Python – Endpoint 101

Automated mains socket power-off for OSMC on a Raspberry Pi

I’ve chosen to replace an ageing mini-PC which I’ve used since 2010 with a new Raspberry Pi 3 B+ running OSMC. It makes for a really capable media centre which can playback newer h.265 HEVC video files at 1080p without any problems, or it can serve 4K files over NFS to a box with a hardware h.265 chip like the Fire TV Box.

This form factor is easy to take on holiday and you can use an old infrared remote control (or Harmony learning remote) with it too.

However, the one thing I’ve struggled with is how to make it easy for my family to use in regard to switching it on and off. The Pi doesn’t have a power button. Some power supplies have an inline rocker switch, which almost fits the bill. I wanted something more automated.

Fortunately I had a spare Energenie power socket from a previous project where I use one to turn off our bass speaker when the TV isn’t on. These power sockets are controlled remotely (over RF) from a Pi which you attach an Energenie control board/shield to.

What I’ve done with the Pi 3 is have it powered through an Energenie socket, and set up a service that executes when it detects OSMC is shutting down. That service will make a quick HTTP call to the Pi with the Energenie controller shield, which will in turn send an RF signal to turn the mains socket off.

osmcshutdown

Here’s how you can set it up like I have…

Scripts for the Pi running OSMC

First, add a new service script.. create a new file in this folder;

/etc/systemd/system/callenergenie.service

	[Unit]
	Description=Energenie Remote Call to Secondary Pi
	Before=multi-user.target
	After=network.target
	Conflicts=shutdown.target

	[Service]
	ExecStart=/bin/true
	ExecStop=/bin/sh /home/osmc/callenergenie.sh
	Type=oneshot
	RemainAfterExit=yes

	[Install]
	WantedBy=multi-user.target

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callenergenie.service

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Then enable it with;

sudo systemctl enable callenergenie.service

There are a couple of useful things happening in this service, the After parameter makes sure the code runs before the network code is shut down, and Conflicts parameter is looking for OSMC shutting down.

Now add a helper script… this will make the webserver call as a background task, so control will be given back to the service immediately, rather than it waiting for the wget to complete.

/home/osmc/callenergenie.sh

	echo Calling energenie socket…
	wget –quiet –background –output-document="callenergenie.log" "http://192.168.1.99/callenergenie.php?delay=10&switch=2&state=off"
	echo Sleeping for 2 seconds
	sleep 2
	echo Done.
	exit 0

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callenergenie.sh

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This calls the PHP script, telling it which socket to turn off, and how long to delay before sending the command, which we’re doing so that the Pi has time to shut down before the power is cut.

Scripts for the Pi with the Energenie shield

This is the PHP script I added to the other Pi which was already configured to be a PHP web server.

/var/www/html/callenergenie.php

	<?php

	/* MattC – Call this with various parameters..

	callenergenie.php?
	delay = time in seconds to sleep before calling Energenie
	switch = which socket to talk to
	state = turn socket on/off

	e.g. callenergenie.php?delay=5&switch=2&state=off

	*/

	print ("Waiting ".$_GET['delay']." seconds");
	sleep($_GET['delay']);
	print ("Switching socket ".$_GET['switch']." ".$_GET['state']);
	exec("sudo python /var/www/callenergenie.py ".$_GET['state']." ".$_GET['switch']);

	?>

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callenergenie.php

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To allow PHP to run the script as root, I needed to add the Apache user to the list of sudo-ers.. not that secure tho :( I’d be interested in anyone who knows how to run the Energenie scripts a regular user.. their Python doesn’t like it when it’s not root.

nano /etc/sudoers

www-data ALL=(ALL) NOPASSWD:ALL

The nice thing about the PHP script is that we can actually call it to turn the Pi on remotely too.. so you could configure that into a widget on your phone, or add it to Alexa.

Controlling Power Sockets using a Raspberry Pi

The subwoofer we had in our home cinema setup died a few weeks ago, so I did my research and found a nice replacement. The only thing I didn’t spot was the fact it never goes into standby if there’s no signal (unlike the old one). I tried using some eco plugs which turn off peripherals when the TV was turned off.. but it learns the TV remote signals, and completely turns the TV off as well.. meaning that you have to hit the power button twice to turn the TV back on; that doesn’t work well with the Harmony all-in-one remote we use.

What I decided to do was buy an Energenie socket, which can be switched on/off wirelessly from a Raspberry Pi. The kit comes with 2 sockets and a transmitter to attach to the GPIO headers on the Pi, and costs about £20.

In my subwoofer scenario I basically want it switched on when the TV is on, and off when the TV is off. The TV has a Chromecast plugged into it which is visible on my local network. If you’ve got a Smart TV on your network, maybe that’ll be visible in the same way. So when the TV/Chromecast appear on the network, we know to switch on the power socket.

Here’s the small Energenie transmitter attached to the GPIO headers on my B+.. it’s pretty tiny and the case I’ve got still fits over the top. Notice the small hole where you can attach an aerial.. if you want extra range, then you’ll need to solder one on.. I added a 135mm wire, since the range I got out of it just wasn’t enough to get from the dining room cupboard to the living room.

To put this together, we can use the Raspberry Pi Network Spy code I wrote in my previous blog posts on element14;

Raspberry Pi Network Spy – Part 1 – Initial Setup + d/b Schema
Raspberry Pi Network Spy – Part 2 – D/b Setup + PHP for the scanner

All we need is a new PHP page that’ll call one of the functions we’ve already written.. we need a list of the MAC addresses that are currently visible on the network, then check whether the Chromecast is there. Once we know whether the TV/Chromecast is on or off, we then call a Python script that will turn the Energenie socket on/off.

Here’s the PHP;

arp-chromecast.php

The Python script could follow the Energenie example script, but there’s actually an even more simple Python package which I’ve used in this project. To install it I did the following;

sudo apt-get install python-pip sudo pip install energenie

Then I wrote this helper Python script (which we’ll call from PHP) that accepts a couple of parameters, like this callenergenie.py [on|off] [switch_number];

callenergenie.py
from energenie import switch_on, switch_off import sys

if len(sys.argv)==1:
print ‘Please specify arguments like this callenergenie.py [on|off] [switch_number]’
print ‘eg. callenergenie.py on 1’
print ‘eg. callenergenie.py off 2′
else:
on_or_off = sys.argv[1]
which_switch = int(sys.argv[2])

if (on_or_off==’on’):
print ‘Switching on ‘, which_switch
switch_on(which_switch)

if (on_or_off==’off’):
print ‘Switching off ‘, which_switch
switch_off(which_switch)

Now that we’ve written the PHP & Python, all we need to do is run the PHP every minute to scan the network and do the switching. We’ll do this using another cron job;

crontab -e

*/1 * * * * sudo /usr/bin/php /var/www/arp-chromecast.php

Project Mentions

This project of mine has been included in PC Pro magazine, and on one of my favourite tech podcasts, TekThing (at 22min 40sec in the video below)!

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