How to run your Raspberry Pi from POE for under £15
I am currently setting up a sensor network to monitor and optimise a data centre as my MSc dissertation and hence had a need to run a number of Raspberry Pi powered sensor nodes off POE enabled infrastructure grade switches. Since POE support exists in various forms for Arduino I had made the fatal assumption that this would be available for the Raspberry Pi platform as well or at least be quick to copy from some pain-free cook book. And so I embarked on the exciting challenge to source POE support for my Pi nodes with manufactured parts behind a scalable supply chain only (<- no soldering is what I mean) and for a target price of under £15.
2. The Solution:
Since I had ruled out batteries and 240V wall wart power supplies as a power source for the nodes POE seemed the only feasible way forward. I searched high and low for something that would offer a USB port from POE but without success. Home-brew type POE systems are well described but I have to build this on top of an existing corporate grade POE infrastructure switch layer so addition of home made POE injectors was out of the question. To keep the system scalable and maintainable by staff without knowledge of handling a soldering iron (good luck finding a sysadmin with that skill these days) I had also decided to pick manufactured components with standard connectors only.
First break through came in form of the TP Link POE splitter which I found by pure luck while shopping for a new IP-webcam. This delightful little box converts down from POE 48V to either 5, 9 or 12V (adjustable with a little switch) and, after sucking out the POE supply voltage, passes through the network connection to another port on the other side. Next to the network out port it has a little barrel DC connector with exactly the type of socket found on the Arduino which supplies the chosen 5-12V DC voltage.
So far so good, I had successfully converted down from POE to 5V so I set out to find some adapters on Amazon. Once I had identified the type of barrel I was working with (the Arduino documentation gave it away as a 2.1mm) it wasn’t too hard to find an adapter from said 2.1mm barrel to a standard USB plug. As far as I know these sort of adapters are/were used to charge older mobile phones off a USB port.
Almost there, just needed to get down to micro USB which is the type of socket used for the power socket on the Raspberry Pi. Guess this stage would already have been sufficient to run the Pi by simply plugging this USB plug into one of the Pi’s USB sockets but I usually try to keep those free to hook up a USB keyboard and mouse in emergencies. The last missing link in form of a USB to micro USB adapter was easy to find. Biggest challenge was to find a nice one that would not come from Hong Kong for £9.50 shipping each…
Right then, all together this is what the result looks like. I’ve successfully tested this today with a recent Pi model
3. Shopping list of links for what I bought to get the above:
StarTech 3 feet USB to Type M Barrel 5V DC Power Cable
Link to Amazon
USB 2.0 A female to Micro USB B male Adapter Cable
Link to Amazon
Unless I’m mistaken this adds up to £14.80 for each set and is hence within my envisaged £15 per set budget