For anyone who needs to test Ethernet cables often and likes to create his own Testing device this is a good way to do it.
On this page i will describe how to make a simple tester to see if all wires are working good. It’s simple and takes only like 20 minutes to make.

Materials you’ll need:
– ISDN (or Ethernet) surface outlet box
– 3x RJ45 Ethernet connectors
– 1x LED (color of your choice)
– 1x +/-680 Ohm resistor
– 1x 9 volts Alkaline powercell (or other battery)
– 1x Powercell connector suited for your battery
– +/-50 cm UTP cable, preferably unshielded

Tools you probably need:
– Flat screwdriver
– Cross screwdriver
– Soldering iron + solder
– Wire cutter
– Knife (sharp!!)
– RJ45 crimping tool
– 5mm drill + drill

– Ethernet inline coupler (from your local electronics store)

We will be making a loopback plug, a small ethernet cable and use the ISDN box to hold the powercell and LED.

because ISDN boxes look very different in every store and differ per brand you might need to find another suitable box or casing. using an ISDN box is more easy because the RJ45 jack is already mounted in it.
Make sure the casing is big enough to hold the powercell and a bunch of solder connections and whatever else you want to put in there.

Mine looks like this:

As you can see the battery is a perfect fit. Up front is the RJ45 jack and on the left of it is a indicator LED.

Here is a simple wiring map of the ISDN box:

Solder where necessary. When you solder the LED and resistor be sure to heat up the Soldering iron and turn it off before you start the actually soldering. This because when you use a very powerfull (or hot) soldering iron it might overheat the components and destroy their inner workings.
Isolate the soldering couplings and cover the resistor entirely with isolation stuff. I used rubber tubing. Other materials to use can be tape, plastic wrapping or cable shielding.

Now fit everything in the ISDN box. This you have to do on your own creative way as every box is different. With the knife and wirecutters remove any plastic debree and rubbish inside making the box empty. Only leave in the parts you need. Like in my case a screw hole for the cap and some walls for the battery and RJ45 jack.
I drilled a small hole next to the RJ45 jack for the LED and glued it in.
Also for the powercell connector i made a little hole stuck the wire though and made a little knot in the wires so they cannot get pulled through the hole. This prevents accidental pull on the soldering and other components. Make sure you have enough wire to the connector so that the battery can come out and you have easy access to it…

The end result could look a bit like this:

Here’s how to make the loopback plug:

To achieve this cut like 10cm of the Ethernet cable and strip the outer isolation layer completely. This should leave you with 4 pairs of wires. Pick out the 2 colors you want and use those pairs for the loopback. Just bend them back in and use the crimping tool to make the connector.

The result could look like this:

And finally the small ethernet cable.
This is a regular straight through ethernet cable, but shorter.
Use the crimping tool and the remaining 2 RJ45 connectors to make this cable.

On both connectors, from left to right with the little flap on top and the cable hole facing backwards use this order in the wires:
1 – Orange/white
2 – Orange
3 – Green/white
4 – Blue
5 – Blue/white
6 – Green
7 – Brown/white
8 – Brown

It’s easiest to make one connector at a time.
So strip one end of the cable for about 2 cm with the knife. Untwist as needed and align the wires as described above and cut them at 1cm straight with the wirecutters and make the connector on it. Repeat this for the other end. Of course cut the cable at your desired length.

And when done it should look a bit like so:

And the final result will be:


This article has 7 comments

  1. HansB

    Neat, but what is the test procedure?
    You have the Lan tester, the inline coupler, the loopback plug and the short ethernet cable. Then what do you do? I need to test a 15 m cable which is mounted through my house, preventing me from getting access to both ends simultaneously. Is there any way your equipment could be used to test the cable?

    • Arnan

      That depends a bit on what you want to test. However to see if the cable and your connectors make a good contact you can use the article here. Simply connect your cable to the tester, connect the loopback to your cable (the other end) via a inline coupler. If the led lights, then your cable is fine as in that it makes connections on all 8 wires.

  2. Jeff

    So what if there is a short in the cable? Your cable will test OK- no good.

    • Arnan

      True, i realized that too, some time ago. It’s still a good measure to see if your connectors are good though.

  3. andrew

    if you added a LED to each pin connection in the ISDN box (2-3, 4-5, 6-7) wouldn’t that let you know if it’s shorting? That way if one is shorting you’ll be missing a lit LED? or am I looking at this wrong?

    • Arnan

      That indeed would give a better measure for the per wire pair.
      Even better would be to put the LEDs on the other end, using each pair as a plus/minus with a led on the end.
      So 4 LEDs total.

  4. abdullatheef

    we need to kno how we can put 8 led