I decided I would make myself a pair of power-over ethernet modules to power an access-point for my point-to-point wireless experiment with Zhiyi. Turned out to be pretty easy. The instructions have been written up by New York City Wireless. They're pretty good, so I won't repeat them. You'll want to have a look at a voltage loss calculator to figure out what your input voltage should be.
For those not in the know about Power-over-Ethernet, it's a means of delivering power to a device (commonly an access-point or network-enabled camera) by appropriating the two unused pairs in an ethernet cable (assuming 100Base-Tx or slower, Gigabit Ethernet (1000Base-T) uses all pairs). This is useful when getting power to often awkward locations would be problematic.
As a description of the image above, the unit on the right is the injector. The DC is simply connected to pairs 1 and 4 (each pair is twisted together, thus creating a balanced wire-pair). The cable that gets plugged in between the OUT port on the injector, and the IN port on the splitter module (top left), carries the data and power. When it enters the splitter module, the pairs that carry the power are simply wired to the DC outlet leading in the access point. Similarly with the ethernet.
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