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Migrate LEDs tutorial from wiki
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‎docs/tutorials/using-ev3-leds.md‎

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---
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title: Using the EV3 LEDs
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group: hardware-standard
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---
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* Table of Contents
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{:toc}
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### Basics
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When the EV3 is booting, the LEDs flash amber (orange). When it is finished
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booting, they turn solid green. This functionality is hard-coded in the kernel
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and in Brickman.
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After the boot is complete, you can control the LEDs yourself. The LEDs live in
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`/sys/class/leds/`.
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```bash
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$ ls /sys/class/leds/
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ev3:left:green:ev3dev ev3:right:green:ev3dev
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ev3:left:red:ev3dev ev3:right:red:ev3dev
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```
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You probably noticed that _amber_ is missing here. We will get to that in a bit.
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Each LED has its own attributes for controlling it. We are using the standard
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Linux LED device class, so there is a bunch of extra stuff that it not really
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useful. Let's figure out what is...
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```bash
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$ ls /sys/class/leds/ev3:left\:green\:ev3dev
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brightness device max_brightness power subsystem trigger uevent
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```
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{% include /style/icon.html type="info" %}
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You only have to escape the colons using the backslash (`\:`) when using a command
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line shell. The actual directory name is `ev3:left:green:ev3dev`.
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{: .alert .alert-info}
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```bash
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# turn the left green LED off
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$ echo 0 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness
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# find out what the maximum brightness value is
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$ cat /sys/class/leds/ev3:left\:green\:ev3dev/max_brightness
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255
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# turn the left green LED back on (255 means 100%)
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$ echo 255 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness
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# dim the left green LED 1/2 way
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$ echo 127 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness
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```
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{% include /style/icon.html type="info" %}
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You you must be a member of the `ev3dev` group (or root) to be able to control the LEDs.
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{: .alert .alert-info}
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### Triggers
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Triggers can make the LED do interesting things.
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```bash
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$ cat /sys/class/leds/ev3:left\:green\:ev3dev/trigger
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[none] mmc0 timer heartbeat default-on transient
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legoev3-battery-charging-or-full legoev3-battery-charging legoev3-battery-full
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legoev3-battery-charging-blink-full-solid rfkill0
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```
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* `none` means we are manually controlling the LED with `brightness` like we just did.
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* `mmc0` makes the LED blink whenever there is SD card activity. This is what the
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LEDs were doing during boot.
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* `timer` makes the LED blink periodically. When we change the trigger, we get
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new attributes for controlling the on and off times. Times are in milliseconds.
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```bash
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$ echo timer > /sys/class/leds/ev3:left\:green\:ev3dev/trigger
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$ ls /sys/class/leds/ev3:left\:green\:ev3dev
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brightness delay_on max_brightness subsystem uevent
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delay_off device power trigger
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$ cat /sys/class/leds/ev3:left\:green\:ev3dev/delay_on
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500
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$ echo 1000 > /sys/class/leds/ev3:left\:green\:ev3dev/delay_on
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$ echo 2000 > /sys/class/leds/ev3:left\:green\:ev3dev/delay_off
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```
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* `heartbeat` makes the LED blink at a rate proportional to CPU usage.
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* `default-on` works just like `none` except that it turns the LED on instead of
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off when the trigger is set.
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* `legoev3-battery-*` are not useful. The batteries (including the rechargeable
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battery pack) do not have a way of telling the EV3 what is going on, so it is
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assumed that the batteries are always discharging. Therefore these triggers
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will always turn the LED off.
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* `rfkill0` is the RF (radio frequency) kill switch for the built-in Bluetooth.
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It should make the LED indicate if the built-in Bluetooth is turned on or not.
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### What about amber/orange?
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To make the LED amber, we just have to turn on both the red and green LEDs at the same time.
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```bash
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# make left LED illuminate amber
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$ echo 255 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness; echo 255 > /sys/class/leds/ev3:left\:red\:ev3dev/brightness
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# turn off left LED
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$ echo 0 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness; echo 0 > /sys/class/leds/ev3:left\:red\:ev3dev/brightness
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```
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How does red + green make amber? If you open up your favorite paint program that
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has a color chooser where you can adjust the individual red, green and blue values,
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we can see what is going on. Make sure blue is on zero, then change red and green
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both to their maximum values. You get yellow. On the EV3, the red LED is a bit
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stronger than than the green LED, so both LEDs at max brightness appears orange
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(which we are calling amber just because it is "supposed" to be sort of yellow-ish).
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In your paint program, if you leave red at the maximum value and start decreasing
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green, the color will start to turn orange and then red. So, playing around with
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the `brightness` values a bit, we can actually get orange and yellow.
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```bash
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# make the left LED really orange
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$ echo 180 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness; echo 255 > /sys/class/leds/ev3:left\:red\:ev3dev/brightness
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# make the left LED yellow (this shows us how much stronger the red led is!)
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$ echo 255 > /sys/class/leds/ev3:left\:green\:ev3dev/brightness; echo 25 > /sys/class/leds/ev3:left\:red\:ev3dev/brightness
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```

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