UTP Cable Tester
This design is more than 10 years old by now.
Although it will still work, I recommend you to buy a ready made cable tester, unless you enjoy building one even if it'll cost you much more than one you can buy ready made.
Nowadays cable testers like this can be bought for as little as €10, give or take a little.
That's far less than you'll have to spend on parts allone, if you want to build one yourself.
The UTP Cable Tester can be used for many purposes.
Mainly to test a UTP network cables of course.
However it can also be used to find the right cable in a large bundle of identical looking cables.
The UTP Cable Tester consists of 2 tiny boxes that have to be connected to each end of the cable under test. One of the boxes contains a signal generator, powered by a standard 9V battery. The other box contains 8 LEDs that indicate the cable's condition.
The principle of operation is very simple: A good cable will show a single walking light. However when the lights are lit out of order you'll know that some wires have been switched in one or both of the connectors. If one or more lights don't light you'll know that one or more wires are cut. If two or more lights light up simultaneously you'll know that two or more wires are shorted together.
This is only a simple tester and therefore it can not detect separated pairs.
Even if all pins are connected, showing the expected walking light pattern it does not automatically mean that the pairs are still pairs in the cable.
Separating or mixing pairs will cause network problems on higher speeds and longer cables.
The UTP tester will not indicate such fault conditions!
Some network adapters and network switches have an auto polarity mode. This means that you may switch the white and coloured wires of a particular twisted pair without causing problems. A cable with wires 1 & 2 switched or 3 & 6 switched will indicate false on the UTP Cable Tester but might still function in some cases. It is not recommended to rely on the ability of the network adapter and network switch to sense the polarity. When one of the devices is replaced by an other one after a while you may be in trouble.
The display box has 3 RJ-45 connectors. The first one, marked 1:1, is intended for testing Straight Through or so called Patch cables. The second one, marked Xll, is used for testing 10baseT and 100baseTX Cross cables. The third one, marked XX, is used for testing 100baseT4 cross cables.
The Signal Box
The Signal Box is powered by a 9V battery B1 which can be switched on and off by switch S1.
Diode D2 protects the circuit in case the battery is connected the wrong way up.
Current drain is minimal, approximately 5mA, which ensures quite a long battery life.
I could have used a programmable controller like a PIC16F84 or AT89C1051. The program would be quite simple. I deliberately chose the discrete approach though for a few important reasons:
But if you are only a little familiar with micro controllers, feel free to program the Signal Box in a processor of your choice.
The Display Box
The Display Box contains 3 RJ-45 connectors.
The first connector, marked 1:1, is used to test Patch cables.
On the second connector, marked Xll, the wires 1 & 2 are crossed with the wires 3 & 6.
It is used to test normal Cross cables for 10baseT and 100baseTX networks.
The third connector, marked XX, is used to test 100baseT4 cables, which have all 4 pairs of wires crossed.
Bill Of Materials
I've created some animated pictures to demonstrate the use of the UTP Cable Tester. Too bad you can't print the animation ;-)
If an error is indicated by the UTP Cable Tester, it may be a bit tricky to find the faulty wire(s) on Cross cables.
The wire number indicated by the LEDs are the wires seen from the connector at the Display Box's end.