Sys Class LEDs

Introduction

Depending on which board you’re using, one or more LEDs are controllable using special files. In this exercise, you’ll learn how to turn them on and off and then how to tell Linux to make them blink on their own.

Try it out

All of the LEDs are found in the /sys/class/leds directory, so lets set a variable so that we don’t have to type that all the time.

First, find out what LEDs you have on your board:

led_base_path = "/sys/class/leds"
File.ls!(led_base_path)

Now, pick one by setting the variable below. We’ll pick "led0", but you’ll need to replace that if you don’t have it.

led = Path.join(led_base_path, "led0")

Reset the LED to a known state for this tutorial. Don’t worry about this yet. If the LED was on, then it will be off after this runs.

File.write(Path.join(led, "trigger"), "none")
File.write(Path.join(led, "brightness"), "0")

Let’s see what files are in the LED directory:

File.ls!(led)

The brightness file lets you turn it on and off by writing a "1" or "0" to it. Try it out:

File.write(Path.join(led, "brightness"), "1")

File.write(Path.join(led, "brightness"), "0")

Linux is happy controlling the LED on its own through the use of triggers. Triggers are quite flexible and can blink the LED on timers, CPU activity, disk activity and more. Here’s how to make it blink at 2 Hz (250 ms on and then 250 ms off) using the timer trigger:

File.write(Path.join(led, "trigger"), "timer")
File.write(Path.join(led, "delay_on"), "250")
File.write(Path.join(led, "delay_off"), "250")

Set the trigger back to "none" to control the LED again using the brightness file. To see what other triggers are available, read the trigger file and try them out.

File.write(Path.join(led, "trigger"), "none")
File.read!(Path.join(led, "trigger"))