Using the LEDs

NOTE: You currently have to be root to use the LEDs.

Basics

When the EV3 is booting, the LEDs flash amber. When it is finished booting, they turn solid green.

The LEDs live in /sys/class/leds.

root@ev3dev:~# cd /sys/class/leds
root@ev3dev:/sys/class/leds# ls
ev3:green:left  ev3:green:right  ev3:red:left  ev3:red:right

You probably noticed that amber is missing here. We will get to that in a bit.

Each LED has it's own attributes for controlling it. We are using the standard linux LED device class, so there is a bunch of extra stuff that it not really useful. Let's figure out what is...

root@ev3dev:/sys/class/leds# cd ev3\:green\:left
root@ev3dev:/sys/class/leds/ev3:green:left# ls
brightness  device  max_brightness  subsystem  trigger  uevent

We can't control brightness, so ignore max_brightness and treat brightness like an on/off switch. Let's turn the LED off and back on.

root@ev3dev:/sys/class/leds/ev3:green:left# echo 0 > brightness
root@ev3dev:/sys/class/leds/ev3:green:left# echo 1 > brightness

Triggers

Triggers can make the LED do interesting things.

root@ev3dev:/sys/class/leds/ev3:green:left# cat trigger
[none] mmc0 timer heartbeat default-on legoev3-battery-charging-or-full legoev3-battery-charging
legoev3-battery-full legoev3-battery-charging-blink-full-solid

• none means we are manually controlling the LED with brightness like we just did.

• mmc0 makes the LED blink whenever there is SD card activity. This is what the right LED was doing during boot.

• timer makes the LED blink. When we change the trigger, we get new attributes for controlling the on and off times. Times are in milliseconds.

  root@ev3dev:/sys/class/leds/ev3:green:left# echo timer > trigger
  root@ev3dev:/sys/class/leds/ev3:green:left# ls
  brightness  delay_on  max_brightness  trigger
  delay_off   device    subsystem       uevent
  root@ev3dev:/sys/class/leds/ev3:green:left# cat delay_on
  500
  root@ev3dev:/sys/class/leds/ev3:green:left# echo 1000 > delay_on
  root@ev3dev:/sys/class/leds/ev3:green:left# echo 2000 > delay_off

• heartbeat makes the LED blink at a rate proportional to CPU usage. This is what the left LED was doing during boot.

• default-on works just like none except that it turns the LED on when the trigger is set.

• legoev3-battery-* are not useful. The batteries (including the rechargeable battery pack) do not have a way of telling the EV3 what is going on, so it is assumed that the batteries are always discharging. Therefore these triggers will always turn the LED off.

What about Amber?

To make the LED amber, we just have to turn on both the red and green LEDs at the same time.

root@ev3dev:/sys/class/leds/ev3:green:left# cd ..
root@ev3dev:/sys/class/leds# echo 1 > ev3\:green\:left/brightness; echo 1 > ev3\:red\:left/brightness
root@ev3dev:/sys/class/leds# echo 0 > ev3\:green\:left/brightness; echo 0 > ev3\:red\:left/brightness

Here is a fun trick. This makes the left LED cycle through each color (green-amber-red-off) every 500 msec.

root@ev3dev:/sys/class/leds# echo timer > ev3\:green\:left/trigger; \
echo 1000 > ev3\:green\:left/delay_on; \
echo 1000 > ev3\:green\:left/delay_off; \
sleep 1.5; \
echo timer > ev3\:red\:left/trigger; \
echo 1000 > ev3\:red\:left/delay_on; \
echo 1000 > ev3\:red\:left/delay_off