When it comes to me getting some work done, there is really only one place in the house where that happens. Coincidentally, it's the same place where I get some sleep, hack about with electronics, write code, record videos, take photos of products, etc. I call it my "room", it gets a little cramped, but that's the least of my issues. What I really don't need are interruptions. Creatives and coders would probably best sympathize with me - once you get in the "groove", find your "muse" or get in the "rhythm", the last thing you need is someone opening your door unexpectedly. Even worse is when you're trying to catch some shuteye, and someone opens the door and barges in. My family has a habit of doing this, and it's not helped by the fact I tend to have odd sleeping hours as I'm not "limited" by regular 9-5 engagements, and my hobbies often dictate being awake at odd hours.

Anyway, it was getting to breaking point several months ago, and I decided to try and solve it once and for all. I looked around my room to find the PiFace Control and Display I had from the Raspberry Pi New Year's Party Pack RoadTest which remained unused, as well as a Raspberry Pi Model A (original) which I bought out of curiosity, having no Ethernet port and just one USB connector. Great! Virtually everything I need to solve this problem - by making an electronic network-connected door sign which I can change at will from any computer on the network (or beyond, by tunneling through another Raspberry Pi set to expose its SSH to the world).

Before I continue further, I must warn everyone that this was a hack borne out of frustration, where time to a solution was the key. As a result, there are known security problems, namely no authentication, user-side validation only and web server running on an account with root privileges. Unless you can guarantee the security and non-maliciousness of the environment where the unit will be deployed, you should NOT use this as an example to set-up your own door sign. Even then, I probably have missed out a few steps, having done this a few months ago, fixing any issues I found on the fly and not writing anything down. It's more an inspiration for others to see what is possible.

Raspberry Pi Model A Set-Up

Setup of the Raspberry Pi Model A is actually quite confusing because of the limited hardware. It's probably best done interactively, so using an HDMI monitor, USB keyboard and mouse, and USB wireless network adapter. In order to do this, you will need a USB hub, preferably powered. Or, if you're sneaky like I am ... just set up the card on the Model B first, then transfer it over to the Model A once you're ready.

The first thing you will need to do all the normal raspi-config configuration. Expanding filesystem, overclocking, enabling SSH, setting locale, timezone, keyboard layout, disable overscan, enable SPI, change hostname, change password. The norm. To simplify wireless configuration, I decided to startx and configure Wi-Fi using the graphical utility, which seems to store any required information in /etc/wpa_supplicant/wpa_supplicant.conf. Once that's done, it's always good to keep yourself up to date with sudo apt-get update and sudo apt-get upgrade.

Because we will be working with the pi remotely, and we would like to find it easily, a static IP was allocated by modifying /etc/network/interfaces - in my case, to 192.168.0.36.

iface wlan0 inet manual

wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf

iface default inet static

address 192.168.0.36

netmask 255.255.255.0

gateway 192.168.0.1

Not only that, we also need to modify /etc/resolv.conf to have one line, nameserver 192.168.0.1 or whichever nameserver you want. In the case of the latest Raspbian distribution, resolv.conf is generated from resolvconf.conf which needs its local nameserver line commented out, and 127.0.0.1 replaced with your nameserver of choice, so that DNS resolves work properly (otherwise you will have trouble installing or updating things. It's good to reboot your unit again and test that it is reachable via SSH on the IP configured. Another tweak you might want to do is to go back to sudo raspi-config to configure the GPU memory split to be 16Mb - as you will probably only be running in text mode, so freeing RAM for other processes. Now that we've got the configuration workable remotely, you can pull the card and shove it into the Model A along with the one wireless adapter and the PiFace Control and Display module.

Then, we need to install the relevant packages. My initial thoughts was that it would be enough to be able to change the sign by SSHing into the box and running a shell script, but that would get tedious very quickly, so I wanted a web interface as well. As a result, I decided to install the support packages for pifacecad and lighttpd as the web server. That's as simple as sudo apt-get install python{,3}-pifacecad lighttpd. At this stage, the PiFace Control and Display module should be working (or might be after a reboot) - a simple Python3 program like the following should write a message onto the screen.

#!/usr/bin/env python3

import pifacecad

cad = pifacecad.PiFaceCAD()

cad.lcd.blink_off()

cad.lcd.cursor_off()

cad.lcd.clear()

cad.lcd.backlight_on()

cad.lcd.write("Gough: AWAKE

Come in!")

If that is working, then we are ready to make the interface work.

Gluing the Web Server to the PiFace Control and Display

I decided to go with lighttpd and the tried and trusted cgi interface to allow the web server to make calls to python. Despite the above example using python3, I decided to go with python (2) instead for this. The first problem I found was the issue of configuring the web server correctly. In the end, my /etc/lighttpd/lighttpd.conf looks like this:

server.modules = (

"mod_access",

"mod_cgi",

"mod_alias",

"mod_compress",

"mod_redirect",

# "mod_rewrite",

)

server.document-root = "/var/www"

server.upload-dirs = ( "/var/cache/lighttpd/uploads" )

server.errorlog = "/var/log/lighttpd/error.log"

server.pid-file = "/var/run/lighttpd.pid"

server.username = "pi"

server.groupname = "pi"

server.port = 80

index-file.names = ( "index.php", "index.html", "index.lighttpd.html" )

url.access-deny = ( "~", ".inc" )

static-file.exclude-extensions = ( ".php", ".pl", ".fcgi" )

compress.cache-dir = "/var/cache/lighttpd/compress/"

compress.filetype = ( "application/javascript", "text/css", "text/html", "text/plain" )

# default listening port for IPv6 falls back to the IPv4 port

include_shell "/usr/share/lighttpd/use-ipv6.pl " + server.port

include_shell "/usr/share/lighttpd/create-mime.assign.pl"

include_shell "/usr/share/lighttpd/include-conf-enabled.pl"

$HTTP["url"] =~ "^/cgi-bin/" {

cgi.assign = ( ".py" => "/usr/bin/python" )

}





Significant changes include making the server use the pi username and pi group, and the addition of mod_cgi, and the last few lines which configure the .py extension to call /usr/bin/python as the interperter. Lighttpd needs to be restarted with a sudo service lighttpd restart for the changes to take effect (or you could try to use reload, but restart is just as easy).

Now we need an interface and the cgi-script to perform the action. I decided to go and write a simple python script for the cgi-handler called pidisplay.py, placed inside the cgi-bin directory under /var/www. Of course, it must be sudo chmod +x pidisplay.py to ensure it is executable.

#!/usr/bin/env python

import cgi

import cgitb

import pifacecad

cgitb.enable()

cad=pifacecad.PiFaceCAD()

cad.lcd.blink_off()

cad.lcd.cursor_off()

cad.lcd.clear()

cad.lcd.backlight_on()

print "Content-type: text/html



<html>"

form=cgi.FieldStorage()

if "lineone" not in form:

print "<h1>Line 1 is Blank</h1>"

else:

text=form["lineone"].value

print cgi.escape(text)

cad.lcd.write(cgi.escape(text))

print "<p>"

cad.lcd.write("

")

if "linetwo" not in form:

print "<h1>Line 2 is Blank</h1>"

else:

text=form["linetwo"].value

print cgi.escape(text)

cad.lcd.write(cgi.escape(text))

print "<p>"

print "Screen Updated!<p><h1><a href=\"/\">Go Back</a></h1></html>"

The interface was coded as a hand-coded HTML file, called index.html in the /var/www directory. There is a form for a custom message to be submitted, and there is a list of URL-encoded presets, both of which provide the lineone and linetwo values to the pidisplay.py file.

<html>

<title>Raspberry Pi Door Display</title>

<h1>Raspberry Pi Door Display</h1>

Enter message and hit submit to be displayed on PiFace Control and Display.

<p>

<form action="/cgi-bin/pidisplay.py" method="POST">

Line 1: <input type="text" name="lineone" maxlength="16" size="20" value=""><p>

Line 2: <input type="text" name="linetwo" maxlength="16" size="20" value=""><p>

<input type="submit" value="Submit">

<input type="reset" value="Reset">

</form>

<p><h1>Presets:</h1>

<ul>

<li><a href="/cgi-bin/pidisplay.py?lineone=Gough: SLEEPING &linetwo=Do not disturb">Sleeping</a></li>

<li><a href="/cgi-bin/pidisplay.py?lineone=Gough: NAPPING &linetwo=Do not disturb">Napping</a></li>

<li><a href="/cgi-bin/pidisplay.py?lineone=Gough: WORKING &linetwo=Do not disturb">Working</a></li>

<li><a href="/cgi-bin/pidisplay.py?lineone=Gough: AWAKE &linetwo=Knock to enter">Awake</a></li>

<li><a href="/cgi-bin/pidisplay.py?lineone=Gough: ON PHONE &linetwo=Do not disturb">On Phone</a></li>

<li><a href="/cgi-bin/pidisplay.py?lineone=Gough: NOT HOME &linetwo=Returning later">Not Home</a></li>

</ul>

</html>

With the files in place, we can give it a go.

Mounting it and Using It

The Raspberry Model A board was mounted in the base on the old Multicomp Raspberry Pi case. This was then Blu-Tacked to the wall above the light switch just outside my door. The power lead was supplied from a manhole into the roof, where I already had a power supply (XP Power 5v 8A, on formerly on clearance from element14) running several other Raspberry Pis in the roof.

The wireless adapter used is one of my favourites for Raspberry Pi - it's a TP-Link TL-WN722N with external antenna connector allowing better antennas to extend range. It's an Atheros based adapter and it's very sensitive. I've been running this for months without any drop-outs!

Visiting the web page on a web browser provides the main interface with a form.

Clicking on one of the presets, causes the web browser to submit a GET request and sit there for a few seconds while the python file executes. Then it returns the successful status.

If we go and find the display, we will see that it has been updated.

The interface works equally well from virtually any web browser because it doesn't use anything special, HTML wise. I could easily whip out my phone just to tell everyone that I'm working.

It might look relatively straightforward, although I did have to spend some time working around user permissions to try and persuade the web server to execute with root privileges, or at least enough privileges to access the hardware directly. If you find it doesn't work, and results in blank pages, or internal server errors, you're probably running into a permissions issue. It's a lot of computing power for a display sign, which might be equally well achievable with the Arduino Yun and a LCD shield, although I had the Model A and PiFace Control and Display, so I used what I had.

Limitations

As this was hacked together in under an hour, it's clear that there is no provision to read-out the screen to determine what it is showing. None of the other PiFace Control and Display peripherals are used. No scrolling of the display is supported at this stage. The web server is being run as the pi user which has root privileges making it insecure and possibly prone to compromise in case of bugs. Validation of the text is done on the user-side for length, extreme lengths could result in unpredictable overwriting of the display. No simultaneous access management is provided - simultaneous calls to the .py file will result in a blanked display or corrupted display. No authentication is provided, although HTTP Basic should be fairly easy to set up. No automatic updating occurs, so you must keep an eye on it and update it from time to time to maintain some security. The interface is not particularly good looking, but it works.

There are probably many more limitations that I haven't thought of, but in the end, it was a quick hack that works. It's fine if you're using it at home, in a network which you keep secure and safe, away from persistent malicious actors.

Conclusion

At last, the PiFace Control and Display finds a use around the house. Now I have a door sign I can control from virtually anywhere in the world with my phone and a data connection (Connectbot SSH tunneling + Web browser). Now, as long as I remember to set it to the right status, I can inform anyone who passes by whether it's okay to come in. Now, if only my family would actually READ the sign ...