How does a cable tone-and-probe kit work?

Question

A useful tool for someone installing electrical cabling of any sort can be a Tone and Probe Tool: it allows one to find individual leads among a possibly hundreds in a cable trunk, without having to either strip or connect the remote end.

With the tone generator part connected to the start of the wire(-s), the probe tool typically generates an audible tone once its tip is in close proximity of the correct wire.

Can someone explain how these work in theory, perhaps with a high-level sketch of the basic circuits of such a tool?

Answer 1

The tone generator injects a signal onto the cable to be traced. Typically it's a Square wave (with high harmonic content), or a warbling tone.

Here's an example:

The probe tool is usually called an Inductive Amplifier, but in reality it works by capacitive coupling from the tone injected onto the wire.

There's a very informative Google group discussion on this, where the probe is described as:

The original 'banana' probe is simply a LM386 driving a mini 1" (25 mm) to 1.3" (30 mm) speaker. The input has an MPF102 JFET as a source follower, with a 4.7k source load resistor and a 10 meg from gate to ground (& collector to +9V). A 47 pf cap in parallel with the 10 M to rolloff highs, and a 1 Meg in series between the gate and probe tip. The source load resistor is coupled to the input of 386 thru a .1 uF ceramic cap. A push button momentary contact switch and 9V battery complete the circuit.

This site describes one guys attempt at building one.