Have you ever wanted a pocket-sized device that could tell you if a network jack was live or not? [TanzerGuy] did and he hacked a piece of old networking gear to do the job.
Today when you think of Ethernet, you probably think of CAT-5 cable or something similar. But it hasn’t always been like that. In the early days of Ethernet networking, an Ethernet cable was a big piece of coax. A media attachment unit (MAU) clamped to the cable and then connected to an attachment unit interface (AUI) that resided in the actual network card. Later standards used thinner coax that attached to the card using a Tee connector, but even these are rare today.
However, there has been a significant demand over time for compatibility between new network gear and old network gear. That means you can find inexpensive surplus MAUs that convert from AUI to modern network cable in the usual places you find surplus (like eBay, for example). These cheap adapters have a network link light on them and all the logic required to drive it.
All you need is a 12V source. [TanzerGuy] had seen an earlier project that used an MAU in a plastic case and did some serious surgery on it that involved drilling and soldering. His MAU had a metal case, so he opted to get a DA-15 connector that fits an AUI connector (despite having 15 pins, these aren’t the same as a VGA connector, which is a DE-15; both are often erroneously called DB-15s). A 12V battery and clip soldered to the DA-15 socket allow [TanzerGuy] to plug a cable into the MAU and determine if the cable is live or not. Including the batteries, he spent less than $20.
If you are interested in learning more about old style Ethernet, [MattMillman] did a 10Base5 project “just because.” Of course, you could carry around a little travel router (or a dedicated tester) to test cables, but what fun is that? If you think CAT5 or CAT6 cable is as good as coax, you might need to consider the application.