How Can I Build a 100kHz LC Circuit for a BFO Metal Detector?

[AndyWeb]

Hello,

I'm looking to build a metal detector. I shant get into all aspects of it, just the first for now.
I have a BSc Physics so you can get mathematical if you want but not much electrical experience.

I need an LC circuit to build a beat frequency oscillator metal detector. It should output ~100kHz but I don't even know where to begin looking for one online. Know how you can get circuitry kits online? What is this type of unit even called??

Also I want to be pumping quite a bit of power through it so I'd need a beefy one.

Any help would be great.

 

[jim hardy]

there are many to choose from

probably thirty years ago i built the one on page 69 of this book
it uses a phase locked loop to give dc voltage proportional to what would be "beat frequency"
https://ia801608.us.archive.org/30/...s_5800304/1972_Signetics_PLL_Applications.pdf

 

[Aaron Crowl]

Are you any good at coding? Lots of people are building Arduino based metal detectors. An Arduino design would be very flexible and customizable. You would have to learn some basics of microcontroller design (understanding interrupts is a must).

 

[Planobilly]

Hi,
Here is a schematic of a pretty simple BFO detector.

Here is a link to the parts list. http://www.hobby-hour.com/electronics/s/bfo-metal-detector.php

In you are poking around into the world of BFO detectors the name Charley Garrett comes to mind. http://www.garrett.com/mediasite/history_en.aspx This will give you a bit of histroical perspective about metal detecting.
Another friend of mine E S Rocky LeGaye wrote a book called The Electronic Metal Detector Handbook with a lot of input from Charley. I did some of the type setting for the book. You would not believe how much work it was to do the typesetting on a Varityper before we had computers...lol

That book was mostly about BFO detectors but also had info about TR detectors. The book can be had on Amazon for next to nothing,$3.95, now days. Charley, Rocky, myself and several other friends have been out on field trips trying out new search coil designs. Fun stuff!

As I think about it, BFO detectors were my first real involvement with electronics.

Cheers,

Billy

 

[AndyWeb]

Hi,

I have done some coding. I have a Raspberry Pi and before I looked into BFO metal detectors I was actually thinking of using the Pi - but - I'm guessing I'd have to use the Pi as a controller / signal reader. Surely for the power needed I couldn't use a Pi?

Speaking of power, what kind of power do you need? 9v battery? I was assuming you'd need quite a big current for the search coil?

 

[Aaron Crowl]

RPi may not be well suited to to the project. The reason is that you don't have access to the low level interrupts from the Linux operating system without some special drivers.

You need to create interrupt routines to create outputs and read inputs in a predictable synchronous way. The rough sketch of what you would need to generate a signal is follows: configure one the chip's timer modules to generate an interrupt at some interval-> point that interrupt at a routine which calculates something like Amplitude=A*sin(Omega*t)-> Upload that binary value to the chips DAC(digital to analog converter). You would do something similar on the input side: Configure the ADC (analog to digital converter) to generate interrupts at some interval-> point that interrupt to some routine that stores the data to an array-> analyze that data in a constantly running program somewhere in user space. If that sounds intimidating in any way then have some hope because there is plenty of help to be had on these forums and also in Arduino forums if you take the Arduino route.

As for signal strength, you can amplify the signal output to any level you like. You can also treat the input to target the voltage range of the chip's analog to digital conversion hardware. You may be able to achieve all of this with Op-Amps which are fairly straightforward. I don't know what kind of power you need to make an effective metal detector. That's what us electricians need you physicists for! I'm sure someone has that information for you.

If you're interested in the digital route then I suggest you take a look at some Arduino metal detector projects. They don't look too complicated.

Also, I think the new Pi 3 addressed its shortcomings somehow but I haven't looked into it yet. I saw that they were advertising better servo control or something along those lines.

 

[Baluncore]

Speaking of power, what kind of power do you need? 9v battery? I was assuming you'd need quite a big current for the search coil?

A high current is not required in the search coil with a BFO detector. The BFO detector is a sensitive inductance meter. The “sensor coil” forms part of an oscillator. The high sensitivity comes from beating another low power oscillator with the sensor oscillator and listening to the slight change in beat frequency. That is why the oscillators are operated at an ultrasonic frequency.

The two oscillators usually employ the same circuit so their frequencies will track with changes in supply voltage and circuit temperature. In the circuit shown in post #4, the varicap D1 is controlled by an unregulated voltage which defeats the benefit of similar design.

The power needed is really determined by the audio amplifier design and the output power required to drive the headphones or speaker. It is not necessary for that audio amplifier to be linear. Unlike the circuit in post #4, a class C amplifier could be used.

 

[AndyWeb]

Ah ok. So you're operating and very high frequencies and then you're detecting the subtlest of changes in the inductance. I guess that makes sense.

And power isn't going to effect the beat...

But! Will a more powerful coil detect deeper metal?

Sorry, I'm a bit like Tim the toolman Taylor when it comes to POWER! :-D

 

[AndyWeb]

Hold the phone!

Is the depth frequency dependent? Like sound waves being transmitted and reflected by a wall? Longer wavelengths go deeper?

 

[Baluncore]

The simple BFO detector is great for finding magnetic items such as iron nails in wall studs, but the BFO is not good at finding small conductive items that are non-magnetic. More expensive and complex methods, such as Phase Discrimination detectors and Pulse Induction detectors are available for finding non-magnetic conductors.

But! Will a more powerful coil detect deeper metal?

Real power is needed to overcome resistive losses in the BFO detector coil and to make up for radiated energy absorbed by the nearby environment. The circulating energy in an LC oscillator is not real power. Unfortunately, the losses to coil resistance and the environment are proportional to the amplitude of the LC oscillation. So more power is not better in a BFO detector.

Is the depth frequency dependent?

As coil frequency rises, skin effect limits the the depth of penetration into the magnetic material, but that is not important since the magnetic material will still be detected. High frequency skin effect also reduces the depth of penetration through non-magnetic conductive materials such as salt water or soil. Magnetic items can be hidden from the BFO detector in salty soil. A lower frequency BFO detector can see deeper through salt, but it is usually better to use a Phase Discriminator or PI detector for prospecting in difficult ground.

https://en.wikipedia.org/wiki/Metal_detector

 

[Planobilly]

The wikipedia link above that Baluncore posted will give you a general idea of the various types of metal detectors and their use that have been developed to date. I looked at the link and it appears to contain accurate information based on my experience with the subject.

Without knowing what you have in mind to detect and and under what conditions it is not possible to say if a BFO detector would be even useable. You also gave no indication of why you were considering a BFO detector.

Without knowing what you are trying to accomplish it is difficult at best to advise you.

Here is a link to a PDF that will give you a good idea of how a BFO detector works with both the electronics and some of the math.

http://www.geotech1.com/pages/metdet/info/bfotheory/bfo.pdf

Cheers,

Billy

 

[jim hardy]

Pulse Induction seems popular nowadays i think it is capable f deeper detection at expense of more power..
here's an old patent that describes the basics
http://pdfpiw.uspto.gov/.piw?PageNu...1=3707672.PN.%26OS=PN/3707672%26RS=PN/3707672

That link is a TIFF file,
patent number is US3707672

With today's incredible power tool batteries and hobbyist microcomputers, it might be interesting...

 

[Baluncore]

A beginner should start by building a BFO detector and finding nails in wall studs. That way they will become familiar with coil geometry, size, movement and interference. It will also be possible to note the difference in detection of, for example, iron and aluminium.

Some experts have trouble getting PI detectors to work.

 

[jim hardy]

A beginner should start by building a BFO detector and finding nails in wall studs.

yep.

I have a BSc Physics so you can get mathematical if you want but not much electrical experience.

Confession time:
I was fishing for insight to "analyze the exponential decay of the echo" in PI technique...
.............................

I need an LC circuit to build a beat frequency oscillator metal detector. It should output ~100kHz but I don't even know where to begin

That Signetics Colpitts i built came out right at 100khz. It'd be a starting point for him.

old jim

 

[Planobilly]

If you really have an interest in metal detectors this site has a ton of info. http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=main&file=main.dat

There are schematics, DIY projects, and about a 100 patents in PDF's listed by type. There is too much to list with both highly technical and easy to understand information.

What type of detector is best is defined by what type of metal or mineral one is looking for and where that metal or mineral is located. Ground mineralization, how deep the target is, size, shape, orientation, amount of oxides on and around the target and how much "trash" is around are only a few of the of the issues to look at.

Here is a PDF for a pulse induction DIY project. http://www.geotech1.com/pages/metdet/projects/hammerhead/HHv1p5.pdf

PI detectors are not easy to build and display good performance. They are not suitable as a first project.

BFO detectors may be considered "old school" devices super-seeded by the latest microprocessor devices but I can assure you thousands of valuable coins and other items have been found with them. I have also found lots of bottle caps and pull tabs using several of the BFO detectors I have owned...lol

Cheers,

Billy