Two-Component Metal Detector

[scarboro]

My special interest in metal detecting is design. Here's a super-simple design (designed by me), which I'm placing here "just for fun". It offers good performance and stability for a beat frequency operation (BFO) metal detector. While BFO in itself is not very sensitive, it is easy, and this design is good enough for a child to have some fun. When my son was small, I built him a Colpitts BFO metal detector (much more complex, and it wasn't as stable), and he enormously enjoyed it. To be specific, this metal detector will pick up a 1" / 25 mm diameter coin at 3½" / 90 mm. The design is based on a simple IC inverter oscillator, which is attached to a Medium Wave (MW) radio aerial. When metal is brought near to coil L1, the frequency of the oscillator (about 200kHz) changes, bringing about a shift in the difference frequency in the MW radio speaker. Solder the circuit (coil winding instructions in a moment) and take a wire to the MW radio aerial as shown. This should be screened, with the screen going to -6V (negative). Switch on both the metal detector and the radio, and tune the radio until a clear heterodyne (a whistle) is heard in the speaker (some heterodynes will work better than others). There will be a "band of silence" at the centre of most heterodynes (a zero beat frequency), and depending which side of this zero beat frequency is tuned in, the tone in the radio speaker will rise or fall at the presence of metal. This metal detector will also respond differently to ferrous and non-ferrous metals, e.g. rusty nails or coins. The coil is 70 turns of 22awg / 30swg enamelled copper wire on a 4¾" / 120 mm diameter former, with a Faraday shield. If you're not that familiar with coils, see here for details (this is my personal blog): http://thomasscarborough.blogspot.com/2008/05/bfo-metal-detector-2.html. Which see also if you'd like to re-publish this, which anyone may do freely with acknowledgement.

 

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[Real de Tayopa Tropical Tramp]

HI, Why not just uses the same IC to generate it's own reflex reference signal to create an audible /difference beat. Of course you would need a simple audio amp for the earphones, say a 555.. this would make it extremely compact and eliminate the need for a separate radio.

Don Jose de La Mancha

 

[scarboro]

Thank you Don Jose. This is a perceptive suggestion. In another design, I use two of these oscillators and mix them, to create what is acknowledged to be the world's simplest self-contained metal detector. See http://thomasscarborough.blogspot.com/2009/03/bb-metal-detector.html. Note, however, that this design is not BFO. It is a different genre called Beat Balance (BB), as explained in the hyperlink. This five-component design will pick up an old English penny at 6" / 150mm, which well exceeds the performance of BFO. With kind regards, Thomas Scarborough.

 

[Real de Tayopa Tropical Tramp]

Good afternoon my friend Scarboro: In that schematic, what I underestand is that you have two identical tuned osc with external matched coils as part of the tuned circuit. The tuned circuits can be adjusted by vdi to allow either of them to be thie higher or lower frequency.

Is the key that together they have a wider frerq. separation due to the influence of an external conductive object causing a wider spread of influence over a simple BFO's in which only one is influenced? But then ---

Don Jose de La Mancha

 

[Carl-NC]

Hi Thomas,

Well, you've just about done it... the 1-component metal detector! One of these days, you're going to figure out how to use a speaker for the search coil, coupled to a single unijunction transistor (or some other obscure device from the 60's), to make a discriminating metal detector.

Heard you went through a recent bout of illness, good to see you back posting.

Regards,
Carl

 

[scarboro]

Thanks Don Jose. One might think that the increased sensitivity of Beat Balance (BB) is due to (simultaneously) the frequency of one oscillator rising and the other dropping. But this is not the major factor, as can be proved by flipping one of the coils over. Or one might think it is due to a disturbance to the balance of the coils, as in IB. But this is not it either, since one can move the coils quite a lot in relation to one another, and sensitivity is not much compromised. What remains is that one oscillator destabilises the other, and vice versa, in such a way that sensitivity is greatly enhanced. To show exactly how, I would need to fire up my dual trace oscilloscope, which I haven’t done for too long! The effect was (probably) first noted by Harold J. Weber in US Patent 4,196,391 of 1980, but Weber could not explain it (he supposed that the effect lay in the brain). It was also of little interest to him, since he was focused on binaural location.

And hello Carl. You seem to be omnipresent!

 

[Real de Tayopa Tropical Tramp]

Good morning Scarboro: K I read you 5 X 5, got it, thanks. Ever follow up on it?

Don Jose de La Mancha

 

[Real de Tayopa Tropical Tramp]

Good morning CARL; How is the new job going? I know that White will be benifited by having you on their staff and we will end up with a new detector design. Again, congratulations.

Don Jose de La Mancha

 

[TerryC]

Hi Thomas,

Well, you've just about done it... the 1-component metal detector! One of these days, you're going to figure out how to use a speaker for the search coil, coupled to a single unijunction transistor (or some other obscure device from the 60's), to make a discriminating metal detector.

Heard you went through a recent bout of illness, good to see you back posting.

Regards,
Carl

Carl,
Sounds more like coma, not illness! Ha ha. Md's have progressed quite a bit, since Thomas "fell asleep"! ( just kidding.. him and Real (Hola!) have a good grasp of that IC stuff) TTC

 

[TerryC]

Scarboro,
How ya doing? Nice thread! Can I ask? Sounds like you work for Whites.... who just came out with the discrimminating pi machine.. was that Whites, or can't you say? TTC

 

[Carl-NC]

Good morning CARL; How is the new job going? I know that White will be benifited by having you on their staff and we will end up with a new detector design. Again, congratulations.

Don Jose de La Mancha

Hi JC,

White's is working me like a pack mule. Couldn't be happier! Several new designs on the drawing board, just takes time. Hope all is well in Meh-heeko.

Thanks,
Carl

 

[scarboro]

Thanks TTC.

My interest is innovation, rather than pushing performance -- and also on compaction or simplicity of design. So I've written perhaps a dozen feature articles for major magazines, incorporating my own designs -- and many smaller articles/designs besides. I'd be interested how discriminating White's PI would be. The principle hasn't had a reputation for good discrimination. I use a PI detector by lesser-known Magenta Electronics in the UK.

With kind regards,
Thomas.

 

[scarboro]

P.S. I couldn't wrap my head around a reduction of two components to one.

 

[Real de Tayopa Tropical Tramp]

Evening Scarburo: Carl can, so can you! me ??

Don Jose de La Mancha

 

[TerryC]

Thanks TTC.

My interest is innovation, rather than pushing performance -- and also on compaction or simplicity of design. So I've written perhaps a dozen feature articles for major magazines, incorporating my own designs -- and many smaller articles/designs besides. I'd be interested how discriminating White's PI would be. The principle hasn't had a reputation for good discrimination. I use a PI detector by lesser-known Magenta Electronics in the UK.

With kind regards,
Thomas.

Thank you very much for the reply, Thomas.
I have owned a Fischer Impulse 8 for many years. Good PI machine. The screws to the battery box, even though stainless, are showing their age. The sea will ultimately claim all! Thanks again. TTC