I built a wire tracer. Help me understand how it works?

[Moph]

Hi guys

First things first - I'm an electronics nub. I have a very basic understanding of electrical theory and can usually fault-find and repair stuff that breaks around here, but mostly I just tinker until it works.

Going back a few years we had some dead irrigation solenoids (buried, of course) at our new house that I needed to find. Hooked up a solenoid chatterbox but no joy, so I had the bright idea that if I could introduce a strong signal to the wire, it would produce stray EMF that I could then pick up with a sensitive receiver. Effectively a tone injector and receiver, I guess.

So I hooked up an old shed stereo to the primary coil of an 8Ω step-down speaker transformer with the secondary coil (5W tap) connected to the wire I wanted to trace and to a ground stake. I burned an repeating two-tone audio track onto a CD, played it via the stereo and figured that would work as a tone injector.

For the receiver I picked up a Jaycar pre-amp and 3.5A amplifier module. I wired up the pre-amp to a bunny ears TV antenna, hooked up the pre-amp output to the amplifier input, and connected a small speaker to the amp output with a 9V battery to power the lot.

The result? This Frankenstein's monster actually worked. With the tone injector in the back shed and hooked up to the active wire of the solenoid that I wanted to trace, I was able to find the dodgy solenoid buried under the front lawn to an accuracy of about 100mm. I could also detect a weaker signal from another solenoid in the garden (known location) that must have been returning via the common neutral wire.

A couple of Youtube vids and pics - I only took them at the time to forward to my dad (ex-sparky; hoped he could tell me what I'd built) so excuse the lack of commentary / detail:

Initial concept test:


Locating the solenoid in the front yard (Youtube description has further details / pics linked):


The receiver / scanner:


Stereo connected to primary side of 8 ohm transformer; wire to be traced and ground stake connected to secondary side:


Can anyone enlighten me as to how exactly this is working? Could I have left out the speaker transformer and just run the 'tone injector' from the stereo amp directly, or was the transformer boosting voltage as I somewhat intended? Was the ground stake necessary or would it work better hooking up the other end of the tone injector to the solenoid neutral? I honestly have no idea what I built or how it works and it's been bugging me for years =)

Cheers
Moph

[Zero999]

Look up capacitive coupling.

The higher the voltage the better, so the speaker transformer to boost the voltage was a good idea.

Connecting the 0V of the receiver would also increase the sensitivity.

[Moph]

Ah thank you, I didn't know what to google. Reading up on it now =)