led

led

This module leverages the LED specific functionality provided by the Linux operating system to control LEDs using JavaScript. The advantage of this technique is speed as the heavy work is off-loaded to the operating system.

Linux boards often have LEDs that can be controlled from userspace. Out of the box, the Raspberry Pi 1 has one such LED labeled ACT or OK, the Raspberry Pi 3 has two such LEDs labeled ACT and PWR. The BeagleBone has four, user led 0 through 3. Some systems allow additional off-board LEDs to be added at runtime using device tree overlays.

The features supported by these LEDs varies from system to system. One system will allow the LEDs to be turned on and off while the next system will support additional fetaures such as heartbeat or hardware accelerated blinking.

Contents

• Installation
• Usage
  • BeagleBone Black
  • Raspberry Pi
  • Notes
• API
• How Does it Work?
• Example - Device Tree Overlay

Installation

npm install led

Usage

BeagleBone Black

Blink all user LEDs on the BeagleBone Black five times a second

var Led = require('led');

['usr0', 'usr1', 'usr2', 'usr3'].forEach(function (name) {
  new Led('beaglebone:green:' + name).blink(100, 100);
});

Heartbeat all user LEDs leds on the BeagleBone Black

var Led = require('led');

['usr0', 'usr1', 'usr2', 'usr3'].forEach(function (name) {
  new Led('beaglebone:green:' + name).heartbeat();
});

Blip all user LEDs on the BeagleBone Black once every two seconds

var Led = require('led');

['usr0', 'usr1', 'usr2', 'usr3'].forEach(function (name) {
  new Led('beaglebone:green:' + name).blink(1, 1999);
});

Raspberry Pi

Turn the ACT LED on the Raspberry Pi on for one second

var Led = require('led'),
  led = new Led('ACT');

led.on();

setTimeout(function () {
  led.off();
}, 1000);

Blink ACT and PWR LEDs on the Raspberry Pi five times a second

var Led = require('led');

['ACT', 'PWR'].forEach(function (name) {
          new Led(name).blink(100, 100);
});

Heartbeat ACT and PWR LEDs on the Raspberry Pi

var Led = require('led');

['ACT', 'PWR'].forEach(function (name) {
          new Led(name).heartbeat();
});

Notes

Although it may not be immediately obvious, the LEDs on the BeagleBone Black or Raspberry Pi will continue to blink, heartbeat, and blip after the corresponding programs have terminated. All the heavy work involved in controlling the LEDs has been off-loaded to the operating system and the number of CPU cycles required to control the LEDs is minimized.

API

Led(name) Returns a new Led object which can be used to control the LED with the specified name. The name to use for a particular LED is the name of the corresponding directory in /sys/class/leds. Examples are ACT on the Raspberry Pi and beaglebone:green:usr0, beaglebone:green:usr1, beaglebone:green:usr2, and beaglebone:green:usr3 on the BeagleBone or BeagleBone Black.

on() Turn the LED on.

off() Turn the LED off.

heartbeat() Heartbeat the LED.

blink(delayOn, delayOff) Blink the LED. delayOn and delayOff specify the on and off time in milliseconds.

delayOn(val) Modify the on time for a blinking LED to the specified value in milliseconds.

delayOff(val) Modify the off time for a blinking LED to the specified value in milliseconds.

How Does It Work?

Linux systems often have files representing LEDs that appear in /sys/class/leds. Such LEDs can be controlled by writing the appropriate values to the appropriate files. More information can be found here

Example - Device Tree Overlay

The example directory contains a device tree overlay called myled.dto which can configure P9_12 on the BeagleBone Black as a pin for controlling an LED from userspace. The script myled performs all the necessray setup and after everything has been setup correctly, there should be an LED called 'my:red:led' in /sys/class/leds.

The following program can be used to let 'my:red:led' glow dimly for a second and then brighlty.

var Led = require('led'),
  led = new Led('my:red:led'),
  dim = true;

led.blink(1, 9);

setInterval(function () {
  if (dim) {
    led.delayOn(9);
    led.delayOff(1);
  } else {
    led.delayOn(1);
    led.delayOff(9);
  }

  dim = !dim;
}, 1000);