Modernizing the BFO

[Yourownfree]

has anyone here ever upgrade a BFO detector to make it better or at least try? Just how many types of metal detectors are there? I am not ,a metal detector professor like some of you. Ive been strolling around here on the site now that I have some time I've never had before and I read some great posts with great info.The reason I ask about how many types there are there seems to be one or two in the mix Ive never heard of. I've heard of Induction Balance(IB),BFO, TR , VLF,Pulse Induction (PI). Is there a shift frequency or something of that naturre one is called? How many more are there? My mind has been going 90 miles an hour thinking of this stuff. I'm a builder of such and have made a few BFO type. and they were useless in real testing. I intend to build another BFO in which I will change it slightly to either be better or end up with some frankenstein unit, the same goes for the PI and VLF type. I have been working on designing myself one of each and will soon test my builds. they are different than most diagrams. First its the theory or visualization in my head then to paper so I can see it with my eyes. I love looking over schematics. Its like eye candy to me. On a couple of my designs I will probably build first then share. When I started, my idea was to design a newer style of unit and make it easy to build for the beginner. Unfortunately I couldn't stop myself as I got further and further into it and it became a monster. Which is ok for me but surpassed my original idea. Still I wont share all my ideas yet until I test them first. I want proof first to prove myself. When I do elaborate on the BFO improvements and how I came about the design, it will need to be a thread by itself. It will be lengthy to explain. I guess I'm really excited in getting started. I may not have created anything different that hasn't been already made but because i've seen a lot of diagrams in my life I always wonder for instance why is the BFO the same now as it has been from the beginning why hasn't anybody modified it. So that's my purpose or drive. I guess is to do just that, for the better. Some of you might say a BFO is just a BFO and that' it. but I am anxious to hear your thoughts are and i can take negative thoughts too. It's all fun.

[Qiaozhi]

I would suggest reading either ITMD2 or ITMD3.
ITMD2 will be the easiest one to read with complete projects to build. Whereas ITMD3 is the bible on metal detecting technology, but may be too much information for you at this time.

https://www.amazon.com/s?k=inside+th...-pd-ops-ranker

[Yourownfree]

Thank you for pointing to these books. thy look like fun to read I'm all for knowledge but for now I found my answer. In todays world there really only three that anybody cares about. The VLF, the PI and the multifrequency.
The BFO is a relic that nobody really cares about. Because people tend to use different ways of stating the same thing, a frequency shift metal detector is essentially a BFO type and one of the same. I know the BFO, its a relic but I like to change thing for fun and experimentation. I fed my information into AI and it returned this: "you are essentially building a high-sensitivity Phase-Shift / Frequency-Window detector." Id say that is different from BFO. I started with the BFO circuit and said to myself this has to change. The BFO circuit has problems, drifting is a big one among other things. Its annoying to try and keep it tuned. Biggest piece of junk that's why everybody said goodbye to it. I build one as a kid, well more than one hoping for better results. waste of my time. The last one I built I made it from whatever I had laying around or could make. So I made this last one and it worked great on the bench, I didn't have a bench but anyhow it worked great in the air and I was pleased and excited. I took it to the park, after a carnival. Sure to find something there. Not a chance as I was looking down watching the coil pass over the grass I would see a coin, penny my luck, before the metal detector saw it. I gave up shortly after that when another person showed up with a D-Tex. I'm not sure what kind it was but the guy didn't look like he knew anything about how to operate it. He was finding very little but more than me. Now here comes another guy with a Compass metal detector with about 6 knobs on it. In the first 5 minutes he had a pouch full of coins, dirty coins not laying on the ground as he was still only less than 10 feet from where he started. It was then I said the heckle with it I'm out of here. Never built one or used one since. I recently had a stroke a few months back and it took me out. Stuff like that can mess with your head bigtime. So I decided to try and sharpen my brain skills and try to grow more dendrites to hep me get some of my lost brains back. By accident as I was working on another project I was prompted to take a look at the simple BFO as I just needed a simple vehicle detection loop. Since I cant focus mentally so good anymore I saw the shiny object and got detoured from the project to looking at the schematic of a BFO. I know I've told you some already and when I looked at the diagram I saw the magic begin and I began working on this new detector for ours and for days now to come up with some design. more so than I have time to explain here now. The BFO uses two oscillators each drifting away any direction, who know but it not stable. I've resolved that. Absolutely no drifting. Removed the reference oscillator. its not needed. I kept going and going until I've totally redone what we knew as a BFO. As you can tell it no longer is a BFO its a Phase Shift-Window detector. I will give you some idea as how I came to the conclusion I didn't need the reference oscillator. I would rather not share my other info at time because I want to build it first. I will tell you this and what i'm about to say and do is not in my detector now but I did design one that does use this idea. Here it is. A BFO uses two oscillators, they mix and the result is an audible difference as you know. Now stare at a diagram for a BFO. ask yourself as I did what if I converted each oscillator into voltage? with an f/v converter. Now I'm on to something right? Now I'm thinking how will I mix two voltages? that doesn't make sense. wait use a comparator. Now that would be cool. a voltage going into the comparator from each oscillator. Then the magical question I asked myself was this is dumb. I cant believe nobody has seen this. If the search coil is changing frequencies when a find is located it is converted to a varying voltage and then goes to the comparator. In which is referencing the voltage on the reference pin from the reference oscillator whos frequency is fixed. What? if the reference oscillator of the BFO is fixed the the voltage after the f/v converter is fixed Right? So in theory you don't need that oscillator anymore. All you need is a potentiometer to replace it. You can take the circuit any direction you want from here. By the way I can say that my detector will either give you a go-no-go result when an object is found and a distance reading (only closer or farther away analog signal. No I am not using a comparator but I have a digital and an analog result indication. Just seemed like the right thing to do. If a person wanted to play around with this design you could get carried away as I did. I kept on going and designing then I shifted to the vlf and PI to improve on those. AI told me as I fed my ideas in that where I am at now is I'm asking it do things Minelab apparently is already doing. So I've taking myself from BFO to cant say but I am pleased I am learning things as I go. Ai has a tendency if you say the right things to tell me what others are doing, but its after the fact I already discussed with Ai my ideas for hours. I do not know what any of metal detector can do, bells and whistles. I have never read about them or any other except I've seen a few schematics of some classic units like the surfmaster. Have to tell you though Ai is not going to design a circuit for you if you ask it to. I mean a circuit beyond what's normally done. You have to convince it to. I use Ai to give me feedback as I design. It is addicting.

[Carl-NC]

I've moved your topic into a separate thread. In general, BFO belongs to the family of "proximity" type metal detectors, where targets alter the transmitter in some fashion. Proximity metal detectors are never as sensitive as IB or PI but they are still being made today, mostly as pinpointers. ITMD3 discusses several types, including modified methods of BFO and applying digital techniques.

[Yourownfree]

you are right about the BFO type, that but its fun for me to glorify this thing. the best part if my design works it shouldn't drift at all which one thing I'm hoping for. That is so annoying to have a machine that drifts. just theory at this point so I will do some testing .

[Carl-NC]

A BFO will always have some drift, but you can minimize it with a good thermal design. You could also add slow feedback to a tuning varactor, sort of like an SAT circuit in PI.

[Yourownfree]

I am not sure how far my idea will go as I waiting for parts to arrive so I can experiment. My hope is to eliminate the drift. as long as the circuitry remains old style there will be drift. I will not have to build the whole detector but just the oscillator section for now. I know what you are thinking because there is so much that can cause drift in the old units. In my new unit I am crossing my fingers I can make the coil work with my circuit not the one metal detector I discussed here but it did lead me in another direction. but I am still working under the title modernizing the BFO, because it still is in that category, the new design. I should have my parts or at least enough to do some testing but I think it could take up to 10 days. I guess I could start the coil design. Just one for bench testing to test. I am not worried about the drift at moment as I am about getting an indication from the coil with the new oscillator design when it sees an object I do appreciate feedback of course. I'm trying to get away from feedback using a varactor. or just drifting. I have some work ahead of me. I will share what I mean if it works or not but I'd like to test my theory first before I start shooting my mouth off too much. The coil design will be as usual with Faraday shield and twisted cabling back to the circuit. I still will make every effort to make a good coil I can use if it works properly. I'm Just thinking about a smaller coil, maybe 4-6 Inches as for testing purposes I need to make sure I get a good powerful frequency response. I'll have to calculate the coil specs in a day or so. I need to work with the circuit values first. I have yet to do that. anyhow back to the drawing board.

[Carl-NC]

Also keep in mind that what works well on the bench may not work in the ground. Proximity detectors are sensitive to ground mineralization which changes the L of the coil. This effect is not so easy to deal with.

[Yourownfree]

thanks carl. I spaced out the mineralization. I totally forgot about it. Well I need to spend a few moments at the drawing board to figure this out and add that to the circuit, hopefully. Always something. And I may not get around it with this type I'm building. Its not over yet. I'll get back with a solution.

[Yourownfree]

Ok it looks like I can pull it off. First I will need to make the Faraday shield using foil or conductive graphite paint. This will work with the problem of the ground capacitance and static, leaving me to deal with the ground balance adjustment. Since my detector has turned into a sort of phase detector, I can manipulate my oscillator to change the angle or phase, matching the angle or phase that the ground is influencing on my coil, then my circuit can see the different phase of the target better. I do have the values calculated and it ends up all I need to do is a pot and a couple of resistors. Fortunate there that it wasn't more complicated. I don't mind adding things that help but sometimes it gets carried away. Thanks to todays technology parts are designed better than they used to be. I will draw out the new addition to the schematic so I can keep up where I'm at.

[Skippy]

Quote: "I always wonder ... why is the BFO the same now as it has been from the beginning, why hasn't anybody modified it?"

There have been some improvements. There is a design that uses digital techniques to multiply the frequency difference between the two oscillators, giving higher sensitivity as a result, or alternatively, allowing a lower search frequency ( like 20kHz ) with the same audio freq shift as a more-typical 100 kHz machine.

I know of one project that does this, the ETI-561 Wait design:
http://<a href="https://www.geotech1... ETI561</a>
Geotech - Metal Detectors - Projects - ETI561

The design as a whole is a bit basic, eg the audio stage, but the digital mixer is the interesting bit, and it's operation is explained in the text.

[Yourownfree]

Thanks. Read it and is quite the detector. Interesting how the builder used the flip-flops to do this. He eliminated the drift from one of the two oscillators. Then turned the search frequency into a square wave to be digital then using the two pulses now injected into the flip-flop where one pulse doesn't change and the other doesn't change and the flip-flops are happy until the search coil changes frequency and throws the flip flop out of sync and then the error is processed as a tone. Looking at it closely it it is very similar to my first design in which I removed the reference oscillator and replaced it with a pot more or less. I need to draw out what it looks like, but keep in mind its never been built. I should build it to see if it works. The second one I am working on, I've never built as it is theory and will take some tweaking to get to work. It will have a crystal controlled oscillator as part of the circuitry. I am also trying to keep the search oscillator fairly stiff from drifting but loose enough to convince a change in the frequency. I suppose it is somewhat like the one above in that I will have it detect a phase difference which will be detected by another part of the circuitry. as a digital signal but I also want to give it the classic up down tone mix the two together so you can hear both at the same tome, digital and analog. You will have control over which one is louder. So you will have a go-no go signal plus a variable tone, one everything is set it wont drift, theoretically. Is it possible ? I don't know for a fact. I am fairly certain there wont be any drift whatsoever and the only drift is what I've set that will only happen when a target is found, then it will allow for drift for 1 second that's it. I'm a little old fashioned but am going to learn to draw schematics using Kicad or something. Otherwise I could just draw it out nice and neat or I guess I could use paint to draw a schematic of the first design. The second one I am still working out the bugs to make it work. Thank for sharing that BFO schematic with me.

[Skippy]

"I could just draw (the schematic) out nice and neat"

This is fine, we're not picky on here. Many older schematics in the 'big archive of old detectors' are hand-drawn. A flat-bed scan works better that a digital camera / smartphone photo, but as long as it's in-focus, it should be OK.

[Yourownfree]

Ok thanks. I will digitize the diagram so easily understood. Working on it. Trying out some software apps to see if I can manage. I could use the old age thing as an excuse as why I'd draw it by hand or the recent stroke but probably wouldn't fly. I will provide a descent diagram for you all to see.

[Carl-NC]

Looking at it closely it it is very similar to my first design in which I removed the reference oscillator and replaced it with a pot more or less.

The most stable BFO design will place the tuning in the reference oscillator and make it digital somehow. Here is a concept from ITMD3:



The search oscillator is squared up and XOR-mixed with a micro-generated square wave. Conceptually you could do the mixing in the micro. You still have to worry about the stability of the search oscillator but at least there are no varactors to deal with. Here's a way to do temperature compensation in a Colpitts oscillator: