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How to make a good wire detector in a wall?

Aleksandar-

New Member
Hi.
For make wire detector in a wall, it is given here that the total length of the coil wire should be 10-12 cm, but no coil diameter was given, how long it should be?

Thanks.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
To detect power cables, it would work by capacitance - the length and shape of the pickup should not be critical, just a short stub or flat spiral should work.

Commercial "voltstick" type non-contact voltage devices only have a very short stub probe, and the audio frequency cable tracers I use also have just short probes, around 20 - 25mm.
eg.

 

rjenkinsgb

Well-Known Member
Most Helpful Member
Just connect it directly across the circuit power, positive after the switch (with buzzer positive) and negative to battery / circuit negative.

What battery are you using? If it's not a type that meter supports, you will not get an accurate percentage.

eg. A "9V" battery starts at around 9V and gradually drops to about 6V over its life, before it is considered completely exhausted.
With that meter, setting it for two lithium cells is probably as near as you will get to an appropriate range for a 9V battery.
 

Aleksandar-

New Member
the length and shape of the pickup should not be critical...
But can anyone is able to estimate the dimensions of that spiral based on the recording, though it was written in the recording that, for example, only a straight wire did not produce a strong enough signal.
 
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rjenkinsgb

Well-Known Member
Most Helpful Member
You are forming a capacitor plate - the spiral is emulating a flat area of metal, that picks up a signal from a powered cable by capacitive coupling.
The size and turns are not critical as it is not a coil in the tuned or resonant sense.

Try something like 10 - 15mm diameter.

If the circuit was better designed and sensitive enough, it should work with a tiny stub of wire.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
There are hundreds of versions of that (very badly designed) circuit on the internet.

It is very bad because the base of the first transistor has no bias and if it detects any voltage and the transistor conducts, the base-emitter diode will cause the sensitivity to drop at the same time, by pushing the base negative of the conduction point.

You cannot expect good operation - a bigger pickup area also means more voltage, more rectifier effect and more "unbiasing" of the transistor, if it was even in a state to conduct to start with...

As a toy, just use any piece of wire or a small area of metal like a small coin. it will work to some extent but never well or reliably.


Use a properly designed circuit and a tiny probe will detect a live cable at a good distance; this looks to be a pretty good design:
 
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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Circuit Cellar had an article about a wire tracer. It wasn;t a few parts. It could do underground as well.

I have an older version of the Ideal Suretrace. https://www.idealindustries.ca/products/electrical/electrical-testing/circuit-tracers.html Not cheap, unfortunately. You can actually find the short.

What happened in my case, is that some manufacture relied on box dimensions and did not protect the side screws on a daisy-chained outlet, so one day a circuit developed a short. I was able to guess it's path and bisected the circuit until I found the short. A screw on an outlet against the enclosure. The box dimensions were off. it was a 1960's enclosure with no tapped ground holes.
 

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