This how-to makes a handy tester that lets you quickly identify if your connection or Ethernet cable that you just made is going to link-up with your network.
If you work in a corporate IT environment you know there are generally more network ports in the building than there are ports on your switches. This means that not all ports are patched in or linked and can be troublesome when setting up a new workstation. Also most managed switches have a feature that allows you to selectively disable ports, so even if the port is patched in it may not be active.
This handy little pocket sized device will allow you to check a port for a link state without having to run down to your wiring closet or having to log into the switch. We use these when prepping server room installs to ensure the networking is good to go when the server may not show up for several weeks or months. (Special thanks to Scott Glick for finding the time to actually make and document one of these)

Step 1: Gather the Goods

Don't throw those old AUI devices out just yet!
You will need :
1 AUI to 10baseT adapter (I have 2 pictured because different models are slightly different)
a soldering gun or pencil
solder
some solder wick or de-soldering tool (wick works best)
1) 9 volt battery
1) 9 volt battery connector
a small length of 18awg wire
and small momentary on switch. These can be had at your local Radio Shack store
a drill and drill bit sized appropriately to accommodate the switch

Step 2: Disassemble and Prep Case

The first thing to do is remove the screw on the back, it`s probably underneath the label.
The case should split into two pieces.
Take the top half (the half where the status lights are located) and find an area to mount the switch. Be sure to allow for clearance between the switch and components on the PCB so everything will go back together easily.
Drill your hole and mount the switch.

Step 3: Melt Some Metal

De-solder and remove the 15 pin AUI connector. (This is where the solder wick come s in handy)

Step 4: Solder

Solder the red wire (+) from the battery connector to one side of the switch
Solder a short length of wire to the other side of the switch (white wire in the picture)
Solder the unconnected wire (white) to the pad where pin 13 of the AUI connector was
Solder the black wire (-) from the battery connector to the pad where pin 6 of the AUI connector was

Step 5: Reassemble and Test

Reassemble the two halves and put the screw back in. Plug in a 9v battery to the battery connector.

Ready to test:

Press the button.
The power light should light (if not try another battery and check you connections)

Plug a short length rj45 patch cable into your switch/patch panel port/wall jack and plug the other end into the link detector and press the button.
The power light should light and if you have a link the link light should light.

Since the AUI adapter is designed for 12V a possible more compact alternative to the 9V is an A23 12V battery. It might actually fit inside the case if you have the right model of adapter and are handy with a dremel tool and soldering iron.