Pi 3B+ External Activity LED

Question

How can I add an external activity LED to Pi 3B+? Like to my Pi casing.

Answer 1

You can re-route the act led to a GPIO pin by adding the act-led overlay to /boot/config.txt. Example setting act-led to GPIO18:

dtoverlay=act-led,gpio=18

Info (see e.g. /boot/overlays/README)

Name:   act-led
Info:   Pi 3B, 3B+, 3A+ and 4B use a GPIO expander to drive the LEDs which can
        only be accessed from the VPU. There is a special driver for this with a
        separate DT node, which has the unfortunate consequence of breaking the
        act_led_gpio and act_led_activelow dtparams.
        This overlay changes the GPIO controller back to the standard one and
        restores the dtparams.
Load:   dtoverlay=act-led,<param>=<val>
Params: activelow               Set to "on" to invert the sense of the LED
                                (default "off")

        gpio                    Set which GPIO to use for the activity LED
                                (in case you want to connect it to an external
                                device)
                                REQUIRED