Popular Electronics, Feb. 1969

[technos]

From here:

It seems to me as double (symetrical VR1+VR2 & VR1a+VR2a with common reference L1 + L2) bridge:
Or phase shift separate two second arms from bridge?

Wow, I must complement my fine colleagues for finding the most unique patents and circuits here! I kid you not I was looking at bridge circuits this last weekend in the patents and missed this one entirely.

Very interesting, this is a metal detector used in food processing. The patent alludes to the probes being a "spatula." The company that owned this patent works in the food processing industry;

http://www.encyclopedia.com/doc/1G1-109569972.html

This could be used in a machine to find metal shavings in a batch of food as they mix it! Yikes!

Actually, the purposes of VR1/VR2 and VR1a/VR2a is to have *two* different bridges. The problem is that the first bridge adjusts to the metal (shavings!) but will stop detecting the metal once the bridge is adjusted quickly. So they have two bridges operating at different speeds. The quick one adjusts, and then the slow one (still detecting the metal) throws the alarm.

The very thing about bridge circuits, their sensitivity, is also the thing that makes them finicky. They tend to need continual adjustment as the components drift around due to the operating conditions (like temperature).

This detector operates not by sensing a VLF signal, but rather the changing inductance of L1/L2. The whole purpose of the phase detectors (and OSC./T1) is to generate a d.c. error voltage that is used to change VR1/VR2 to bring the bridge back into balance.

So this circuit has more in common with a B.F.O. than a T.R. detector in that respect.

A very interesting circuit with some good ideas. You might could build a regular B.F.O. metal detector that doesn't have any knobs!

[technos]

Hi technos. Coil Coupled Operation (CCO) Metal Detector:Search oscillator IC1a oscillates at around 500kHz, depending on the positioning of the coils on the search head. The presence of metal induces changes both in the inductance and coupling of the two coils, thereby inducing a shift in oscillator IC1a’s frequency.A second oscillator, IC1b, together with buffer IC1c, serves as a beat frequency oscillator, oscillating at around 2·7MHz. This higher frequency avoids frequency lock, and also increases sensitivity. An LC oscillator was chosen here for its stability, to avoid problems with drift. The CCO MD in postings # 39 and # 48.

That's an interesting circuit. But it still is shifting frequencies. It may be more sensitive than a regular BFO simply because you have *two* coils, instead of one. I still think that a regular TR will still outperform the CCO. You need to build the regular one transistor BFO and then the CCO. Then air-test a quarter and report back to us!

[WM6]

So this circuit has more in common with a B.F.O. than a T.R. detector in that respect.

After all your kind explanations here, it is hard to imagine for me how regular T/R detector can be made in sort of bridge implementation.

A very interesting circuit with some good ideas. You might could build a regular B.F.O. metal detector that doesn't have any knobs!

Without knobs? Good to see that possibilities in schematic, maybe REMI group will take this idea and put some working in life?

I still think that a regular TR will still outperform the CCO.

Me too. I am searching to pull T/R solution out of mikebg creative ideas (depth as first comparative criteria)

[Grinok]

In Chemelec's web page you can find the Vehicle Loop Detector. This detector if for big mass as car, ideal for to counting it. There are in the web page other simple detectors.

http://www3.telus.net/chemelec/Projects/Projects.htm

[maikl]

That's an interesting circuit. But it still is shifting frequencies. It may be more sensitive than a regular BFO simply because you have *two* coils, instead of one. I still think that a regular TR will still outperform the CCO. You need to build the regular one transistor BFO and then the CCO. Then air-test a quarter and report back to us!

Hi technos.I built a metal detector CCO (good sensitivity and good discriminate sound). Unfortunately, the maximum detected Victoria penny 180mm in air. Is there any idea to increase the 300-400mm?

[technos]

Hi technos.I built a metal detector CCO (good sensitivity and good discriminate sound). Unfortunately, the maximum detected Victoria penny 180mm in air. Is there any idea to increase the 300-400mm?

Great job on building the circuit, and thanks for reporting back!

Your circuit is doing pretty good! It probably has the same depth as a regular T/R for that size of coil! But the CCO is a BFO detector which is hard to improve. Besides if you ad a bunch of components, you might as well build a T/R of VLF even!

But if you insist, I would try making bigger coils. But it will be hard making it to 300 mm!

[technos]

After all your kind explanations here, it is hard to imagine for me how regular T/R detector can be made in sort of bridge implementation.

Me too. I am searching to pull T/R solution out of mikebg creative ideas (depth as first comparative criteria)

I'm working on this - a bridge circuit may not be the best way to go. But it will take some time and some testing. I would like to run some tests on a network analyzer (which I don't have yet).

[WM6]

I'm working on this - a bridge circuit may not be the best way to go. But it will take some time and some testing. I would like to run some tests on a network analyzer (which I don't have yet).

Bravo technos! Right decision. Maybe "REMI Group" cad do some help here by network analyzer?

[mikebg]

BG mine locator

This is the genuine circuit diagram of "Miniature transistor metal detector" (see the PDF file in posting #7 by Qiaozhi or posting #42 by Technos).
Mike BG

[WM6]

This is the genuine circuit diagram of "Miniature transistor metal detector" (see the PDF file in posting #7 by Qiaozhi or posting #42 by Technos).
Mike BG

Nice mikebg, but are there known to REMI group where single line in schematic is crossing and wehere it is connected?

[mikebg]

WM6, where is the single line? All lines are connected.

[mikebg]

Sorry, now I understand the question. This is the method for circuit drawing without to use pont in wire conection.

[WM6]

WM6, where is the single line? All lines are connected.

If so then must be some shortings in schematic.

like this:

[mikebg]

Nice mikebg, but are there known to REMI group where single line in schematic is crossing and wehere it is connected?

The method without points is ugly, but it avoid errors at printing and scaling.

[Qiaozhi]

The method without points is ugly, but it avoid errors at printing and scaling.

Actually, the left-hand drawing is also ugly. You should never use 4-way crossovers in schematics, and you should always use tie-dots in order to avoid confusion.