 1000).Forever();

The jled::Esp32Hal(pin, chan) constructor takes the pin number as the first argument and the ESP32 ledc channel number on the second position. Note that using the above-mentioned constructor results in non-platform independent code, so it should be avoided and is normally not necessary.

For completeness, the full signature of the Esp32Hal constructor is

Esp32Hal(PinType pin, 
         int chan = kAutoSelectChan, 
         uint16_t freq = 5000,
         ledc_timer_t timer = LEDC_TIMER_0)

which also allows to override the default frequency and timer used, when needed.


Since JLed uses the ESP-IDF SDK, JLed can also be directly used in ESP-IDF projects, without the need of using the Arduino Framework (which is also possible). See these repositories for example projects:


Arduino framework

I had success running JLed on a STM32 Nucleo64 F401RE board using this STM32 Arduino core and compiling examples from the Arduino IDE. Note that the stlink is necessary to upload sketches to the microcontroller.

Raspberry Pi Pico

When using JLed on a Raspberry Pi Pico, the Pico-SDK and tools must be installed. The Pico supports up to 16 PWM channels in parallel. See the pico-demo for an example and build instructions.

Example sketches

Example sketches are provided in the examples directory.


To build an example using the PlatformIO ide, uncomment the example to be built in the platformio.ini project file, e.g.:

; uncomment example to build
src_dir = examples/hello
;src_dir = examples/breathe

Arduino IDE

To build an example sketch in the Arduino IDE, select an example from the File > Examples > JLed menu.


Support new hardware

JLed uses a very thin hardware abstraction layer (hal) to abstract access to the actual MCU/framework used (e.g. ESP32, ESP8266). The hal object encapsulate access to the GPIO and time functionality of the MCU under the framework being used. During the unit test, mocked hal instances are used, enabling tests to check the generated output. The Custom HAL project provides an example for a user define HAL.

Unit tests

JLed comes with an exhaustive host-based unit test suite. Info on how to run the host-based provided unit tests is provided here.


  • fork this repository
  • create your feature branch
  • add code
  • add unit test(s)
  • add documentation
  • make sure the cpp linter does not report any problems (run make lint). Hint: use clang-format with the provided settings
  • commit changes
  • submit a PR


How do I check if a JLed object is still being updated?

  • Check the return value of the JLed::Update method: the method returns true if the effect is still running, otherwise false.
  • The JLed::IsRunning method returns true if an effect is running, else false.

How do I restart an effect?

Call Reset() on a JLed object to start over.

How do I change a running effect?

Just 'reconfigure' the JLed with any of the effect methods (e.g. FadeOn, Breathe, Blink etc). Time-wise, the effect will start over.

Author and Copyright

Copyright 2017, 2018 by Jan Delgado, jdelgado[at]



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