[EE]:: Simple DIY TDR - Time Domain Reflectometer.
There are a zillion TDR circuits 'out there' - I'm highlighting this one
due to the good writeup, example images and references. It's about as good
an introduction as you'll get to a simple and useful device.
A TDR is used to determine characteristics of cables and to determine the
distance of "features" from the test point. This may include shorts, opens,
impedance discontinuities, taps, ... .
A simple TDR typically applies a sharp "square" edge signal or a pulse to
the cable and then observes the resultant voltage at the drive point -
often using an oscilloscope.
This TDR circuit uses mainly a 74AC14 hex Schmitt inverter IC.