How to use PoE to power an additional device?

Question

I have an Ethernet (100 Mbps) camera powered through PoE by a PoE enabled switch. The camera is turned on and shut down by configuring the switch to cut the power on and off on the given port.

I want to have a visual indicator that the camera is on. So I did the following:

• Assembled a 9V-120V to 5V step down module, a resistor, and a LED.
• With a multimeter, determined which wires are used by the camera to get its 54V (which weren't, by the way, the commonly used wires 4 and 8, but rather wires 2 and 6).
• Connected the step down module to the wires 2 and 6.

However, it doesn't work. The switch doesn't seem to detect anything. If I unplug it, unsolder one of the cables leading to the step down module, and plug it again, the switch does detect the camera.

What can I do to find where's the problem?

Answer 1

In PoE, you don't put a voltage to single conductors, but always to pairs. The standards allow two configurations: either pairs 1/2 and 3/6 OR 4/5 and 7/8. So, it sounds like this is a normal wiring.

Connected the step down module to the wires 2 and 6.

You can't do that: in PoE you need a center-tapped transformer to extract the DC offset that the switch puts onto the wire pair, and can't directly solder to the wires.

The reason is kind of easy to explain: if you directly attach a load to either of the differential cables, you're just shorting the high-frequency signal through your load.

A switch, by design, has to detect such a "signals shorted" situation and thus disables the port, typically. Even if it didn't do that, it couldn't use the port for ethernet anymore.

So, instead of doing what you're doing, you'll have to find a supply voltage inside your camera, and then you can attach your LED to that.