Simple Nand metal detector problems...

[Dr Vel]

Carl : "I don't recall any BFO detector from the 70's hitting coins at 8" "

My Bounty Hunter III with a 12 inch Jetco coil will do it with a quarter or larger in air, 6 to 7 on the ground. Anything smaller in diameter it rapidly drops to 3 or 4 inches on the ground. Seems there is a threshold right at the diameter of a quarter since distance abruptly falls off for smaller diameters no doubt due to the large coil. I do not know what it would do with the original 6" coil as mine was cracked. Looks like someone tried to take the coil apart not knowing it was a 6 inch diam. PCB ring where the turns were foil paths, the board embedded in a solid plastic ring. The moment they pried on the coil they cracked the PCB making the coil an open circuit. I traced it out by hand, if someone wishes to redo the schematic using a CAD program it would be nice if you posted it here. I will never know how well it operated using the original 6" coil but I am amazed how far this BFO detects with the 12" coil. I can hear my car several feet away (holding unit out horizontally) and looking for objects like cans it way out detects all my modern MD's for distance. Being my first BFO I am really annoyed with the constant retuning. Very sensitive to every change around the search coil. The ratio for the mechanical vernier is so high there is a wide dead spot at zero beat. Scale is 0-100, I set the trimmer inside the coil ( see pic) for zero beat at 50 on the scale just off the ground. I kind of like it since I think it would find the lid on a mason jar full of Gold Double Eagles that was deeper than all my newer models could. I believe in the 30's when they outlawed owning Gold a lot of people would have buried theirs. I know I would have buried mine so my theory is so would everyone alive back in those days. The handle design combined with the heavy coil make this model painful to use for long and the constant tuning is no fun either yet I think I will keep it for certain searches such as finding that metal lid buried deeper than most average finds would be.

[Dr Vel]

Just realized I made an error on that schematic, it uses eight double A batteries not triple A's.

[Jeroi]

We look good pre-amp mono circuit and then an amp circuit for replacing the transistor and adding preamp cut-off and then amp it for headphones or speaker.

Can you guys provide good and simple schematics?

[Jeroi]

Can this work? Now it makes 15mA current to coil. I added +10V thrue 10M resistor and moved coil out of paraller with 100 ohm's. Now the circuit adds 100 times more current to the coil. But does the coil produce still sine wave to the lower circuit now?

If I insted connect the coil to gnd I get 28mA running trhue coil. So basically does the oscilator sircuitry do it's job still?

[porkluvr]

You have missed the point of Carl's post. 1.11 nH is next to nothing for a coil. It's equivalent to a straight #28 wire about 0.1" long. That is only a rough guess but a still reasonable ballpark number.

Multiply by 1000 to get a 1.11uF coil and you might start to see results.
Download Qiaozhi's coil calculator and start using some useful numbers.

You may need to use an "Initial Conditions" statement in your simulation to strike a small coil current to get the oscillator working.

[porkluvr]

You have missed the point of Carl's post. 1.11 nH is next to nothing for a coil. It's equivalent to a straight #28 wire about 0.1" long. That is only a rough guess but a still reasonable ballpark number.

Multiply by 1000 to get a 1.11uH coil and you might start to see results.
Download Qiaozhi's coil calculator and start using some useful numbers.

You may need to use an "Initial Conditions" statement in your simulation to strike a small coil current to get the oscillator working.

edit: Come to think of it, the coil in your simulation circuit is valued as 1.11nF. Don't you see a bigger problem with that? (Think units of measure.)

[Jeroi]

DIid not affect to change it to 1.11nH.

Bah I need to check what the ohm load of it is and claculate the Henrys out of it. **** **** ****.

[Qiaozhi]

The main point you've missed here is the fact that you're flogging a dead horse.
If your intention is to produce a circuit with greater depth, then you need to look at a completely different design. In particular, forget about the BFO.

[Skippy]

Your simulation is full of errors. You have 15mA through your coil, with 10Mohms resistance in series with it? No. Also,where's the ceramic resonator in the reference oscillator? Try simulating the original circuit, you may learn something.

[Jeroi]

There is sine wave source connected upperside to gnd and upper sircuit. It provides 470khz sinewave.

[Carl-NC]

"10M" is 10 milliohms, not 10 Megohms. No, none of this will work.

[Dr Vel]

"10M" is 10 milliohms, not 10 Megohms. No, none of this will work.

I do not see how M is not meg. From the diagram starting this thread that would put 0.02 ohms from + to ground blowing the supply. Since the 60's working in electronics in many thousands of schematics I have studied and every book on electronics I can recall a capital M has stood for millions of ohms not thousandths of an ohm. No idea where other than from some number system nomenclature 'M' would be so described. Yet in electronics a standard of conventions which has existed for decades such as E not V for volts and M for meg-ohms has been the norm everywhere I have known. As to the Nand MD circuit I agree give it up the energy in the coil is too low and the area covered by the field is too minute to be useful. I was kind of hoping He would look at my drawing of the BH III and go in that direction since it actually is a useful BFO circuit which is capable of being used to find something of value assuming you can stand the endless retuning due to every fluctuation in ground composition as well as search coil height.

[Carl-NC]

In most simulators "m" (or "M") is "milli". If you want "Meg" then you have to use "meg". His schematic has simulation annotations, and it's clear from the numbers that "10M" is 10 milliohms.

[Qiaozhi]

To make this a bit clearer:

In schematic land, it is normal to use m for milli and M for mega.

However, SPICE syntax is not case-sensitive. That is why m (or M) stands for milli (10E-3), and meg (or MEG) stands for mega (10E6). This is a common error that beginners experience when learning about SPICE simulation.

[Dr Vel]

Makes sense if you consider that the software writers did not come from a conventional electronics background. If you look at the first post it is clear that the 2 10 meg's from + to ground would work in the circuit but two 0.01 ohm resistors would dead short the rail. I would like it better if the authors would alter their programs to reflect nomenclature which has been standardized in the electronics industry rather than changing symbols to suit their programs. All a moot point I suppose since I fail to see how the Nand idea is ever going to be useful in finding lost treasure.