This circuit allows simple testing of Ethernet cables; by using other sockets, it is possible to test other cables with up to 8 wires, such as telephone cables. It can detect shorts, breaks and mis-wiring.

(for photos, scroll down)

Circuit diagram

Click to enlarge. You can also download the schematic in Eagle .sch format.


IC1 generates a square wave at approximately 5Hz, which is fed to IC2 as a clock signal. Output Q8 is wired to the reset input, effectively causing IC2 to cycle through the first 8 outputs. With a good (straight) cable, LED1..LED8 should light up in sequence. The order in which the wires are connected causes a crossover cable to light the LEDs in reverse (LED8..LED1). I used 4 diodes because that's how it fit my board; there can be as little as 2, or ideally 8.

Note! The datasheet for CD4017 says it can sink or source approximately 1mA for each output. This design stresses the IC with about 4 times as much. Although I haven't had any problems with that, YMMV. You may want to add a buffer IC, such as the 74HC243 or 74HC541.

The circuit is powered by a 9V battery, and draws about 7mA of current. The active (powered) section fits nicely into a RJ45 socket case (photo). The LEDs were placed on a small board and connected to a small length of cable (photo); a cable extender plug can be used to connect it to the cable under test.