For a while now I’ve been intrigued by the idea of home automation, but despite a large amount of research I’ve yet to take the plunge and buy anything. The one thing I do have is a bunch of these Maplin remote control sockets. These are simple things which just plug into any wall socket, then have a socket which you can plug any device into, a simple RF remote offers 8 buttons, on and off for each of the four sockets. The remote lets you choose from one of four channels meaning you can have four sets of four remotes all working within range of one another. I bought these originally because we had a few power sockets in awkward to reach places - who wants to move the sofa every time you want to turn a light on? - but never considered the possibility of using a computer to control them.

That was however, until I spotted a intriguing sounding tweet from Chris Swan directed at Andy Stanford-Clark:

@andysc I've previously used https://t.co/boK7BrIKI9 to control Maplin sockets, so I just need to track down (or analyse) the monitor signal

— Chris Swan 💙💛 (@cpswan) December 3, 2014

Oh really Mr. Swan, now this does sound very interesting. I asked for some details and Chris pointed me at this blog post which explains pretty much everything I needed to know. The post linked to this library from Duncan McGregor which did the hard work and linked to the source of the transmitter I would need. Sorted then, this sounded simple enough!

I bought one of the transmitters and a ribbon cable with a plug that fitted the GPIO connector of the Pi. I planned to snip all but three cables from the ribbon, shorten the remaining ones and solder these to the 3 pins on the TX, the hope was with a very short wire from the GPIO plug to the TX I’d be able to hide it inside my existing case. The pin out of the GPIO and the connections you need to make are listed on Duncan GitHub repo in the Readme. But in short it goes like this:

Once I’d soldered the connections, I plugged the ribbon cable onto the GPIO and booted the Pi. One quick clone of Duncans repo later and I ran the following:

sudo ./strogonanoff_sender.py --channel 1 --button 2 --gpio 0 on

And hey presto my fish tank light turned on. minor celebration 🙂

I wanted to try and avoid sudo access, but reading a few, things, decided that sudo was probably safest! So be it.

I’m pretty terrible at remembering that the fish tank lights are channel 1, button 2 so I wrote a short python script which accepts room names and a simple on or off:

It essentially just uses a Python dictionary to store the room to channel and button mapping:

switches = {
'bookshelf': { 'channel' : 4, 'button' : 1 },
'tank':      { 'channel' : 4, 'button' : 2 },
'sofa':      { 'channel' : 4, 'button' : 3 },
'outside':   { 'channel' : 4, 'button' : 4 },
'spareroom': { 'channel' : 3, 'button' : 1 },
'all' :      True
}

I’ve put this script ‘switch.py’ into a repo in my Github account here. You run it like this:

./switch.py -s tank -a on

The success of all this lead me to consider actually building a remote control for these things that I could use on my phone, partly because it sounded like a neat thing to do and partly because finding the remote when the kids have been using it is non-trivial. The added, actually useful feature would be to turn lights off remotely when I go out and forget to turn them off.

So I pondered for a bit and sketched out a plan for a simple HTML UI running on a web server, somehow talking to the Pi to do the work. I didn’t want to expose the Pi to the internet because I already have a Linux box with a public IP and apache running on it and didn’t want two entry points into the house. I just needed a way of getting a command from a client browser, via the other linux box and onto the Pi. Then it dawned on me, I already use Mosquitto, the open source MQTT broker for moving temperature readings around, I could just use that. A bit more hacking lead me to write maplin-mqtt.py which is a variation of switch.py and simply subscribes to the MQTT broker and listens for messages containing commands.

I wrote it to expect JSON messages in the following format:

{ "channel": 3,"button": 2,"action": "off"}

Testing this is as simple as using the command line MQTT publish client:

% mosquitto_pub -h trin -t foo/bar -m '{"action": "off", "switch": "spareroom"}'

So now I just had to write a simple web UI which could publish messages of the correct format. I’ll write about that at another time because I hit upon something which changed my plans slightly 🙂