Garrett BFO schematic

[Carl-NC]

I'm 100 miles from El Paso.

I'm at the First Texas factory generally the first 2 weeks of every month. If you have a hankering, stop by and I'll show you around. Make sure to ping me first to see if I'm really in town.

Thanks for posting the schematic carl. My first metal detector build was a bfo. I may just build this one to see out of curiosity. I just can't resist seeing a circuit, even a bfo, without making the dam thing. I think it's a disease.

That's not such a bad disease to have.

[dbanner]

You said it Carl, keeps me out of town and out of trouble!

[homefire]

I'm at the First Texas factory generally the first 2 weeks of every month. If you have a hankering, stop by and I'll show you around. Make sure to ping me first to see if I'm really in town.



That's not such a bad disease to have.

I'll take you up on that one of these days Carl. That would be a Cool Experience for sure.

[6666]

I'll take you up on that one of these days Carl. That would be a Cool Experience for sure.

DO IT

[aa1ww]

Circuit Questions

Carl-NC, et. al.,


Thanks for doing the circuit tracing. It's a nice circuit for inexperienced detector fans like myself to analyze and possibly build. I was an EE for 100 years before becoming becoming an electronics professor at a community college. This is a quality hobby and technical forum thanks to those willing to submit circuits and info. Thanks also to those adding coil details.


Looking at Carl's schematic in the area of the search oscillator, there is nominally 1nF (C6) + 10nF (C15, at least on the +sourcing transitions) + maybe 200pF (C4+C5) connected in parallel with one of the two coils. Using 120uH (from Carl's previous reply) and the venerable Fres = 1/(2pi*sqrt(LC)) I get approx. 137kHz.


Carl's remembrance of 200+kHz may point to an inflation on my behalf of C15's contribution. Given only ~400pF of adjustment my estimate of Ctotal may indeed be on the high side.


I see a +6V stabilized supply and a +3V virtual ground. Might I ask what the Vcc (Pin 8 and GND/Vee (Pin 4) connections on the LM358's are?

Thanks folks ... for the thread and the forum in general,
Coop, aa1ww

[chuubaba]

Dear sir can you tell me from where you buy your bread etc+ (what brand or flour mill) and any other tips for living 100+ years. Regards

Carl-NC, et. al.,


Thanks for doing the circuit tracing. It's a nice circuit for inexperienced detector fans like myself to analyze and possibly build. I was an EE for 100 years before becoming becoming an electronics professor at a community college. This is a quality hobby and technical forum thanks to those willing to submit circuits and info. Thanks also to those adding coil details.


Looking at Carl's schematic in the area of the search oscillator, there is nominally 1nF (C6) + 10nF (C15, at least on the +sourcing transitions) + maybe 200pF (C4+C5) connected in parallel with one of the two coils. Using 120uH (from Carl's previous reply) and the venerable Fres = 1/(2pi*sqrt(LC)) I get approx. 137kHz.


Carl's remembrance of 200+kHz may point to an inflation on my behalf of C15's contribution. Given only ~400pF of adjustment my estimate of Ctotal may indeed be on the high side.


I see a +6V stabilized supply and a +3V virtual ground. Might I ask what the Vcc (Pin 8 and GND/Vee (Pin 4) connections on the LM358's are?

Thanks folks ... for the thread and the forum in general,
Coop, aa1ww

[dbanner]

Carl-NC, et. al.,


Thanks for doing the circuit tracing. It's a nice circuit for inexperienced detector fans like myself to analyze and possibly build. I was an EE for 100 years before becoming becoming an electronics professor at a community college. This is a quality hobby and technical forum thanks to those willing to submit circuits and info. Thanks also to those adding coil details.


Looking at Carl's schematic in the area of the search oscillator, there is nominally 1nF (C6) + 10nF (C15, at least on the +sourcing transitions) + maybe 200pF (C4+C5) connected in parallel with one of the two coils. Using 120uH (from Carl's previous reply) and the venerable Fres = 1/(2pi*sqrt(LC)) I get approx. 137kHz.


Carl's remembrance of 200+kHz may point to an inflation on my behalf of C15's contribution. Given only ~400pF of adjustment my estimate of Ctotal may indeed be on the high side.


I see a +6V stabilized supply and a +3V virtual ground. Might I ask what the Vcc (Pin 8 and GND/Vee (Pin 4) connections on the LM358's are?

Thanks folks ... for the thread and the forum in general,
Coop, aa1ww

I think it would be +9volts and ground.

[dbanner]

Carl, can you say the value of the voltage rails of the Lm358's

[Melbeta]

Garrett, back in 1974, produced for Ray Smith, this BFO/TR booklet. The co-author was Roy Lagel, and through the expertise of both of these famous old treasure hunters, they shared their knowledge and information. The booklet covered both the BFO models and the TR models. These are very rare copies and extremely hard to find copies. Now Garrett only printed a few of the booklets, and Garrett copies were all in black text, no color.

But I was a good stocking dealer for Garrett detectors. At that time I used a Garrett Master Hunter package, and had the Super Pro package in my stock. Back then I could pick up the phone, phone the Garrett company, as to be connected to Charles Garret, and I had my conversations with Charlie. He was a wonderful honest person. As Garrett and Lagel, did not copyright the booklets, I was permitted to reprint for my own needs my own copies, and my copies employed the green and black text. I sold no copies for money. My copies were intended to be gifts to a buyer. The knowledge contained in this booklet is priceless. The number of my copies was even less then Garrett's printing of copies. I gave away the last copy years ago. I did keep the master image copies so I could print some again in the future though, but never did it. You see, I trained as a printer's devil, at Bradford Robison Printing Company in Denver, and started out melting linotype lead in a type melting room. From there I progressed out onto the print shop floor, where they printed law books and other legal books. They started me out on an vintage Heidleburg letter press, and I printed embossed business cards and embossed letterheads and embossed envelopes. Each was printed, one at a time, by hand, inserting into the letter press, print one copy, extact it, then print the next copy. When done, the copies went into a heating unit, where the ink with the embossing material, was heated, and the type and images rose up in height. Do they do this today? No, it was a skill that has vanished into the past. Today the machines did what a single printer did with love.
Melbeta

[aa1ww]

@chuubaba:

Dear sir can you tell me from where you buy your bread etc+ (what brand or flour mill) and any other tips for living 100+ years. Regards

Why, the flour was ground by Charles "Wheatstone", of course!
Coop, aa1ww

[chuubaba]

CHARLES WHEATSTONE .... GREAT SCOTT !!

@chuubaba:

Why, the flour was ground by Charles "Wheatstone", of course!
Coop, aa1ww

[aa1ww]

.... GREAT SCOTT !!

@chuubaba ... nicely done

[Carl-NC]

Looking at Carl's schematic in the area of the search oscillator, there is nominally 1nF (C6) + 10nF (C15, at least on the +sourcing transitions) + maybe 200pF (C4+C5) connected in parallel with one of the two coils. Using 120uH (from Carl's previous reply) and the venerable Fres = 1/(2pi*sqrt(LC)) I get approx. 137kHz.

Carl's remembrance of 200+kHz may point to an inflation on my behalf of C15's contribution. Given only ~400pF of adjustment my estimate of Ctotal may indeed be on the high side.

I measured the frequency to be 270kHz, which gives a Ctotal of about 2.9nF. The circuit is similar to a Colpitts but different, and I'm not sure how to calculate Ctotal.

I see a +6V stabilized supply and a +3V virtual ground. Might I ask what the Vcc (Pin 8 and GND/Vee (Pin 4) connections on the LM358's are?

The '358s have VCC=6V & VEE=0V.

[aa1ww]

Search oscillator musings

So ..... Here's my take on the search oscillator and coil electronics, etc. ...

I tried some LTspice simulation (I know lies, damned lies, and simulations) and the D1, R18, C15 level detector is just super low Q and doesn't effect tuning. So how am I going to BS my way from ~1.19nF to Carl's 2.9nF? How about search coil self capacitance? Here's a sim screenshot of my LTspice sim:


The forum did not care for my LTspice ".asc" file type so maybe I can add a link to that tomorrow or Friday.

Thank again, Carl-NC, et. al.,

Coop, aa1ww