Early 1934 PI?

[Qiaozhi]

Don't know the performance. Article refers that is important to reduce in the minimum the vibrator sound, maybe in special receptacle. This was published on Science&Invention magazine, 1921.

But, as you said, can be re-adapted with modern components.

Do you want the article?

Yes please.

It looks like the buzzer would need be to set to a very low frequency to provide enough time for current to build up in the coils. The search coils are shown as having either 100 or 60 turns depending on the thickness of wire used.

[Esteban]

60 turns requires # 14 wire, 1.62 mm diam!, 100 turns requires # 10 wire, 2.58 mm diam! Great current consumption need for to be operative this electrical MD. Email me. Is in public domain your email?

[Max]

This is quite an interesting circuit. In fact, it is most similar to a P.I., as it transmits pulses and uses the same coil(s) to transmit and receive. However, unlike a normal P.I. with a monocoil, there are 2 TX coils that do not need to be balanced, so it's definitely not an I.B. system. At first glance it looks like there is no circuitry to sample the decay curve, but this is not true. If a metal target is located below one of the coils, then the decay time of this coil will be different to the other coil, and a signal will be heard in the telephone receiver. The transformer with 2 primary windings in opposition is used to cancel the signals from the two halves of the TX circuit. The meter is simply zeroed using the rheostat.

It could be quite interesting to build a 2 coil P.I. based on this concept, but obviously using more modern technology. It might even work quite well.

Esteban - how about this as a project?

Hi Qiaozhi,
"In fact, it is most similar to a P.I., as it transmits pulses and uses the same coil(s)
to transmit and receive."

I don't agree here: yes transmit pulses, but using the same coil to transmit and receive
don't mean it's a pulse induction. Older BFO or simpler VLF use also a monocoil to transmit
and receive but aren't pulse induction--> using a monocoil don't mean having a PI
And also transmitting pulses don't mean a PI detector: I think they used this kind of
circuitry because they haven't electronics (tubes, transistors etc) and so some kind of
ac signal must be generated by switching, like in Rumkorf's transformers.

"However, unlike a normal P.I. with a monocoil, there are 2 TX
coils that do not need to be balanced, so it's definitely not an I.B. system."

Well, I don't think so. Coils MUST be balanced here, they MUST have same drive current,
same number of turns, same wire , same shape, same orientation ...so it's a balanced
circuit. Actually I think that this circuit is more IB than many other modern stuff out
there, because induction need really to be accurate balanced here because not
electronic filter or dsp or anything else can align slight differences in this circuit
--> so coils are "phisically" balanced and all the circuit needs perfect balancing !

I think that in use, the unit must be held as in the picture, mean quite distant from ground
like in our T/R detectors, because helding too close to ground one coil respect to the other,
thus disaligning could give false signals due to magnetic or capacitive effects.
Seems also no faraday shield present.

"At first glance it looks like there is no circuitry to sample the decay curve,
but this is not true. If a metal target is located below one of the coils,
then the decay time of this coil will be different to the other coil, and a
signal will be heard in the telephone receiver. The transformer with 2 primary
windings in opposition is used to cancel the signals from the two halves of the
TX circuit. "

I agree only on signal cancellation. Yes, the decay time will be different BUT the
circuit haven't any control/intelligence on timed sampling--> means it works continuosly,
with no care of TX phase and RX phase separation.

Actually there isn't TX phase or RX phase, but just, like in a continuos wave detector,
a unique TX-RX detection process. I think it's not a contiunuos wave, because no continuos
wave is present, but "continuos detection" and even if "pulses of induction" were present
--> can't be time domain / pulse induction as we know it (with tx / rx phase separation)
Much like in "continuos wave radars", where no tx-rx phases separation occours, but
multi-pattern antennae and dsp are used to locate peaks continously.

Another issue : is revelation based on decay curves at the two coils ???
I think that decay curve is only a part of the whole principle of operation and not so
significative here like in our PI detectors. I mean that main differences between the
two halves could be in top of voltage spikes. When e.g. an iron mass is under one of
the coils, then inductance raise and so (by Lenz's law) the voltage spike is higher (abs)
respect to the other.

|V| = L * di/dt --> if di/dt is a constant V increase in magnitude with inductance increase

Means that all the curve "increase", but if you consider power you'll find that more
power (say 90%) is in few us after switchoff.

This fact can be more times more revealable than microvolts small
decay variation after say 15 uS or more in this kind a circuit,
where amplification is done by a diff. transformer.

"The meter is simply zeroed using the rheostat"

No. Meter serves only to check if the device is working ok (1 to 2 amperes) and rheostat
serves to give same ammount of current to the two halves, acting like a shunt regulator for
the two halves so minimizing small resistance asymmetries. If one reads more than 2 amp
something wrong happens e.g. a short at "buzzer" occours and one need to disconnect main
battery to prevent fire on search coils...

"It looks like the buzzer would need be to set to a very low frequency to provide enough time
for current to build up in the coils."

Yes, I think so also. but another issue is arcing...I mean spikes on the switch can cause
burnings at contacts--> an early good practice in Rumkorf was to avoid too buzzing sounds
at switches because the hi current involved will soon burn contacts if too arcing present.
In some cases this will produce a short circuit at the switch damaging the apparatus.

Here there isn't any anti-arcing capacitor so earing a minimum sound is quite critical.
Then the "buzzer" screw is important in correct (and safer) "minimum sound" regulation.

Seems that with modern components will be simple to avoid all this complications, but I
dubt this device could produce good results on small targets e.g. a coin.

Anyway, I think signal cancelling is a good idea ever and maybe with newer stuff, electronics
etc it could be funny to test such a thing on the field.
--

Esteban, I'll like to see the article too --> [email protected]

Best regards,
Max

[Qiaozhi]

Hi Qiaozhi,
"In fact, it is most similar to a P.I., as it transmits pulses and uses the same coil(s)
to transmit and receive."

I don't agree here: yes transmit pulses, but using the same coil to transmit and receive
don't mean it's a pulse induction. Older BFO or simpler VLF use also a monocoil to transmit
and receive but aren't pulse induction--> using a monocoil don't mean having a PI
And also transmitting pulses don't mean a PI detector: I think they used this kind of
circuitry because they haven't electronics (tubes, transistors etc) and so some kind of
ac signal must be generated by switching, like in Rumkorf's transformers.

"However, unlike a normal P.I. with a monocoil, there are 2 TX
coils that do not need to be balanced, so it's definitely not an I.B. system."

Well, I don't think so. Coils MUST be balanced here, they MUST have same drive current,
same number of turns, same wire , same shape, same orientation ...so it's a balanced
circuit. Actually I think that this circuit is more IB than many other modern stuff out
there, because induction need really to be accurate balanced here because not
electronic filter or dsp or anything else can align slight differences in this circuit
--> so coils are "phisically" balanced and all the circuit needs perfect balancing !

I think that in use, the unit must be held as in the picture, mean quite distant from ground
like in our T/R detectors, because helding too close to ground one coil respect to the other,
thus disaligning could give false signals due to magnetic or capacitive effects.
Seems also no faraday shield present.

"At first glance it looks like there is no circuitry to sample the decay curve,
but this is not true. If a metal target is located below one of the coils,
then the decay time of this coil will be different to the other coil, and a
signal will be heard in the telephone receiver. The transformer with 2 primary
windings in opposition is used to cancel the signals from the two halves of the
TX circuit. "

I agree only on signal cancellation. Yes, the decay time will be different BUT the
circuit haven't any control/intelligence on timed sampling--> means it works continuosly,
with no care of TX phase and RX phase separation.

Actually there isn't TX phase or RX phase, but just, like in a continuos wave detector,
a unique TX-RX detection process. I think it's not a contiunuos wave, because no continuos
wave is present, but "continuos detection" and even if "pulses of induction" were present
--> can't be time domain / pulse induction as we know it (with tx / rx phase separation)
Much like in "continuos wave radars", where no tx-rx phases separation occours, but
multi-pattern antennae and dsp are used to locate peaks continously.

Another issue : is revelation based on decay curves at the two coils ???
I think that decay curve is only a part of the whole principle of operation and not so
significative here like in our PI detectors. I mean that main differences between the
two halves could be in top of voltage spikes. When e.g. an iron mass is under one of
the coils, then inductance raise and so (by Lenz's law) the voltage spike is higher (abs)
respect to the other.

|V| = L * di/dt --> if di/dt is a constant V increase in magnitude with inductance increase

Means that all the curve "increase", but if you consider power you'll find that more
power (say 90%) is in few us after switchoff.

This fact can be more times more revealable than microvolts small
decay variation after say 15 uS or more in this kind a circuit,
where amplification is done by a diff. transformer.

"The meter is simply zeroed using the rheostat"

No. Meter serves only to check if the device is working ok (1 to 2 amperes) and rheostat
serves to give same ammount of current to the two halves, acting like a shunt regulator for
the two halves so minimizing small resistance asymmetries. If one reads more than 2 amp
something wrong happens e.g. a short at "buzzer" occours and one need to disconnect main
battery to prevent fire on search coils...

"It looks like the buzzer would need be to set to a very low frequency to provide enough time
for current to build up in the coils."

Yes, I think so also. but another issue is arcing...I mean spikes on the switch can cause
burnings at contacts--> an early good practice in Rumkorf was to avoid too buzzing sounds
at switches because the hi current involved will soon burn contacts if too arcing present.
In some cases this will produce a short circuit at the switch damaging the apparatus.

Here there isn't any anti-arcing capacitor so earing a minimum sound is quite critical.
Then the "buzzer" screw is important in correct (and safer) "minimum sound" regulation.

Seems that with modern components will be simple to avoid all this complications, but I
dubt this device could produce good results on small targets e.g. a coin.

Anyway, I think signal cancelling is a good idea ever and maybe with newer stuff, electronics
etc it could be funny to test such a thing on the field.
--

Esteban, I'll like to see the article too --> [email protected]

Best regards,
Max

A BFO detects changes in frequency of the transmitted signal due to the presence of a magnetic target that alters the inductance. This device is detecting a change in amplitude, so it is not a BFO.

It is true that this design relies on an imbalance between the coils to detect a target, but the balancing is done by the two counterwound primaries on the transformer. It appears to me that the meter simply indicates which coil is being unbalanced and the strength of the target signal. Of course, the coils may also be constructed in an IB arrangement, but I don't think that's necessary here.

At the moment I'm in the middle of some important design work, and don't want to get sidetracked by this. But I'd like to investigate this design further at a later date using more modern technology.

[Max]

A BFO detects changes in frequency of the transmitted signal due to the presence of a magnetic target that alters the inductance. This device is detecting a change in amplitude, so it is not a BFO.

It is true that this design relies on an imbalance between the coils to detect a target, but the balancing is done by the two counterwound primaries on the transformer. It appears to me that the meter simply indicates which coil is being unbalanced and the strength of the target signal. Of course, the coils may also be constructed in an IB arrangement, but I don't think that's necessary here.

At the moment I'm in the middle of some important design work, and don't want to get sidetracked by this. But I'd like to investigate this design further at a later date using more modern technology.

Hi Qiaozhi,

"This device is detecting a change in amplitude, so it is not a BFO."

Never said that it's a BFO. Just in previous posts that could have BFO-like output (beating and change in tone), thinking at some etherodyne stage, before seeing the schematic.

"It is true that this design relies on an imbalance between the coils to detect a target, but the balancing is done by the two counterwound primaries on the transformer."

So it's a balanced type. I called this "Rumkorf balanced style"...RBS, because I've read of some of these devices that used similar stuff (vibrating switches etc) on a pdf also here on geotech. If I remember well it was "roberts150.pdf" or something and treats the history of metal detecting (Bell and others).

"done by the two counterwound primaries on the transformer" yes, but only way to balance at these is by using balanced search coils also, in balanced two halves schema.

"It appears to me that the meter simply indicates which coil is being unbalanced and the strength of the target signal. "

No. For sure, can't work like this. Meter is connected in series with both sub-circuits so measures only total current delivered.

"Of course, the coils may also be constructed in an IB arrangement, but I don't think that's necessary here."

Well, maybe not necessary this way and other configurations possible but if one have to balance the circuit and reveal unbalances produced by metal targets some kind of geometric balance is required in any case e.g. to balance ground or Eart's magnetic field or capacitive effects etc. Also in Robert's document there are some coil balanced similar devices descriptions e.g. George Hopkins or also M.C. Gutton from France --> they used induction
bridges and balanced schema also.


Unfortunately, I'm also too busy in Bandido and other stuff + work to make a serious research on this topic.

Best regards,
Max

[Qiaozhi]

No. For sure, can't work like this. Meter is connected in series with both sub-circuits so measures only total current delivered.

Hi Max,

Yes, you are correct. The meter is a 20A type, and is not centre-zero. So this simply indicates the current consumption.
I would definitely like to investigate this further.
Must resist .... must resist...

[Esteban]

The meter also help for to adjust in good point the screw vibrator, in the range 1-2 A.

[Max]

The meter also help for to adjust in good point the screw vibrator, in the range 1-2 A.

Hi guys,
Yeah. I think one can simulate switch using an onld car relay...the type used e.g. in front light commutation (20A +) that in good quality ones have anti-arcking plates. Another stuff useful could be some rotating points used in very old rotating car spark generators as switches/interrupter that again are anti-arcking plated (good ones with very resistant anti-arking alloys/metals).

Anyway, in car relay no adjustment screw... and Ruhmkorff (now I write it ok) stuff needs this kind of regulation.

Also other stuff can be from a dismantled old car...well...not everything.
Most critical is rheostat...a 3 ohm rheostat is almost impossible to find now except in some school physics lab I think...

Well also simple electronic components can do everything...but I think a major issue is voltage breakdown...on switching device.

Must resist too !

Best regards,
Max

[Esteban]

The other substitute of the buzzer is the electromechanical vibrator. You can "control" the oscillation adding weight, if I remember.

But, off course, the ideal is replace the old system by semiconductors.

[Esteban]

The name of a "more modern" project with buzzer is (translation from Spanish) "Simplified Metal Detector".

Original was published on Radio Electronics. I have the reprinted version in Spanish, published in 1968.

Author: B. F. Miessner. This is like a two boxes??? (racktet form) wich receiver is a AM radio. The parts are shielded by aquadag. In the article you can find steps for to silent at the maximum the buzzer's noise (mechanical noise). Author refers that the most good buzzer he found for this project was made for Ericsson Telephone Manufacturing Co. from Sweden.

The size of the wand of this detector is 76 cm.

[Max]

The name of a "more modern" project with buzzer is (translation from Spanish) "Simplified Metal Detector".

Original was published on Radio Electronics. I have the reprinted version in Spanish, published in 1968.

Author: B. F. Miessner. This is like a two boxes??? (racktet form) wich receiver is a AM radio. The parts are shielded by aquadag. In the article you can find steps for to silent at the maximum the buzzer's noise (mechanical noise). Author refers that the most good buzzer he found for this project was made for Ericsson Telephone Manufacturing Co. from Sweden.

The size of the wand of this detector is 76 cm.

Hi Esteban,
yes, it's a Two-box, I have realized one similar using a long wave receiver but with
a continuos wave transmitter (homemade) around 130Khz, some years ago as a rainy sunday
technology obsession.
I've found it was good to finding e.g. water pipes and the like under 2+ meters of
ground, but then dismantled because I don't like 2-box.

But here the "oscillator" is a commutation one...like in the other project. Here also
a radio (AM) receiver is used like in my device and nulling coupling between tx and rx
is by 90 degree "antenna" offset, I think, like other two-boxes. Thus decoupling
geometriacally field lines with receiver "coil".

Main difference with first electric circuit: THERE IS A CAPACITOR --> tuned circuit
where in the other there isn't - but keep in mind parassistic effects...so there is too
(but here let's talk about the newer circuit)

This circuit is less interesting because it's not so esotheric.... like previous!

Here actually there is only the big TX round shaped coil and rx-antenna is the ferrite
stick of the radio. Seems that a "regulation" metal plate is used as magnetic field
"deflector", thus giving some kind of major nulling in eventually coupled flux lines.

Ferrite is highly directive so I think the unit will work like other 2-box with conventional
rx loop but with maybe improved directivity and less lateral coupling, but this stuff can't
work under some hundred khz because leaks by magnetic hysteresis in the core increase at lower
frequencies, thus saturating rx-stage.

Tx works on step function energizing the tuning circuit (coil+capacitor) then peaks at
resonance frequency + lot of harmonics are obtained after switchoff. Then the radio
revealer (AM) detect imbalances.

Radio works in two ways :
First is off-resonance: I've made one when I was 13 , after watching a tv series ...anyway
I've used also AM radio like in tv, with ferrite coil near the ground at the end of a pole.
No separate transmitter, only rx osc. and local osc. needed --> so only a portable AM radio
Just tune a radio station, then when a metal was present a slight frequency shift (you ear
your radio station vanishing...) reveals a change in inductance.
Not so sensible. Say under 3inches for a coin.

Second is BFO. Main tx freq. and harmonics are detected by revealer when an imbalance
happens, audible sound detected by BF stage as low freq. beating by the above and local
osc. running e.g. at 455KHz. One can ear distinct the commutation frequency here (some
decades/hundreds hertz) that is a kind of rough modulation of tx-freq(s).
Not too bad in performance if well constructed.
All like in other CW 2-boxes.

"The parts are shielded by aquadag."

just varnished: this is to protect from rain , dust, etc and keep coil turns fixed and close.

"you can find steps for to silent at the maximum the buzzer's noise (mechanical noise). "

this is to avoid too arcking and then too "commutation noise", when no target present,in output

"Ericsson Telephone Manufacturing Co. from Sweden"

I had many of these in past running at 24v...from an old telephone central commutation PCB -
I think also this buzzer is just a telephone relay in self-commutation. They have very good
contacts, plated for extra heavy duty. I don't remember where my 24v ones are
...maybe just launched in the trash can some years ago.

Here is another in the picture. This is bigger than ericsson ones but higher quality:
It's a Siemens hi-quality relay, with regulation srews and anti arcking plates.
Dimension about 9cm x 4cm x 2.5 cm.
Now it's not so easy to find, I have just this one - in my "collection" - and don't want
to use !

Must resist !!! Must resist !!!

Best regards,
Max

ps: the Roberts article is here
http://geotech.thunting.com/pages/me...roberts150.pdf

[Esteban]

Here actually there is only the big TX round shaped coil and rx-antenna is the ferrite stick of the radio. Seems that a "regulation" metal plate is used as magnetic field "deflector", thus giving some kind of major nulling in eventually coupled flux lines.

Yes, exactly, the metal plate actuates as deflector.


Just varnished: this is to protect from rain , dust, etc and keep coil turns fixed and close.

Words of the author:

The receiver is in a small wood box cover with aquadag, 500-1,000 resistence, measured between opposite extreme part. Is for to shield against the electrostatic component of the transmitter, but permits the electromagnetic portion of the signal. Without this, will be impossible to obtain an inaudible zero audio.


this is to avoid too arcking and then too "commutation noise", when no target present,in output

No, the author make mechanical shield: gum, wax, etc., into a glass receptacle, because the mechanical noise is an obstacle for hear clearly the zero point.

I read than a method for to reduce the spark (and consequentely the interference causes by the contacts) is to connect a 100 ohm resistor in the contacts. This is the "damping resistor".

[Max]

Here actually there is only the big TX round shaped coil and rx-antenna is the ferrite stick of the radio. Seems that a "regulation" metal plate is used as magnetic field "deflector", thus giving some kind of major nulling in eventually coupled flux lines.

Yes, exactly, the metal plate actuates as deflector.


Just varnished: this is to protect from rain , dust, etc and keep coil turns fixed and close.

Words of the author:

The receiver is in a small wood box cover with aquadag, 500-1,000 resistence, measured between opposite extreme part. Is for to shield against the electrostatic component of the transmitter, but permits the electromagnetic portion of the signal. Without this, will be impossible to obtain an inaudible zero audio.


this is to avoid too arcking and then too "commutation noise", when no target present,in output

No, the author make mechanical shield: gum, wax, etc., into a glass receptacle, because the mechanical noise is an obstacle for hear clearly the zero point.

I read than a method for to reduce the spark (and consequentely the interference causes by the contacts) is to connect a 100 ohm resistor in the contacts. This is the "damping resistor".

Hi Esteban,
have some dubts...sometimes electronics is funny!

"
The receiver is in a small wood box cover with aquadag, 500-1,000 resistence, measured between opposite extreme part. Is for to shield against the electrostatic component of the transmitter, but permits the electromagnetic portion of the signal. Without this, will be impossible to obtain an inaudible zero audio.
"

??? what ??? The receiver is faraday shielded ??? There will be a kind of noise source...
somewhere.. but where ??? Can't see...ehm...maybe some device...the buzzer, yeah !
I never see a fully enclosed-faraday shielded receiver ! Well, maybe a potato chip bag could be used too...
and is also cost-effective! Very strange. Seems black magic using some nickel coating.
But this stuff couldn't affect performance ? ::confused:: life is strange...mr. meissner
too...maybe he thinked about something useful here I can't see.

"
No, the author make mechanical shield: gum, wax, etc., into a glass receptacle, because the mechanical noise is an obstacle for hear clearly the zero point.
"

Well, yes makes sense. Also because mechanical buzzing has same frequency of expected
output signal...ehm difficault to distinguish between
Also because electrical commutation noise has same frequency of expected output signal...
ehm...even more difficault...
(seems "murphy's law of induction" here:
- the more the amplification the more shielding required...wow!)

"I read than a method for to reduce the spark (and consequentely the interference causes by the contacts) is to connect a 100 ohm resistor in the contacts. This is the "damping resistor"."

Sarcasm ?
Well, yes ...resistor dumps oscillations...but there are more suitable devices if
you want to switch hi-current without arking...examples:
- argon filled cavity enclosed switch
- hi-vacuum switches
- electrolitical switch
- some newer solid-state with hi breakdown
- ******-switch ehm...well this kind can't you find anywhere - just forget !
...etc etc etc
I suggest strongly argon filled cavity that are very good and relatively cheapy!

Best regards,
Max

[Esteban]

??? what ??? The receiver is faraday shielded ??? There will be a kind of noise source... somewhere.. but where ??? Can't see...ehm...maybe some device...the buzzer, yeah !
I never see a fully enclosed-faraday shielded receiver ! Well, maybe a potato chip bag could be used too... and is also cost-effective! Very strange. Seems black magic using some nickel coating.
But this stuff couldn't affect performance ? ::confused:: life is strange...mr. meissner too...maybe he thinked about something useful here I can't see.

Is not fully shielded, only is painting with graphite coloidal solution. The resistence of painting is 500 to - 1,000 ohms. Also you can use this aquadag for PI coils.

[Max]

??? what ??? The receiver is faraday shielded ??? There will be a kind of noise source... somewhere.. but where ??? Can't see...ehm...maybe some device...the buzzer, yeah !
I never see a fully enclosed-faraday shielded receiver ! Well, maybe a potato chip bag could be used too... and is also cost-effective! Very strange. Seems black magic using some nickel coating.
But this stuff couldn't affect performance ? ::confused:: life is strange...mr. meissner too...maybe he thinked about something useful here I can't see.

Is not fully shielded, only is painting with graphite coloidal solution. The resistence of painting is 500 to - 1,000 ohms. Also you can use this aquadag for PI coils.

Hi Esteban,
now I think I understand. If open somewhere and not too thick will be good insulation from too noise. OK.
Graphite is ok for this kind of stuff.

Best regards,
Max