DIY LAN Tester for cable that runs long distance

Keep it Simple, Stupid
Do it Yourself LAN tester!

I had a LAN cable not working and it went from one floor to another. So any testing was impossible without any proper gear. And who really needs expensive gear for just one occasion? So I thought up a simple and fast DIY tool to test LAN cables over distance and it takes about 3 minutes to put together. I have never looked closely to a LAN tester so they can be working differently or with the same principle. Probably they work better as they detect the wrong wires and all that jazz, but hey, they cost money 🙂


  • Empty 9V battery (yes, empty)
  • 9V battery wired plug
  • RJ45 plug (without transformers inside)


  • Any Cheap-O-Multimeter (Voltages)
  • Soldering iron
  • Solder
  • Possibly some piece of wire needed

Let’s start by looking at what LAN cable wires do. If you expected more info on lan cables, you should check this page on CAT5 LAN cables.
So there are 8 wires in a LAN cable and if one of them is in wrong place or the wire does not have a A to B setup or has at least one cut wire, the cable does not work.

First was to test if the wires are not cut. So I came up with this idea:
LAN tester idea

On one end of the cable would be a simple RJ45 jack with an 9V battery. The battery doesn’t need to be full even, a empty battery will work as you only need some voltage across the cable. How you connect to the wires is not important really, only that you know how it is done and you can test it from other end. I made the connection so that from the testing I could simply test 4 positive and 4 ground pins easily.

The RJ45 plugThe (+) pins were bent to one side and (-) pins were bent to other side and the battery wires were connected to them.

Empty 9V battery read 5.59V but that is enoughThe 9V battery was thrown away but it holded still enough volts and was just good for testing the LAN cable. So use the battery to the max!

final productAnd the final product!

And thats it! I used a short red CAT5 cable to test the pins and then I went to test the broken LAN cable that was drilled through a floor. The result: two wires were broken.

OK, so my idea does not test the different wires if they are connected correctly, if the wires are switched for PC-SWITCH configuration or is directly connected for PC-PC setup. Or if you make the LAN cable yourself and mess up some wires. Well, the wires are color coded so you can run that part of LAN testing by eye 🙂

My cable wires were correct. I noticed that some wires were bare near the cable sleeve and I found out that the LAN cable crimpers actually cut into the wire when I cut the cable to right size.

If this LAN tester helps you out one day or you have ideas of making this testing to even simpler (over long distance), let others know by leaving a comment below 🙂


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