BFO

[giuhalftrack]

anyone here interested in BFO detectors ?

 

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[vpnavy]

Those that don't know what a BFO Metal Detector is (like myself) - take a look at: BFO Metal Detectors (Beat Frequency Oscillator). Here part of the article:

"...BFO Metal Detectors is one of the most rudimentary metal detection systems. It’s processed by six analog circuits, one digital circuit, and several basic components...more..."

 

[jeff of pa]

nope, not here in the North East. I had RELCO's and the iron ore played havoc with them.

I had Both

The $19.95 and (I think) the $198 Models .
the $19.95 model turned out to be a kit, needed , soldering, wrapping the coil, Building a tuner and a Small AM Transistor Radio, to get and broadcast the Signal.
After a Year they sent me an Offer to Trade for the Frontiersman at a Discount, I think $125. give or take. Which may explain why a Pic of the $19.95 Model is Super Rare. Everyone sent them in.
{this is Not Mine}

they may be ok in certain parts of the U.S. Like Texas,
but not here the Frontiersman was not an improvement.
because of large areas of iron ore it sounded like a theremin

 

[giuhalftrack]

Because they are easy to build, I love build them, reading the book of karl von mueller, I recover the design of one he builds and build myself.

Unfortunately here the metal detector hobby come many years after in respect to USA so sometimes I think that the treasures that a BFO can find are never been found
on eBay etc. they come sometimes at very low price but the shipping cost to Italy are enormous so I give up to buy complete units.

In general I'm fascinated by all retro-technologies.

 

[Ed144]

anyone here interested in BFO detectors ?

I started out with a Bounty Hunter BFO back in the 70s. BFO detectors can be very sensitive since the human ear is very good at hearing a slight momentary change in pitch. I remember hitting pennies at 6". I upgraded to a Compass induction balance machine. Although the Compass was easier and faster, it could not get the depth of careful BFO use. It would be interesting to design an analog machine using either phase lock loop detection or adjustable active narrow audio bandpass filtering. Another approach might be a BFO at VLF with regeneration and down conversion to audio. Keep the soldering iron hot! Ed.

 

[boogeyman]

I started out with a Bounty Hunter BFO back in the 70s. BFO detectors can be very sensitive since the human ear is very good at hearing a slight momentary change in pitch. I remember hitting pennies at 6". I upgraded to a Compass induction balance machine. Although the Compass was easier and faster, it could not get the depth of careful BFO use. It would be interesting to design an analog machine using either phase lock loop detection or adjustable active narrow audio bandpass filtering. Another approach might be a BFO at VLF with regeneration and down conversion to audio. Keep the soldering iron hot! Ed.

Got a Relco Pacesetter right here! wow! forgot I had it. Had a Bounty Hunter in the late 60s. Best hot rock (leaverite) locator ever made😋

On the other hand, knew an old boy that wouldn't give up his BFO even for a VLF TR machine & he'd hunt circles around us all day.

 

[giuhalftrack]

I started out with a Bounty Hunter BFO back in the 70s. BFO detectors can be very sensitive since the human ear is very good at hearing a slight momentary change in pitch. I remember hitting pennies at 6". I upgraded to a Compass induction balance machine. Although the Compass was easier and faster, it could not get the depth of careful BFO use. It would be interesting to design an analog machine using either phase lock loop detection or adjustable active narrow audio bandpass filtering. Another approach might be a BFO at VLF with regeneration and down conversion to audio. Keep the soldering iron hot! Ed.

very cool memories.

for the "soldering iron" part I'm playing with the digital domain, even if I'm an analog guy after reading and build several circuit I came out with an idea to use software to carry out much of the BFO function.

Let me briefly explain what i mean... let's say that if i compare the search frequency with a set value in software (reference frequency) i don't need a stable reference oscillator so i have eliminated all the problems connect to it.

soon or later i came out with a prototype so stay tuned

 

[jeff of pa]

Got a Relco Pacesetter right here! wow! forgot I had it. Had a Bounty Hunter in the late 60s. Best hot rock (leaverite) locator ever made😋

I haven't thought of it, but may also be the best choice for meteorite too. I'm not a meteorite hunter and don't know them that well, but considering the iron content and usual magnetic qualities But Still Best to use them in States without High
Iron Ore content

 

[Carl-NC]

Let me briefly explain what i mean... let's say that if i compare the search frequency with a set value in software (reference frequency) i don't need a stable reference oscillator so i have eliminated all the problems connect to it.

Because BFO operates on proximity effect it will probably never equal (or even come close) to what IB can do. That said, there are some fun things you can do with BFO.
Yes, you can use the micro to generate the reference frequency. An NCO will give you the resolution you need with the bonus that you can implement an instant retune button. You can also use an input compare/capture channel to just measure the TX frequency directly, no need to compare to anything. Of course in both case you are really comparing the TX frequency to the micro's clock.
Furthermore, you can make a VLF-BFO. You can also make an IB-BFO. You can make a multifrequency BFO. You can combine them all to make a VLF-IB-MF-GB BFO. Lots of fun things to play with. Visit the Geotech web site (geotech1.com) for more info.

 

[giuhalftrack]

Because BFO operates on proximity effect it will probably never equal (or even come close) to what IB can do. That said, there are some fun things you can do with BFO.
Yes, you can use the micro to generate the reference frequency. An NCO will give you the resolution you need with the bonus that you can implement an instant retune button. You can also use an input compare/capture channel to just measure the TX frequency directly, no need to compare to anything. Of course in both case you are really comparing the TX frequency to the micro's clock.
Furthermore, you can make a VLF-BFO. You can also make an IB-BFO. You can make a multifrequency BFO. You can combine them all to make a VLF-IB-MF-GB BFO. Lots of fun things to play with. Visit the Geotech web site (geotech1.com) for more info.

thank you Carl your post is of great inspiration to me.
yes i know geotech1, I'm a member, I read a lot and post rarely, I got your book too...
I'm not looking to revive an old technology to discover some secret advantage hided by evil corporations.. I'm only an enthusiast, let's compare my interest to people that love shooting muzzle loader rifles... it doesen't make sense go hunting with those rifles but.. they like it.

 

[BILL96]

I had a Garrett BFO in the middle 70's, haven't thought about it years, wish I still had it.

 

[boogeyman]

thank you Carl your post is of great inspiration to me.
yes i know geotech1, I'm a member, I read a lot and post rarely, I got your book too...
I'm not looking to revive an old technology to discover some secret advantage hided by evil corporations.. I'm only an enthusiast, let's compare my interest to people that love shooting muzzle loader rifles... it doesen't make sense go hunting with those rifles but.. they like it.

Got a Relco Pacesetter right here! wow! forgot I had it. Had a Bounty Hunter in the late 60s. Best hot rock (leaverite) locator ever made😋

On the other hand, knew an old boy that wouldn't give up his BFO even for a VLF TR machine & he'd hunt circles around us all day.

Now that I think of it, I've got a Relco Pacesetter in the closet. If anyone is looking for one for their collection etc. Pm an offer.

 

[TheGreenBoy]

best BFO i ever made was ETI 561, actualy a usefoul design with solid depth too. Very sensitive on small items, very stabile and quite useless on grass.

 

[giuhalftrack]

best BFO i ever made was ETI 561, actualy a usefoul design with solid depth too. Very sensitive on small items, very stabile and quite useless on grass.

yeah build this one too... the most interesting feature was the multiplication of frequency shift. For 1Hz of frequency shift at the coil you ear 8Hz on the earphone this overcome the limitation of the human ear to distinguish small changes in the frequency (tone). the quartz reference oscillator is another improvement.

 

[Carl-NC]

best BFO i ever made was ETI 561, actualy a usefoul design with solid depth too. Very sensitive on small items, very stabile and quite useless on grass.

Shield thy coil.

yeah build this one too... the most interesting feature was the multiplication of frequency shift. For 1Hz of frequency shift at the coil you ear 8Hz on the earphone this overcome the limitation of the human ear to distinguish small changes in the frequency (tone). the quartz reference oscillator is another improvement.

Using a PLL frequency multiplier is how you do a VLF-BFO. Some BFOs also operate on mixer overtones, I don't recall what the ETI561 did.

 

[gunsil]

Man, I started with a Garrett Master Hunter BFO in 1970, it was one of the best machines I ever had with the dual coils. I found lots with it but of course there was little clad or pull tabs although rusty bottle caps were awful. With the two size coils and the ability to toggle between then you could get a good idea of depth and sometimes size of object. The main drawback was they could not ground balance and I am in an area with lots of iron in the soil. I still have it although it is not working. There were no discriminating detectors then so it was normal to dig almost all signals and we were used to it, few today dig all targets. Oh, just remembered, someimes my BFO would pick up police radio car radio transmissions. I thought it was pretty funny when I heard a police dispatcher tell a radio car to investigate strange goings on at the school I was detecting and sure enough two minutes later a car shows up and they ask me what I was doing. They didn't care and left, but back then nobody knew what we were up to and better yet, nobody cared.

 

[TecSpi]

Hi This is an old thread, But I am certainly interested in BFO detectors.( I designed one, if you search for 'super-bfo backyard test' on youtube). The original idea of BFO , in its very name, is neat, as are the suggestions to further improve the power of a BFO. But let me tell you how easy and powerful a BFO can be now; As I write this, I have a microprocessor on my desk that cost 2 dollars, and It can measure the slight change in frequency of one- part-in-20 million. This chip has inside it, all the components that used to be external and had to be carefully designed back in the early 2000s, including the 20MHz oscillator and precision voltage references, so it was unimaginable back in the day when BFOs were first invented. Long-story short, if you believe 'a few transistors and some resistors' is a simple circuit. My design has basically one chip. To get technical; but as you may already know, if you have your BFO running at a frequency , say 15kHz, and you want to measure a slight change in frequency. Then the trick was to 'beat it' (mix it) with another oscillator and get a difference, an audio tone. Impressive how this was achieved with analog circuitry. Now, you can 'just measure the frequency directly', and detect when it changes by even by 0.000001 Hz. The amplification and Audio tone , can be anything you like. Hence how my design has a different beep for IRON. (Because iron increases the inductance of the coil (at close range), and non-iron conductors tend to decrease the inductance). For reference, the exact micro I'm talking about is an atmel 'ATTiny'


Drawbacks of BFO: Sensitive to the ground, 'Capacitance' and conductivity. Very touchy and noisy in general.

But, these issues can be solved.

 

[giuhalftrack]

Hi This is an old thread, But I am certainly interested in BFO detectors.( I designed one, if you search for 'super-bfo backyard test' on youtube). The original idea of BFO , in its very name, is neat, as are the suggestions to further improve the power of a BFO. But let me tell you how easy and powerful a BFO can be now; As I write this, I have a microprocessor on my desk that cost 2 dollars, and It can measure the slight change in frequency of one- part-in-20 million. This chip has inside it, all the components that used to be external and had to be carefully designed back in the early 2000s, including the 20MHz oscillator and precision voltage references, so it was unimaginable back in the day when BFOs were first invented. Long-story short, if you believe 'a few transistors and some resistors' is a simple circuit. My design has basically one chip. To get technical; but as you may already know, if you have your BFO running at a frequency , say 15kHz, and you want to measure a slight change in frequency. Then the trick was to 'beat it' (mix it) with another oscillator and get a difference, an audio tone. Impressive how this was achieved with analog circuitry. Now, you can 'just measure the frequency directly', and detect when it changes by even by 0.000001 Hz. The amplification and Audio tone , can be anything you like. Hence how my design has a different beep for IRON. (Because iron increases the inductance of the coil (at close range), and non-iron conductors tend to decrease the inductance). For reference, the exact micro I'm talking about is an atmel 'ATTiny'


Drawbacks of BFO: Sensitive to the ground, 'Capacitance' and conductivity. Very touchy and noisy in general.

But, these issues can be solved.

this is very cool. I saw your video and subscribe your channel, I hope you post more videos "in action". I made a "digital BFO" by myself too... nothing special I think it is normal that we all go to the same "idea", the possibilities offered by the MCU's really cut out many plagues of the BFO implementation.
Unfortunately we cannot do much against the phenomena that change inductance of the coil (proximity) si BFO will be allways less sensitive against they IB counterparts.

In any case I like them, and I think detecting can be fun even using old instruments. For many of us is the detecting itself that is interesting despite the finds.