---
name: linux-led
description: The Arduino Uno Q's MPU-side RGB LEDs, driven from Linux via /sys/class/leds (active-low). Load this to control the on-board Linux LEDs, as opposed to LEDs wired to Arduino pins.
---

# Linux-side RGB LEDs (MPU)

The Uno Q has RGB LEDs owned by the **MPU (Linux)**, exposed under
`/sys/class/leds/*` (e.g. `red:user`, `green:user`, `blue:bt`). These are separate
from the MCU-owned LEDs you drive with `digitalWrite`/`analogWrite` in a sketch.

## Use the `sysfs_led` tool

- `action=list` — enumerate available LEDs.
- `action=set`, `led=<name>`, `brightness=0..255` — set brightness in **natural
  sense** (0 = off, 255 = full). The tool handles the active-low inversion and
  scales to each LED's `max_brightness` for you.

## Under the hood

The LEDs are **active-low**: writing `0` to `<led>/brightness` turns it fully ON,
`max_brightness` turns it OFF. `sysfs_led` inverts your natural-sense value so you
never have to think about it.

## Which LED tool to use

- On-board **Linux** RGB LEDs → `sysfs_led` (this skill).
- LEDs wired to **Arduino pins**, or the built-in D13 → flash a sketch with
  `uno_q_flash`, or toggle via `gpio_write` on a running sketch.
- The 13×8 blue **LED matrix** → the `led-matrix` skill (it's an MCU device).