Falcon MD20

[ODM]

That copper tape for a shield probably needs to be the thinnest one you can find, or other materials. Very thin lead sheet or nickel spray. Probably still better than aluminium foil because it's easier to make a small and clean solder joint to it instead of contact joining alu foil.

A bobbin like that murata will compress the field to its sides though for probing in sand that might not be bad. One ferrite shape that could be interesting is one half of a pot core. They use such ferrite cores for some inductive proximity sensors.

[Teleno]

What are you planning on useing for the inductor , at 400 megs it will be a hairpin.

I meant 400 KHz, sorry. At present I'm experimenting with Murata 1415440C.

[jladre]

Coil info.

thanks a lot 6666!

[6666]

For clarification, Can someone post the correct schematic and coil construction for this device?

Coil info.

[6666]

Vackar oscillator at 400 MHz (amplitude is rather constant with frequency)

What are you planning on useing for the inductor , at 400 megs it will be a hairpin.

[ODM]

I think the sensitivity ring was also called focus adjust at some point as well (by users, not the manufacturer, at least in their recent manuals). It would make sense, a shunted turn does have an effect on field shape. Something to try in a field shape analyzer program, perhaps counteracting ground "compression" that can be noticed with rising frequency, to some extent. Probably not increasing sensitivity since it's a lossy element but possibly giving better pinpoint.

[Teleno]

A good grade probe design with modern parts would be interesting to see. The Falcon design was based on amplitude/bias change, around 300kHz, but a pitch shift would likely do just as well. Self tuning is a big help with pitchshifters and relatively easy to do with modern MCU.

Perhaps the most unique part in Falcon is what they call sensitivity ring adjustment on the ferrite cored search head, and relatively many adjustments (due to rarely seen design) for a pin point probe to dial out the sensitivity. Many pinpointers are relatively insensitive to make them more straightforward to use.

The effect of the "sensitivity ring" (high conductivity loop) is to increase the oscillator's frequency. The same efect could be achieved by tuning via varicaps or MCU-selectable capacitors. In any case, at 400K Hz the level of sensitivity (minimum acceptable freq. shift) can be decided in software and selectable via (+-) push switches.

[ODM]

A good grade probe design with modern parts would be interesting to see. The Falcon design was based on amplitude/bias change, around 300kHz, but a pitch shift would likely do just as well. Self tuning is a big help with pitchshifters and relatively easy to do with modern MCU.

Perhaps the most unique part in Falcon is what they call sensitivity ring adjustment on the ferrite cored search head, and relatively many adjustments (due to rarely seen design) for a pin point probe to dial out the sensitivity. Many pinpointers are relatively insensitive to make them more straightforward to use.

[jladre]

For clarification, Can someone post the correct schematic and coil construction for this device?

[Teleno]

The Falcon MD20 is extremely simple and depth is ridiculous, yet my first ever gold nugget was found using this device. It was on the slate bedrock of a ditch next to a primary deposit (gold-bearing arsenopyrite in quartz - Teleno peak - León - Spain).

Working principle: mineral or a metallic objects near the ferrite head change its inductance which causes a change in the amplitude of a Colpitt's oscillator. This kind of oscillator varies its amplitude with frequency. Metals increase the frequency and decrease the amplitude, minerals do the opposite. The waveform is then rectified and used to modulate a buzzer.

Because this machine has its well defined niche in locating superficial small gold and gold specks embedded in rock, I've decided to build my own version with the following (radical) modifications.

- Vackar oscillator at 400 MHz (amplitude is rather constant with frequency)
- Frequency counter based on ATtiny85.

It will work by continously measuring the frequency and comparing it to a sliding-window time-average. The MCU can be used to produce a sound indication (wavetable) and/or visual indication.

Will keep you posted.

[Noro]

for example pin of 5 of ci 555 is free but in your schematic is galvanic coupled to colector Q5!
C5 33 micro in schematic is couple to ground, but in pcb is coupling to sensibiliti potentiometer

[Friiman]

Friiman you have a schematic? because i I suppose my schematic is wrong.Regard Noro.(
it does not fit for this forum schematic)[ATTACH]34865[/ATTACH]

I posted it earlier in this thread.

[Noro]

Friiman you have a schematic? because i I suppose my schematic is wrong.Regard Noro.(
it does not fit for this forum schematic)

[Friiman]

This is photo of MD falcon test ,Friiman your coil is shelted in tinker cooper foil ? tnx for photo![ATTACH]34863[/ATTACH][ATTACH]34864[/ATTACH]

Yes Noro it's shelted in some copper foil. Is the pcb image of your md falcon design?
I have tested winding some coils and i am almost there with the count of winding.
Anybody know if a resistor in the pcb with a different tolerance will be a bad choice. I have tested resistors with 1% tolerance and 5% on the prototype pcb. What can be different with other tolerance?

[Noro]

This is photo of MD falcon test ,Friiman your coil is shelted in tinker cooper foil ? tnx for photo!