RJ-45 cable tester

Though a cable tester is actually a simple device, it can save you a lot of time compared to using an ohmmeter to test the wires one by one. I stubbornly decided to build one myself, even though very cheap testers can be bought nowadays.

This cable tester can test continuity and short circuits in wall cabling and patch cords that have RJ-45 connectors. It consists of two units:

• A battery-powered sequencer to connect to one end of the cable under test,
• A read-out box with 8 LEDs to connect to the other end.

Two units are needed to perform and end-to-end check of the cable and assess continuity of all 8 wires individually.

In the autumn of 2007 I replaced all the old phone wiring in my house by Cat5E cabling, suitable for Ethernet. I tried the cabling by connecting my laptop and my network router. Unfortunately, a few cable runs did not work, and it wasn't clear why. I also had some unreliable Ethernet connection cables lying around.

I could use my faithful ohmmeter to test the cables, but that really costs time. To do the job efficiently, I needed a cable tester. Of course, it would be cool to do a reflection test to localise shorts or subtle cable faults, but most cable problems are simple continuity problems that can be tested more easily using a continuity tester. I decided that I needed a circuit that connects a voltage to one of the wires in sequence at one end and a display unit that shows which wires are selected at the other end. Perhaps I could think of a solution where I could have the display unit at the same end as the sequencer, but then I would still have to connect some kind of terminator at the other end. The bottom line is, if you want to do an end-to-end test, you will have to onnect equipment to both ends of the cable.

I came up with a circuit using 7 LEDs at one end, using one of the wires as a return (“-”) wire. That left me with a problem: what if the return wire is interrupted or non-existent? It is not enough to know that the return wire is interrupted, as the cable can have more than one fault. I thought of a few ways to split the test in two, using different return wires. But I found that unelegant.

I almost went to buy a simple cable tester. These are available for less than 10 Euros. But no. I wanted to build one myself. That is more fun.

Then I decided to do a websearch to see if others had had these ideas. On SB projects I found a circuit that was very similar to my idea, designed by San. But he had also found an elegant solution to the return wire problem: he used all of the unused wires as a return, putting them in parallel. I thought of ways to improve the circuit, but decided to hurry up and build it. After all, I had a cabling problem waiting.

I used a small plastic case for the display unit. On a piece of perfboard, I mounted 8 green high-efficiency LEDs, peeking through 8 holes in the case.

In order to fit the display unit in a very flat case, I did not put an RJ-45 receptacle in it. Instead, a piece of flat UTP cable is protruding from the box, with a male RJ-45 connector at the end. The sequencer is housed in a slightly larger box. It contains an oscillator circuit and a 4017 decimal counter-decoder. Every pin of the RJ45 connector is connected to one of the outputs of the 4017 decoder through a resistor, allowing the 4017 to sequence the pins. The display unit can be connected to the sequencer directly. A piece of RJ-45 cable and a female-female connector are needed to complete the test set and test wall cables as well as patch cables.

The tester is really easy to use. Connect the sequencer to one end and the display unit to the other end. If everything is OK, you'll see all 8 LEDs blink sequentially. If one LED remains dark there is an interruption. If two LEDs light simultaneously, there is a short (by the way: in case of a short, the LEDs have to share the current so they are dimmer than normal). If the LEDs light out of order, two or more wires have been swapped. There is always a small pause (during 2 of the 10 counts of the sequencer, no wire is activated) so you can see which is the first LED in the sequence (the first one after the pause). One of the two remaining outputs of the decoder is used to drive a yellow LED on the sequencer box to indicate it is on. Flashing it instead of having it on continuously saves batteries.