Two hosts power switch
######################

General principle
=================

This project detects up to two host power state through the their usb port and
switch on their associated devices' mains outlets (screen, sound amplifier,
printer, etc).

Devices might be shared (the same printer is used by both computers), in this
case, the device is switched on by whatever computer is detected on.

Flowchart
=========

When detecting a host state change, the following happen::


    .------------------.  .-------------------.
    |    Host state    |  | Skip filter input |
    | change detection |  |   (usb control)   |
    '------------------'  '-------------------'        .--------------.
              |   .-------------.   |          .------>| Switch relay |
              '-->| Skip filter |<--'          |       '--------------'
                  '-------------'              |
                         |          .---------------------.
                         v          | Start on/off timers |
                 .---------------.  '---------------------'
                 | Update target |             ^
                 |     table     |-------------'
                 '---------------'


Feature list
============

* Detect when one of the host is up (not in standby), switching on its
  associated devices (mains outlet).

* Each computer has its own set of associated outlets to switch on.

* An outlet may be shared between computers, if at least one of the computers
  is on the outlet is on (logical OR).

* Each outlet have an optional delay before it switch on or off. Delay for on
  and off may be different.

* Configuration for outlet associations and for the switch delay is saved in
  non-volatile memory.

* Computer standby detection is isolated and done by a mini-usb plug
  (calibrated for standard 5 volts supply, but you might change the input
  tension according to the optical coupler and input resistor, see schematic
  for more details). Molex to usb cable might be done if all usb ports of the
  computer are always on and its standby is never detected.

* There is one button for each computer to toggle its state (thus overriding
  its current state). The override is reset at the next computer state change.
  If the override reset is the same as the current state, you will see no
  change: if an override set the state to on, and the computer is switched on,
  the state change will not be seen until computer is in standby or off again
  or if another override is done.

* There is one button for each outlet so it can be toggled off or on. This
  toggle bypass the configured delay. The outlet can be toggled again normally
  according to the associated computer(s) state change.

* If at least one of the computers is on, global led is on. An individual led
  indicate the state of each outlet.

* Very low power then no outlet nor computer is on: the MCU will sleep and
  consume near nothing and all led will be off.

* Instead of pressing button, the dedicated control usb connection can be used
  to toggle state of things and to set a detection skip for the next off to on
  detection of a computer.

* A low consumption power converter (near 0 watt in full standby) is used to
  power the MCU. Advantages compared to being powered by control port include
  being autonomous, not using control port to switch relay (might induce
  little current spikes), and keeping current state when unplugging
  controlling computer.


Rationale
=========

The power switch could have triggered computer wakeup instead of doing
computer standby detection, but it is more complicated and it prevents using
the keyboard or another source of wakeup to switch on its associated devices.

The disadvantage is to need a way to skip the switch-on of the associated
devices when it is not needed (for example when doing wakeonlan to read data
in another location, one does not need the associated devices to be on).

In my case, the wake-on-lan was an exception to the normal usage of my
computer, so I made the skip choice to be in the workflow.