First off, kudos to Psyber for https://www.instructables.com/id/Ethernet-link-tes...
He's the one who came up with this brilliant idea. I have hopefully made is easier to do and present this as an improvement.
I'd been looking for a way to test whether or not a given network port was live since 1997, when I got yelled at by a scientist at EPA/NOAA when his network connectivity suddenly went down. His computer was in a hutch with a short network cable so I couldn't pull out the PC to see link lights on his NIC card, the wall jack was behind the hutch so I couldn't pull it out to get to the jack. Finally the network guys admitted they'd been working in the closet and must have disabled his connection. I might have suspected this earlier, but I was new to network support at the time (my previous job didn't have a PC network) and when I asked for help, testing the network connectivity was not suggested by my supervisors.
After the dust settled, I asked if there was a way to have tested connectivity and was told that only the network guys could handle the Fluke tester. We desktop support folks eventually got permission to use it for testing after I raised enough hell given this incident, but I wanted something small and inexpensive to test basic connectivity. It sure seemed like a good idea and simple enough to do, but I never found anything until I stumbled across Psyber's Instructable sixteen years later.
I ordered the MAU recently off of eBay, but despite having handled a soldering iron on and off since the 70s I wasn't real happy about desoldering fifteen pins off a circuit board. When I got the MAU, I also found that while Psyber's was in a plastic case, mine was sheet metal, and drilling holes through that wasn't going to be much fun either. So I noodled out the alternative described below, and it worked on the first try.
Step 1: What You'll Need
(1) MAU; I was more familiar with and so bought a Centrecom 210TS for $7.99 new on eBay.
(1) 15-pin female D-connector and shell. Be sure to get a two-row female connector that fits the MAU and not the three-row VGA-style connector. I bought mine for $4 on eBay.
(1) A23 12-volt battery; two of them cost $3.74 at Wal-Mart.
(1) A23 battery holder; I bought five in a pack for $3 on eBay.
Soldering iron and solder.
Step 2: Assembling the 15 Pin Female D-Connector Power Pack
Take one of the A23 battery holders, and solder the black ground wire into socket 6 of the D-connector, and solder the red wire into socket 13. Remember that as you're viewing the soldering side of the D-connector, the ground wire goes into the third socket from the right in the row of eight sockets and the red wire goes into the third from the right in the row of seven sockets, as seen in the diagram.
Assemble the shell around the D-connector, with the battery holder outside the shell.
Use the Permabond to glue the battery holder to the shell.
Insert an A23 battery properly into the holder.
Step 3: How to Use
Plug the newly finished battery/D-connector into the plug on the MAU, You should immediately see two lights come on as shown - the Power light and the Polarity light. (Don't worry about the Polarity light - (MAUs are apparently usually used in pairs, and that light comes on if it doesn't detect the other MAU. It's nothing to worry about in this application. And yes, I had to go look to make sure everything was okay the first time I tested it.)
Just clip in an Ethernet cable connected to a network. If that cable is live and capable of delivering network to a computer, the Link light will light up.
Don't forget to pull the battery/D-connector out of the MAU when you're done or you'll run the battery down. I thought about adding a momentary switch but there isn't much room for it. But the battery basically can't get shorted out since the D-connector has sockets instead of pins.