Author......Ikaros Kappler
                                                                            Date............2015-03-26
									    Modified........2015-04-13
                                                                            Version................1.0
  

On my ASRock motherboard I found a header like this with the 'Power Switch' connector (Pin 6 and 8) which are connected to the – big surprise – power switch of my computer case.
I just opened the wires and soldered a bypass into them, the blue/white cable in the pictures below :)
Short-circuiting the blue-white wires sends the signal to the controller and commands it to boot the computer.

I used GPIO Pin 11 (blue wire) to give the signal, U_c 3.3V+ (red clamp) and GND (white clamp) for connections.

For the (very simple) circuit I used two BC547-NPN transistors, a 220Ω resistor and a LED to indicate when signal in ON.





Fritzing file: Raspi_PC_Startup.fzz

Create a simple python script startup.py that sets your GPIO pin 11 to HI for one second and then switches it back to LO:

#!/usr/bin/python

# enable debugging
import cgitb
cgitb.enable()

# The comma prevents 'print' to add a line break.
print "Content-Type: text/plain; charset=utf-8\r\n\r\n",

# date 2015-01-25
# A test script to set voltage=HI on your RaspberryPi (pin 11) for 1 second, then LO again.

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)

GPIO.setup( 11, GPIO.OUT );

try:
    print "Pin 11 HI"
    GPIO.output( 11, True )
    time.sleep( 1 )
    print "Pin 11 LO"
    GPIO.output( 11, False )
    time.sleep( 2 )
    GPIO.cleanup()
except KeyboardInterrupt:
    GPIO.output( 11, False )
    GPIO.cleanup()
    # print "Program exit by keyboard interrupt."

Check if your python script runs: Output:

  Content-Type: text/plain; charset=utf-8

  Pin 11 HI
  Pin 11 LO

Note: we will need the Conent-Type header later when calling the script via HTTP.

Go to your /etc/apache2/sites-enabled/000-default configuration file. Add the line
AddHandler cgi-script .cgi .py
to this section, it will enable the CGI handler for python scripts:

    [...]
        ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
        <Directory "/usr/lib/cgi-bin">

                # Added 2015-03-26 by Ika
                AddHandler cgi-script .cgi .py

                AllowOverride None
                Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
                Order allow,deny
                Allow from all
        </Directory>
  

Now add a sub directory for your CGI script because we don't want to protected the whole CGI path:

Place your startup.py file here and set the executable flag so Apache ca use it:

The python script is now placed in Apache's cgi-bin directory and can be called from your web browser (though it still throws errors). To prevent unauthorized access you should set up a password for your system's protection.

Open your /etc/apache2/sites-enabled/000-default file again and add this section. It will set up a basic password protection to prohibit unauthorized access to the script. After all the script controls your hardware!

        # Added 2015-03-26 Ika (procted cgi-bin/startup/ with a password)
        <Directory "/usr/lib/cgi-bin/startup">
                AllowOverride None
                Options None
                Order allow,deny
                Allow from all

                AuthType Basic
                AuthName "Scripts"
                AuthUserFile /usr/lib/cgi-bin/startup/.htpasswd

                Require valid-user
                Satisfy All
        </Directory>

The /usr/lib/cgi-bin/startup/.htpasswd file is a plain apache/htaccess password file.

Do this on your own responsibility and only if you know what you do. We will give the www-user the permission to execute a shell script with sudo privilegues!

As we want to access /dev/mem our www user (Apache) requires sudo permissions. So we need a wrapper to call python with sudo. Create /usr/lib/cgi-bin/startup/startup.sh:

#!/bin/sh

sudo python startup.py

Now we need to give the user 'www-data' the privilegue to execute this script with sudo permissions without asking for a password. Change to root (the pi user is allowed to do so):
Now edit your /etc/sudoers file (this file has to be edited with visudo):
and add these two lines to the end of the file:
www-data ALL=(ALL) NOPASSWD: /usr/lib/cgi-bin/startup/startup.sh
www-data ALL=(ALL) NOPASSWD: /usr/bin/python

[...]
# User privilege specification
root    ALL=(ALL:ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d
pi ALL=(ALL) NOPASSWD: ALL

www-data ALL=(ALL) NOPASSWD: /usr/lib/cgi-bin/startup/startup.sh
www-data ALL=(ALL) NOPASSWD: /usr/bin/python

Now Apache runs the startup.sh script with the permission to fork sudo processes (python), just by passing its request URL! That's why we have set up the password.

Open your browser and test if your CGI script is available:
http://127.0.0.1/cgi-bin/startup/startup.sh
Enter the username and password. The document should show

    Pin 11 HI
    Pin 11 LO

and your PC/other connected device should now (re-)boot.

I recently soldered the circuit onto a small board.