Some notes on a cheap 4-Bifferboard PoE setup. This can be used for remote sensing, webcam applications and so on.
I make no assertion of reliability of any of the mentioned Ebay sellers as I haven't ordered from them. You will have to check out their feedback and decide for yourself. I'm only outlining what you need here, the costs involved and possible sellers of the gear.
First you need a ethernet hub:
Next you need a high voltage power supply. I decided to standardise on the 5.5/2.1mm power jacks used for CCTV applications to reduce the amount of soldering I needed to do. They look like this:
They are not the same as the Bifferboard power jack, which is 3.5mm. The 5.5/2.1mm jacks are quite common but there are some non-standard 'internal' diameter sizes out there so be careful. If a seller doesn't state all three dimensions best to double-check with them.
To supply 4x Bifferboards requires 1.2A x 4 at 5V, giving 4.4 amps at 5v. Assuming 80% conversion efficiency this equates to 0.73 amps at 24v, so a 24v 1A PSU would be sufficient, however I decided to get a 2A PSU to ensure I had capacity for expansion. Seller mie-cctv  is a UK supplier of these PSUs, and they look like this:
Cost is £10.49 for the 2A version.
Next up, we must split the power four ways with a splitter, also available from the same seller:
The link is here:
The cost of injector/extractor pair is £2.95 (free postage). The injector is on the right, extractor on the left. Total cost for 4 ports gives £11.80
Finally, we need to drop the line voltage of 24v down to 5v to power the Bifferboard, and we can use a 24v->5v converter from emarket4un:
Cost is £2.60 with free postage, making the cost of four £10.40.
You can see this has a screw terminal for the 24v input, and USB Host (A) connector for the 5v output. To connect the PoE extractor to this device requires a 5.5/2.1mm jack with fly lead, fortunately this is supplied my mie-cctv:
Cost for 4 of these is £3.94.
And lastly, you need to connect the USB 'A' connector on the converter to the Bifferboard itself, which requires one of these:
You can obtain these from rfbat, cost for a pair is £4.32, although I'm sure you can probably find them cheaper. Cost for four would be: £8.64.
So in total this setup will set you back £46.85 for power to four Bifferboards. That's £11.71 each. Don't forget it replaces the cost of four Bifferboard PSUs, and if the units are sited outside could save you quite a bit for underground armoured mains cable (and the glands to connect it up), not to mention being a lot safer.