Last updated on October 11, 2018

First, find the corresponding directory of this LED device in /sys/class/leds, such as /sys/class/leds/device_name, and switch your working directory to this directory. The device_name may be similar to the device driver of your wifi adapter. You should have a file named trigger and a file named brightness in this directory. cat trigger shows you available triggers to trigger the LED light, with the current trigger surrounded by brackets. Different triggers will turn on and off the LED in different cases. Use echo new-trigger > trigger to change trigger. cat brightness shows the current brightness of the LED. Use echo N > brightness to change brightness, where N is an integer.

For example, I have a USB wifi adapter with an Atheros AR9271 chip, which is powered by the driver ath9k_htc on Linux. When I plug in the device, I have a new directory /sys/class/leds/ath9k_htc-phy0 created on my system. cat trigger outputs

none cpu0 cpu1 cpu2 cpu3 usb-gadget usb-host rfkill0 phy0rx phy0tx phy0assoc phy0radio [phy0tpt]

The default, phy0tpt, causes the LED to blink when there are activities. echo phy0radio > trigger causes the LED light to be on when this wifi interface is on. echo phy0assoc > trigger causes the LED to be on when connected to an access point. You can check the relevant document for explanation of phy0rx, phy0tx, phy0assoc, phy0radio, phy0tpt (Check the corresponding functions to see what these triggers mean). echo none > trigger means no trigger. That is, LED will be controlled manually. In this case, echo 255 > brightness can be used to turn on the LED, and echo 0 >brightness to turn it off.

If you want to automatically use a different trigger other than the default trigger when the adapter is plugged in, you can make a udev rule to do this. Note that this link is about flash drive, but the rule is the same for any USB devices. In my example I above, I created a script wifi-led.sh to update the trigger:

#!/bin/bash

for trigger in /sys/class/leds/ath9k_htc-phy*/trigger; do
    id=$(echo $trigger | sed -e 's/.*ath9k_htc-phy\([0-9]\).*/\1/g')
    echo "phy${id}radio" > $trigger
done

And add a new udev rule 100-usbwifi.rules to /etc/udev/rules.d to run this script when the adapter is plugged:

ACTION=="add", ATTRS{idVendor}=="venid", ATTRS{idProduct}=="prodid", RUN+="/path/to/wifi-led.sh"

Replace venid and prodid with your actually usb device ID. You can see the ID by running lsusb. For example, I have this following line output by lsusb corresponding to my USB wifi adapter.

Bus 001 Device 004: ID 0cf3:9271 Atheros Communications, Inc. AR9271 802.11n

where 0cf3 is the vendor ID, 9271 is the product ID.