There's something of a gap in the technology market still when it comes to powering embedded devices, such as sensors, etc.
You have a few options for that networked sensor node you just built.
You can invest in battery cells, either primary or rechargable. Not only is this an additional initial investment, but you then have the added service burden of eventually needing to change them out for freshly charged once depleted.
Perhaps if your power budget is suitably slim, you could enlist the services of solar, or try out some cool thermoelectric energy harvesting techniques to keep your gadget ticking, but that's not going to cut it for larger loads, such as LEDs or electromechanical actuators, with their high-current drain characteristics making short work of any stored energy within the system.
Of course, you can always run some additional power wiring to the node, but then you might question why you specified wireless connectivity in the first place. After all, if you're going to be constrained by needing a copper connection for your power, you could always just run another wire alongside to cover your additional needs. Copper cables carry considerable costs however, so that's not always an attractive option!
But, if you've got a Feather project that's sat on Cat5 for the backhaul, you might find the PoE FeatherWing from Patrick Van Oosterwijck is the answer to your problems!
For those unfamiliar with it, PoE stands for "Power over Ethernet," and it's a nifty way to utilize the unused copper pairs of a standard ethernet cable and RJ45 connector set, to pass power along with data, all within the single connection.
You are free to use conventional Cat5 cabling that you likely already have meters of, kicking about in the parts bin, and for a "passive" PoE setup, simply splicing out the unused pairs to to a DC source on one end, and your low-powered device's input on the other, technically, counts as PoE. Cables, like the ones pictured below, do exactly what they look like they do — there's no "brains" in that cable, and in my opinion, it's a no-brained solution
You can imagine the limitations this passive approach poses. Trying to simply pass DC over these unused pairs will quickly result in power losses, with current flow being crippled by the cables resistance.
Then there's the "dumb" nature of the just forcing DC into a cable, leavening any fault protection down to whatever power supply is Heath Robinson-ed onto the other end. Are you sure you want to trust that sort of lash up to a sensor that might find itself stowed away in your drywall? While hammering a nail through such a cable isn't going to trip a breaker, it instead can sit there, generating significant heat from a dead short across the rails.
It's safe to say that there are some interesting... adaptations of what it means to be PoE within the electronics world. Some are safe, some are... not quite sure what that word means.
It's quite pleasing to see this polished approach from Van Oosterwijck; his PoE powered board is a professional approach to powering your projects, and it's even being put forward as a product offering, soon available for your purchase!
While we've recently touched on a very nicely designed PoE powered device design from Glen Atkins, which is well worth a look, Van Oosterwijck has managed to cram the same functionality, albeit with slightly less power into an impressive amount of space
Designed to give your more permanently placed projects and prototypes a single port of call for both data and power, this handy dandy part takes the form of a Wing, the Adafruit Feather form factor expansion board format of choice.
With interfacing to the WWW handled by a extremely well supported wonder-chip, the WIZnet W5500, this board is able to make use of the widespread availability of pre-written code and libraries, which means you'll never be left wondering "how do I wire this thing up?"
We can see the board happily waving hello to the web with Adafruit's example Arduino sketch below.
The thing is, there are already W5500 wings, and ENC28j60 expansion cards. Where this design really shines is in its implementation of the 802.3at PoE standard!
Not only has Van Oosterwijck managed to identify a PoE PD interface that supports a appropriate amount of power for this level of application (13W!), he's also managed to cram this convertec, complete with the ferrite-cored transformer, into the few square cm available on the front face of the Feather, having already lost more than half the board to the RJ45 jack and W5500 parts!
That doesn't leave much space for a converter and some sizeable magnetics, as we can see from the highlight below!
But the TPS23758 from Texas Instruments tucks neatly away in to the tiny amount of remaining board real estate, thanks to the 0.4mm pad pitch on its 4x6mm VSON package.
That's a tiny part, but it's truly a tribute to what can be achieved with some well thought tracking and layout. With its additional corner pads jumping onto a non-typical pitch, there's some effort required in laying out the part footprint that already goes above and beyond a simple SOIC pad set, but we'd say that the total footprint of the PoE implementation makes that all worthwhile!
What is more, the TPS23785 allows for a fully isolated converter design, with regulation managed through feedback from the primary side of the magnetics, removing the need for the typically chunky optoisolators that are often associated with isolated design.
Although there are smaller packaged converters on the market, the integration offered by the TPS23785 leads to a minimal amount of additional parts being required to implement an isolated PoE device, which can result in a smaller overall total component footprint
With a rated power delivery of approx 13W, some back of a beermat math says the perfect implementation would maybe give us 2A, at 5V, accounting for a conservative amount of losses.
However, this level of space-saving layout, has an effect on the circuit operation, as does the reasonable choice of parts. Normally, such a converter would specify transformers with coupling coefficients optimised to provided the required feedback. In a bid to keep costs down, Van Oosterwijck is using a more generic part to get the job done, and this also has an effect on efficiencies.
Some load testing indicates that a modest 1A will sag the output of the converter to approximately 4V. 4W is plenty enough for even the most power sapping of sensor nodes, so I think this is a pretty good result for this tiny little module!
In order to make sure there's no mixup of MACs, Van Oosterwijck has chosen a sure-fire solution that will make certain there are no cases of confused identity: the Microchip 24AA02E48.
Effectively a 2Kb I2C Serial EEPROM, the special feature of this part is that it comes with a factory pre-programmed EUI-48™ MAC ID, guaranteed to be unique from any other. This hardware based solution means that the assembly and setup of this board is purely hardware — there's no need to program on test or otherwise. A small price to pay to avoid what would otherwise be a purely electrical test procedure.
Speaking of colors, I'm a massive fan of the yellow silk on matte black silk screen that has been chosen for the production run of these boards. Complimented by the gold finish on the plated through hole pads, it's a gorgeous looking board!
With a pre-production batch of assembled boards in hand, Van Oosterwijck is in the process of performing acceptance testing and validation of the design. I can't wait to see these for sale, and the resulting projects that people come up with them!
Be sure to follow along with updates from Van Oosterwijck and the PoE FeatherWing over on the project page!