DIY cable detector

[panoss]

I want to build a DIY cable detector (I don't wanna buy one, even though there are cheap ones).
e.g I have a cable coming out of a wall and I want to find out where it ends.

I tried this one, but no...it  can't detect a cable.
So (trying to avoid building a bunch of 'detectors' that don't work), have you tried something that you could suggest me building?
I 'm thinking about this one (creates a signal which is fed to the cable being detected. An AM reciever picks up the signal. Original article here), what do you think?
Or this one.
Or this one (on page 2).
Has anyone tried any of these? (or something else and worked fine)

[CopperCone]

seanb said he plugged in wire to his motorcycles ignition coil and used a AM radio

[panoss]

Ok, but I was thinking about something...more...descend... ...or let's say, more 'professional'.

[CopperCone]

hard to beat the power and cheapness of a motorcycle coil for something as crude as cable finding. Apparantly strong enough to work outdoors to find underground shit iirc.

I kinda want to make a box with a ignition coil mechanism to work as a transmitter with some kind of way to adjust coupling (maybe big resistors?).

I think that using a small hand held radio as a receiver is pretty professional.

Otherwise some kind of sparkgap transmitter I guess.

[Marco]

What's the point in RF? Won't you be detecting based purely on capacitive coupling until you get to UHF? If it's capacitive any way, might as well stick to audio frequency.

I'd build the one which alternates between 2100 and 2200 Hz. Build the receiver into metal box connected to circuit ground, so your body can provide the capacitance to earth it needs to work when you hold it. Also has the benefit of not being airwave polluting, which seems relevant in the HAM forum.

[panoss]

Ok I 'll built the one that alternates between 2100Hz and 2200Hz.
The receiver is just an audio amplifier with A1 (Antenna),  C1 and R1 in it's non inverting input, right?
So I can replace it with an LM386 amplifier, right?
Something like the circuit in the attachment.

[CopperCone]

What's the point in RF? Won't you be detecting based purely on capacitive coupling until you get to UHF? If it's capacitive any way, might as well stick to audio frequency.

I'd build the one which alternates between 2100 and 2200 Hz. Build the receiver into metal box connected to circuit ground, so your body can provide the capacitance to earth it needs to work when you hold it. Also has the benefit of not being airwave polluting, which seems relevant in the HAM forum.

marco I think its so that you can get a reasonable idea of where it is even if its going through a conduit or underground I think.

[MathWizard]

What's the point in RF? Won't you be detecting based purely on capacitive coupling until you get to UHF? If it's capacitive any way, might as well stick to audio frequency.

I'd build the one which alternates between 2100 and 2200 Hz. Build the receiver into metal box connected to circuit ground, so your body can provide the capacitance to earth it needs to work when you hold it. Also has the benefit of not being airwave polluting, which seems relevant in the HAM forum.

Yeah I made a TX with a 556 timer, alternating between something like 1200 and 1400Hz, that just attaches to a single wire. For a RX, I'm using my homemade version of Mr.Carlson's Super probe, for AF/RF signal tracing. They work pretty good together, I haven't looked at either one in detail on a scope, IDK how much of higher frequency harmonics from the "square-wave" output of the 556 there is.

But just powering the Super probe from a battery, and the TX from a bench PSU, a lot of signal can couple from me into the probe tip, with it all on the desk anyways. It doesn't work well through the gyproc walls, IDK the physics well enough to know why. But yeah I'll try earth grounding the Super probe next time.

I did add some RF oscillator to the AF TX PCB, with a crystal and CD40xx logic chip, but it never worked. But yeah how much better would some RF signal go through walls ? And then are you saying people would usually use a coil to detect that through a wall ? All that's in my radio books I guess. I should hook one of my working MHz osc. circuits to a dead housewire and see how good my RX projects pick it up with coils. One day I made a big loop antenna to try and estimate the magnetic field strength or mag. flux of a local radio station.

The superprobe picks up 60Hz no problem from live wires, I haven't experimented with how the current affects the volume or anything, but yeah I don't remember the superprobe being able to follow live wires in the walls very good, not with my parts or lack of proper shielding.

[Marco]

marco I think its so that you can get a reasonable idea of where it is even if its going through a conduit or underground I think.

I don't see how that works when it has the wavelength of your entire house and is emitting from a wire, I can't see EM coupling ever being relevant ... if the signal gets through a grounded conduit it's simply because of the grounding resistance and it's still going to be capacitive. 1000 Hz or 1 MHz is a small difference.

[jpanhalt]

@panoss

Are you attaching your transmitter ground to house ground or any other Earth?  Most of the affordable tracers I have seen require that, e.g., Klein ET450 (https://data.kleintools.com/sites/all/product_assets/documents/instructions/klein/ET450%20Manual-web.pdf ).

Second, while implied, I assume the circuit is not powered currently?  If it is, then you want protection from the power.

[richnormand]

Last time I needed to locate what looked like an abandoned (unpowered) cable from the basement to where it was going in the house I used my old RF generator set for a low output.

Tuned on the AM band to a quiet spot between two stations. Used the 1kHz internal audio modulation of the RF generator (old Eico) with the RF hooked on the wire (did not bothered with ground).

AM transistor radio located it in the garage at the other end of the house. Wire was terminated behind an old wall repair.
Never would have found it otherwise.

If you don't have a simple RF gen they are easy to build cheaply.

[Kleinstein]

What's the point in RF? Won't you be detecting based purely on capacitive coupling until you get to UHF? If it's capacitive any way, might as well stick to audio frequency.

The AM band is largely abondoned in Germany and radio receivers are still often at hand or found. So it could make sense to use such a receiver - though many use a magnetic antenna and thus don't detect the electric but magnitic field. This can still be usefull, though may want a short at the end or the line.  Otherwise a frequency in the upper audio range could work as well.

[shabaz]

I was desperate for tracing a wire a few years ago, and ended up with the attached circuit powered from a 9V PP3 battery. I used a portable AM receiver to pick up the tone, tuning in at the source and then following where it went.

I used parts I already had, and it was a throwaway circuit built on stripboard, not overly tested to see how well it would work with different loads, but it was stable enough for my task though.

[Marco]

though many use a magnetic antenna and thus don't detect the electric but magnitic field.

If we're talking in absolutes, the electrical wire is an electrical antenna so it doesn't transmit magnetic field. I guess the truth is somewhere in the middle.