On Sun, 21 Jan 2018 21:47:06 +0100, R.Wieser wrote:

> Thats odd, as the red led is directly connected to the 3.3v power, and
> thus *cannot* be switched off.

Not according to the Pi 3 schematic. There is FET between the LED and
ground, the gate fed from the active low !RST output of a
APX803-46SAG supply supervisor device. If the 5 V supply drops below
4.6 V it asserts the active low !RST and thus turns the LED off. If
the supply rises above 4.6 V it waits at least 140 ms before lowering
!RST. The same signal goes elsewhere as the PWR_LOW_N line.

The documentation is incomplete but dig about for gpiovirtbuf to at
least monitor the PWR_LOW_N line and in principle be able to control
the LED via that line.

--
Cheers
Dave.

--- SoupGate-Win32 v1.05