Can I easily get DC out from a PoE AP?

Hardware Questions and Recommendations
1

It's been awhile since I've used OpenWrt and I'm looking for a good outdoor AP to start deploying. I want to use PoE and I need to hang a microcontroller off each one with minimal equipment and complexity.

I can imagine that an AP that supports both PoE and direct DC-in (through a barrel jack) might not isolate the two power inputs. This would allow me to pull some power out through the DC-in port. (Or I might be able to short a diode to get what I want?)

Are there OpenWrt-compatible APs that would give me "power out" like this without modification? It's not something I'm likely to find in documentation unless it's through a "Do not use PoE and DC-in at the same time!" warning.

Next, how much power can I safely pull? I'm guessing that if, for example, the AP uses 12W and supports 18-57V on the PoE, I might be able to use a few watts if I stay on the high end of the voltage.

I'm hopeful that this is not such a weird request and people have been doing it forever. I'm not finding any mention of it, though. I welcome pointers.

Thanks!

--kyler

2

There are PoE splitters, where you could use a Y-cable, to get two DC plugs.

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3

I would buy a real dedicated by design outdoor AP.

First of all the device must work with the environment factors like moisture, rain, hot, cold, sun etc.

It need over voltage protection for outdoor use to get some protection from lightning storms. That includes also a TFP ethernet cable.

And it has POE input to begin with.

Think we have some of the TP-link EAP outdoor models in the ToH.

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4

Are you trying to power the microcontroller physically from the same PoE source and cable as the AP?

I don't believe that any PoE powered APs that also have DC-input would pass voltage through from PoE to the DC barrel jack. There would be, at a minimum, blocking diodes to prevent this.

I think that the most straight forward approach would be to run 2 ethernet lines to each AP. One of them goes to the AP itself (and per @flygarn12 's comments, if you're using this outdoors, get a properly rated purpose-built outdoor AP). The other one could go to a PoE extractor (ideally one that takes an 802.11af/at input; although you can get some that are simply passive injector/extractor pairs, but those aren't as good).

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5

You're about electrical engineering and modding territory. Or get an AP that is indoor and has a USB port, or has PoE passthrough too? (indoor AP's then do your own enclosure?) I'm not aware of outdoor AP's which have USB ports or GPIO stuff....

I'd suggest get a PoE powered ethernet switch with PoE passthrough as well. Then use a converter on another switch port for the microcontroller. You can get outdoor ones that are for CCTV, or use enclosures.

Another option is use passive PoE splitters or active PoE splitters then split the DC and then use the DC input into the AP.

Is your environment condensing, freezing, 40 degrees C +, in the sun? Is there salt water spray? Once it's outdoor things get hard.

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6

Having USB would be ideal - better than Ethernet if it has data, too. I recall having APs with USB that I used for 3G data sticks, so I suppose they're out there but I hadn't considered it.

I was hoping to find an "outdoor" unit so that I wouldn't have to add an enclosure, but it's sounding like I'm likely to return to doing that. I'll need it more space anyway in some situations for solar/battery installs.

Thanks for the ideas.

7

How tiny a microcontroller are we talking here? Will you be placing the micro inside the AP enclosure or outside?

Depending on how much juice the microcontroller draws, perhaps you could just hook it up to the console pins that most routerboards have internally?

Here's a random router from the table of hardware:

img_4843
img_4843960×1280 330 KB

Look closely above the yellow plug, there is a 4th pin.

I bet that the pin above "RX" is either 3.3 or 5 volts, and that you could draw a tiny amount of power from it.

Here's another one:

gs1900_serial3
gs1900_serial31618×805 186 KB

That particular one has a more clearly marked VCC on the console pins.

You'd have to browse the ToH and find a device which actually have those 4 pins exposed for the console.

:person_shrugging:

8

VoCore makes some small Mediatek-based devices that run OpenWRT, 32MB flash / 128MB RAM.

PoE-powered and lots of GPIOs for the sensor, but only 802.11n (wifi) and 802.3u (eth).

Don't know, might be worth a look.

https://vocore.io/v2u.html

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Yes, I want to embed the microcontroller, perhaps in a custom lid with a bulge. JTAG is an interesting source of power.

I got a little frustrated thinking about it so I bought some BE200 cards. I'm guessing it'll be easier to roll my own solution than try to modify something that's out there...which will then be discontinued.

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An SBC/network dev board and rolling your own is certainly an option. IMO I'd go for something like a reference board or SoM board that's already supported. I've been looking HLK-RM65 as I had good experiences with the HLK mt7621 board and SOM. (i.e. hi-link boards) but Banana Pi might be a good option.