Sure, you can buy a cable tester, but what fun is that? [Ashish] posted a nice looking cable tester that you can build with or without an onboard Arduino. If you don’t use an Arduino, the project uses a 555 chip to test the eight wires in an Ethernet cable. The readout is simple. When testing a conductor, one of 8 LEDs will light. If one doesn’t light, the cable is open. If more than one light up, there is a short. Mixed up pins will cause the LEDs to light out of sequence. You can see the device in the video below.

The 555 device is fine for the design and we were surprised that the project had provisions for using an Arduino as nothing more than a pulse generator. It could replace most of the circuit which is pretty simple. A decade counter converts the pulses into 8 pulses (a wiring change makes it reset on the 9th count). The rest of the circuit is nothing more than LEDs, resistors, and diodes.

This is a great example of how a few simple components can come together to do something significant. It would be tempting to use the Arduino to create the 8 or 9 output pulses and then measure them, but that would be a lot of I/O for a small Arduino. You could watch the return with analog inputs though, so that might form a further refinement of the circuit. There could even be an advantage of allowing a more detailed analysis on each pin.

Still, such flights of fancy aren’t nearly as simple, and this is above all a straightforward and pleasant project. Sure, it won’t replace a $12,000 cable tester, but it doesn’t have to. Another simple circuit (if you don’t count the scope) that is useful for cable testing is a time domain reflectometer.