Basic Detector Question...

[Wooden Nickel]

Detector electronics question......

What does the threshold circuit do in a detector besides providing a "base"or "reference tone" to use to compare with the tones generated by a disruption in the detector's field?

I frequently see comments about raising or lowering the threshold to increase or decrease the detector's ability to find a target, akin to changing the sensitivity or discrimination setting. The "low threshold" vs. the "high threshold,' and the ability to hear very subtle to large differences is obvious.

Is there something else going on? Am I missing something?

Don't shy away from being technical.

mike.....

 

[kai_gold]

I am not an electronics guru and don't know about all detectors but on my gold bug 2 the threshold is only heard in all-metal. In iron disc mode the threshold is gone and it is silent search. However, if you crank up the threshold knob while in disc the audio response for a faint target gets louder so based on observation of this particular detector it would seem that it can be more than just setting a audio reference point and may have some influences similar to gain. I too would be interested in hearing the technical reasons for this.

 

[skookum]

High,
I will try to put this in laymen terms as best to my ability.
A detector puts out a frequency at a certain strength from the coil transmit windings which upon reflection by the ground or an object buried in it is then detected by the coil receiver windings.
In some coils, the transmit and receice windings are the same/one unit.
Most coils made nowadays have separate transmit and receive windings.
Some detectors have a fixed transmit frequency strength - here you adjust the receive value/strength.
Some detectors have a fixed receive frequency strength - here you adjust the transmit value/strength.
The value, where you just hear a signal when air balancing is the amount that the coil is at its

 

[Wooden Nickel]

Skookum - I'm sorry, but I wasn't able to pull an answer out of all of that. I especially didn't understand the term "air balancing." Maybe you can try the technical version.

Thanks. mike.....

 

[skookum]

High mike,
Of course.
Air balancing is balancing your coil away from any ground/magnetic interference. It is done by holding your coil away from the ground.
You turn up on your threshold knob, until you just hear a distinct background signal. Should you not like that sound, back off just a tad.
Then you can proceed to ground balance.
This is where you "bob" the coil and listen to increasing/decreasing signals emitted and make further adjustments.
I am not an electical engineer - but I can ground balance my 6000 or my 1270 within max 10 seconds - usually even slightly faster than that.
I am sure, that someone here on this forum will be able to give you the real-McCoy technical download with all sorts of technical terms on the exact happenings/procedure.
Greetings
skookum

 

[Wooden Nickel]

Well, I didn't intend for this to branch out into other things, so surely someone can answer what I thought was a simple question. OK, maybe it wasn't;t asked simply, but I suspect it has a fairly simple answer.

Again..... what does the threshold circuit do besides providing a "base" or "reference" tone which varies with the detector's response to a target?

Thanks to all who responded.

Let's try again.

mike.....

 

[Larry (IL)]

The threshold circuit monitors the ground balance point and gives you an audio signal as a reference. If the audio changes in pitch, the GB point is changing. Many users like to listen to the threshold as a signal if we are going over an iron target which you will lose your threshold and it should come back when you pass the iron. If your threshold rises or drops and stays there, you might be wise to do another GB. Hope this helps.

 

[Mike Hillis]

I don't think you are missing anything

I submit for your reading pleasure courtesy of Tony's Bandito II Umax Website, which is no longer around.

HH
Mike



[size=large]Threshhold-the reason why[/size]

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The reason behind having a threshhold, is to basically compensate and calibrate the detector for it to work properly.......Why? Well lets start off at the beginning, I believe that if, you the reader , understand the reason why you have to set a proper threshhold, it will be easier in setting up the threshold. It is the base setting that the rest of your initial setup of the detector will depend on, and I believe it is important enough to have a better understanding. The setting of a threshhold is basically compensating for differences of electronics, the differences in human anatomy/quality of one's hearing, and our individual preferences.

[attachment 226502 pic1.jpg]

Threshhold refers to a phenemonon of the silicon transistor. One of its characteristics is that it will not conduct until the magic voltage of 0.7 volts is exceeded.Once it exceeds this voltage, the transistor will then begin to conduct and amplify any signal. The final output stage consists of a final audio transistor with two input sources. One from the audio oscillator and one from the filtered detected signal output. When we say filtered detected signal output, a network of 2 or 4 pole capacitor filters that take the amplified detected signal and make it a DC(direct current) This is the basis of one of the inputs to the transistor. The second is the audio oscillator-this is the tone we hear coming out of the detector.(NOTE: the audio coming out of the control box is not the "operating frequency" of the detector-this normal "operating frequency" is around 10khz thats 10,000 hertz and would be almost a high screech- almost impossible to hear -the audio oscillator's frequency is probably around 800-1200 hertz-I havent actually measured it but as a radio tech for many years and having the ability to whistle a 1khz tone-the standard test tone to test a radio- is kinda like a job requirement - )

[attachment 226503 Pic2.jpg]

The pic above rates the the principle and basis of a induction balanced metal detector's threshold. The yellow portion represents the filtered output from the recieve amplifiers output- with no signal/aka target present.of course the grey wall represents our "threshold" of the final audio amplifiers transistor. The signal level of the audio transistor is not large enough to exceed the threshold, and we have no audio signal out. This also would be representative of when the threshold is turned fully counter clockwise. When we adjust the threshold we are actually inserting an additional dc bias into the input or base of the transistor, this allows the audio oscillator's signal to exceed the threshold and be amplified in a logarimthic way.This would be equivalent to tuning the threshold clockwise until we hear an audible constant hum.

[attachment 226504 pic3.jpg]

In a nutshell the tuning of the threshold will actually is compensating for the differences in our hearing, because no two human beings posses the same quality of hearing. What may be loud to some one , may be too soft for another person. Another way it compensates is for personal preferences, some people like the threshold to be a little louder, and some people like it a little softer. I personally like the threshold just loud enough to not having to strain to hear it i.e. not too loud, and not too soft. This, is like I said, a matter of personal preference. In conclusion the threshold compensates for the slight differences in the electronics, the slight differences the way the coil was constructed(no two coils are alike), and the differences in the individuals human ears. Setting the threshold is important, because the rest of the detector set up is determined and based on the setting.
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[Wooden Nickel]

Mike,

A G-R-E-A-T answer explained in a great way. So the threshold is really nothing but a volume control when you get down to it, although a very specialized voluming control. Without it we would probably not hear anything unless our detector was sitting on top of a car.

I get it. The threshold is part of an amplifier circuit. By adjusting the pot we overcome the bias on the NPN transistor allowing us to hear the audio oscillator and the audio from the processed signal of the receiver. The audio volume is determined by the position of the pot. If it is adjusted too low, we won't hear anything (unless the addition of the target response audio just happens to be enough to overcome the transistor bias). If it is adjusted too high, those weak signals will be lost in the loudness of the threshold audio. That's why it's best to adjust the threshold to a low level, which is different for different folks. The threshold adjust also allows one to compensate for circuit and component differences.

Too, changes in the threshold audit level will indicate that something has changed, whether it be from a target, something else out of adjustment, or even a change in the soil conditions.

Is there a place to get more of this stuff?

Thanks a ton !!!

mike.....

 

[Mike Hillis]

Hi Mike,
What are you interested in?

HH
Mike

 

[Wooden Nickel]

HH Mike - is there a place to get more explanations about how a detector works, like the info you posted above? That was a terrific explanation of how the threshold works. How does the ground balance circuitry work? The discriminator circuit? Other circuits?

I was afraid that if I asked about the electronic workings that I would get the real technical info, but your post was perfect. My electronics knowledge is very dated, from my military days of more years ago than I would like to admit to, but it was sufficient to understand the post info.

I can use more if you can give me a source.

Thanks a million!

mike.....

 

[Blank Planet]

http://www.minelab.com/__files/f/11043/METAL%20DETECTOR%20BASICS%20AND%20THEORY.pdf