Archive for March, 2013

This post will detail how I created a simple home automation project to control multiple power outlets using an Arduino Mega microcontroller.

Project highlights:
•    Reduce the amount of systems left on standby and ultimately reduce the amount of wasted energy.
•    Remotely control the power state of multiple devices.
•    Power on and off a media server by using the Power button on the front of the computer.
•    Control a collection of radio frequency (RF) devices, including single sockets and power strips.
•    Control a couple of Power Distribution Unit’s (PDU).
•    Collect and present power state information via a web page that is iDevice mobile friendly.
•    Control all the above using a single Arduino Mega with an Arduino Wiznet Ethernet Shield.

The ability to remotely manage the power states of a computer or external device over the internet can be very expensive using traditional off the shelf products, in this post I’m going to show you how I achieved this using a single Arduino. I use wireless RF and IP controlled devices for all mains power related switching so there is no risk of high voltage shocks.
This project could be recreated exactly by others but I believe that it’s more likely that people will pick and choose areas of this project and implement these into their own. The code has been made available and is fairly simple to follow due to the use of functions and comments.

As this is a physical computing project I decided to do a quick overview video to accompany this post.

This video is best viewed in 1080p HD.

Bill of materials:

# Name Price Shop
1 Arduino Mega £20 Google/eBay
2 433 Mhz Transmitter and Receiver £3 eBay: search for “Arduino 433″ or “RF Link kit”
3 Energenie Trailing Gang with Four Radio Controlled Surge Protected Sockets £20
4 Opengear IP PDU ?
5 Breadboard and a few jumper cables Google/eBay
6 2N2222 NPN bipolar junction transistor £1

Step One – Directly connected PC
I thought it would be interesting to use a remote method to power up and shutdown a PC by directly connecting to the power button. I opened the case and popped the power switch out, on inspection it was clear that this is just a little push switch. I cut into the cable and added an electrical block connector and gave a quick test to confirm I could power it on and off by simply connecting these two wires. As I wanted to have an Arduino create the connection I decided to use a 2N2222 transistor as I had a couple of these already, you could just as easily use a relay. The connections were as follows:

Pin (flat side to front) Name  Arduino
E – Left Emitter Ground
B – Middle Base Signal (from Arduino)
C – Right Collector Power

I used a 10k resistor between the Arduino and the base of the transistor to add a little protection to the Arduino. You can see from the code that I connected the base pin to pin 8 on the Arduino. Pin 8 need to be set to OUTPUT (pinMode(8, OUTPUT);) and then I to simulate a button press I used the following code:

delay(500);               // wait for half a second

To check that the operating system is up I wanted to send ping packets and report on the response, I found a ping library at Blake created a similar project to this step and I encourage you to check out his site.

Step Two – Energenie RF Power Sockets
This step has been covered in separate blog posts so as not to repeat myself I will provide the links below.
Capture the RF codes for your Energenie Power Strip
Configure the circuit like I did in the following post, the code/sketch is similar with the most obvious change being the use of serial as input over web which I’m using in this project.

Step Three – IP PDU’s
Again, this step has been covered in a previous post – As I needed to have the Arduino do the authentication to the IP PDUs I needed to form an HTTP GET request with the username and password encoded. I used the website which allowed me to encode the default credentials, for example
Concatenated with a colon join: snmp:1234
Base64 encoded string: c25tcDoxMjM0

To complete the example, below is a code sample of one of the functions I use to collect state information from the PDUs. This clearly shows the use of the authentication code:

client.print("GET /");
client.print("Host: ");
client.println("Authorization: Basic c25tcDoxMjM0");
client.println("User-Agent: Arduino Sketch/1.0 snmp@1234");

Step Four – The Web Portal/Server
Writing a web server for an Arduino is challenging mainly due to the amount of RAM available. Originally I started this project with an Arduino Uno which is limited to 2K, I maxed this out almost immediately* so I decided to get an Arduino Mega with 8k of RAM. Using the Simple Web Server sketch provided in the Arduino IDE as the starting point I created an HTML table to display the power buttons of the Energenie power strip, this code used up a lot of the available RAM. When I started to add further table rows I soon realised that I would not be able to fit the desired 21 I needed using this format. The solution I used is to have the unique information about these devices within an array and then loop through them to build the page for each client request. This made the code a little more complex but allowed massive reductions in the amount of code I needed to store within RAM.

* see the freeRam code block at – for an easy way to see memory utilization

Using the original web portal interface for the IP PDUs it required multiple actions to change a power state which I didn’t want and as I have two PDUs I needed to log into each one individually. This should not be considered as a design issue with the devices it’s just that they were not designed for such frequent use or outside a server cabinet for that matter!


Instead I wanted it to be mobile friendly so I needed large buttons that were colour coded to their respective devices power state, the below screenshot is taken from my iPad


You will notice that the page fits perfectly; this was achieved by adding this line to the HTML page

<meta name="viewport" content="width=device-width" />

It basically resizes the table to fit to the mobile devices landscape screen resolution and works well on all my iDevices.

In effort to keep the code line count and size down you will notice I created three functions within the Arduino sketch
1.    PDUWebStatus  – Enumerate power states on PDUs
This code queries the current power state of the PDUs and stores the results in an array for use by the aforementioned HTML table loops.
2.    PDUWebCommand – Issue power state changes on the PDUs
This code actions PDU power state change requests.
3.    PDUWebCommandPush – Collect current power state and rotate
This code uses the above two functions to action the HTML button pushes.

Whenever a user presses one of the HTML buttons they call a small javascript function that then calls the PDUWebCommandPush function mentioned above. This makes the users browser send a GET request to the Arduino which then gets matched to one of the predefined actions, for example Energenie Socket 1 On button would trigger the following action

if(buffer.indexOf("GET /?E1=ON")&gt;=0) {
mySwitch.send(4314015, 24);
refreshRequired = 1;

You will also notice that this sets the refreshRequired variable, this is used to force the client back to the webservers root directory and allow the changes of power states to be show on the page. I found that redirecting the users back to the root also prevented the refresh button on a browser sending an additional GET request to an already completed request.

At the time of writing this blog post I only have a single Set as listed on the above image as “Set – 1” but I have plans to add several more. Sets allow me change the power state on multiple devices at once even if they differ power source and/or control method. I have used this to power on my desktop computer and all associated monitors, this is shown in the demo video which accompanies this post.

A note on security
I have controlled access to the Arduino web server using the firewall on my router. This has allowed me to restrict access to my place of work and my internal LAN only.

The Code
The Arduino Sketch can be found over on my GitHub

Well I hope you’ve enjoyed reading about this project as much as I enjoyed putting it together. Please use any of the ideas and code in your own projects but please do give credit to the original author.

Kind Regards