WARNING!
The PiPower board is outputting 5V, which can kill your Pi according
to the specs.

Either add a logic level converter chip or a simple resistor based
voltage divider:


                    2.2k    3.3k
  PiPower signal ---~~~--+--~~~--- GND
                         |
                         |
                        GPIO

Source: https://www.raspberrypi.org/forums/viewtopic.php?f=45&t=22841



To install as a service, save pipower.service to /lib/systemd/system/ and execute:

sudo chmod 644 /lib/systemd/system/pipower.service
sudo systemctl daemon-reload
sudo systemctl enable pipower.service
sudo systemctl start pipower.service



Please check /boot/config.txt for a line defining enable_uart.
If this is set to 0, change it to 1! If not defined, add it just
to be sure:

enable_uart=1

I have encountered this default of 0 in RetroPie, but I've seen
reports for a standard installation as well:
https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=141195





You can connect power directly to PP1 or PP2 (+5V) and PP5 (GND).

Add some capacitors between de ATTiny85's +5V and GND pins.
I learned the hard way that if you don't, the power will drop to the
ATTiny and it'll reboot as soon as the Pi draws power, taking the
relay and thus the Pi with it. This cycle repeated and killed my Pi.

I bodged it and added in order two 100 nF ceramic capacitors, one 10 uF
electrolytic and one 470 uF electrolytic to get stable results.
I'm still learning electronics basics, but I recognize this is bad design.
Maybe a diode would've helped to prevent the Pi from draining the
capacitors immediately?