Can a meter be added to a non-meter VLF?

[Steve in (MS)]

i just wanted to ask this to all of the electronic people that frequent this site. The reason I ask is many prefer non-meter units because they have characterists that may not be found on metered units.
I was wondering something as simple as a volt/ohm meter could be tied into the discrimination control and get some kind of reading from it. There's got to be either a voltage or resistance going to the disc control, right? Or maybe tie one in one the circuit where the disc control is a part of? Ok, maybe it is not that simple but still there could be a big market for one on the likes of a X-5, Nautilus,Tesoro,etc?
Steve in (MS)

[Max]

Hi Steve,
I'm not a specialist in this field but I'll try to answer your questions. First,
in every kind of VLF manifactured before the 80's the visual indicator was a must.
I know of many of those (bfo, -vlf- or ib-t/r) that were equipped with a kind of
meter, for strenght of signal indication or for disc purposes. This was becouse
every manifacturer agreed (at that time) that this feature was adding something
good to their detectors, becouse from the start of m.d. age (bfo in 1920 or so)
almost any landmine detector has one, and so every hobbist m.d. must have one to
look and feel not just a toy but a hi quality unit.
In the 70's having such a kind of indication was a kind of marketing strategy:
advertising was on easing pin-pointing and discrimination by the built in meter.
Most of these were 100uA to 1mA instruments devices.
Anyway when the digital electronics become a must (first in the marketing strategy
, obviusly) any older style indicator become something to avoid in the control
panel of newer detectors, so they become rare and eventually miniaturized to avoid
extra weight (consider that old fashioned indicators may reach 1/2 pound in
weight or more) and to focus the attention of customers to the newer "controls".
Modern electronics wants lighter, simpler, fool-addicted design.
In any case they serves only for 4 funtcions (as I know):
- battery status indication
- signal strenght indication and pinpointing
- ground balancing indication (where present)
- discrimination of metals indication (where present)
But in any case only very good manifactured m.d. had a good functionality on this
component; in most cases it was simply unuseful or too poor in quality to serve
for something useful.
Modern units (most of) haven't visual indication for the reasons above, and also
becouse the newer circuits can produce an acoustic indication that is everything one
would have to discriminate. No analog battery tester needed (usually it is a led on
the main panel) - no pinpointing by meter needed (concentric coils or other
configurations make simple to pinpoint; extern pinpointing devices are cheapy now).
BUT come back to the question: "Can a meter be added to a non-meter VLF?"
or maybe better "IT'S POSSIBLE TO ADD ONE TO A NEWER VLF ?"
The answer could be "maybe", let's explain.
In older VLF (BFO, or IB, or T/R) the circuit was analog: discrimination was made by
phase shifting (actually also in modern units, but elaboration is often made by a
microprocessor)
--> so a circuit that uses a phase comparator drive the audio output stage that
is (mostly) a VCO (voltage controlled oscillator) , that generates different tones
for different kind of metal (iron vs non-iron at least)
If one gain access the phase shift path then he can build a visual interface for that
signal e.g. a meter (or an LCD or whatever), using simple components like a buffer amp
and a resistor / diode network or interface at the vco output using a e.g.
frequency-to-voltage converter to drive the meter etc).
The problem is ,then, watching at the schematic and
1: find if it's possible jump in the analog path and then interfacing the meter here
2: or, if a DSP (digital signal processor) is present, how to elaborate output signals to
have a corresponding visual target indication.
(So using a different approach, becouse the output signal could be a coded
digital train of pulses that drives the digital audio processor on the base of the
"program" in use).
1 is really simple to do for a skilled engineer
2 could be hard even for a skilled eng(becouse one have to reverse engineering etc etc)
So if you have enough skill, you for sure can do it.
(I think one can do everything if has enough skill )

Anyway, I think that one should buy only a detector that look and feels just as he expect,
and that's becouse , generally speaking, it's not a good idea to open the cover of your
detector if you aren't a really skilled ee.
I think you can search if someone realized the mod you need on your particular
unit so avoiding the too much time waste involved or better if some dealer makes this
mod to the public with little expense.

Best regards,
Max

[Max]

Hi,
here is a simple example of how to add a dvm meter to a minelab unit. Well, Minelab manifactures a particular type (say "smart") of search coils.

http://members.tripod.com/~dlclark/meter/meter.htm

Best regards,
Max

[Sean_Goddard]

Steve in answer to you question, NO the way you propose (except maybe to some Whites machines with a little internal modification) and YES to ANY single frequency machine.. BUT.......I've no finished the design yet, but all I can sya is that the finished unit will be commercial sell for around $120 and add a target ID to ANY single frequency machine. The prototype have been tested on a Fisher and a Tesoro and works fine on both. Can't say any more than that .

Sorry nothing "smart" about Minelab designs (coils that is) the processing is done in the control box and fed OUT on the coil connector.

[Steve in MS]

Yes, I knew about...

the Sov and after market meter. I think it is stuck between the coil and box? Sean I don't know if you are serious or not about having a product that will work with a single frequency, I think if you really have one there would be a lot of interest in it and that is what I was referring to, not the Multi-frq units. Say send me one I will try it out,lol. You won't have any problem selling it.
Steve in MS

[Sean_Goddard]

I've worked out how it can be done and the work is spin off from the Sovereign meter I was designing, but that never got finished due to other commitments. i.e. working to pay bills.

IF I ever get the time to finish this project, then I would be more than happy to let you have one to have a "play" with.

If you think about how it can be done however, it's really quite simple and i'm very surprised no one has done it before.

[Steve in MS]

That sounds good..

Sean, let me know if you ever get around to it, I will even pay for it, I haven't registered here but you can get in touch with me on TD and FM forums thru PM or email, I go under the name Steve(MS). I would be glad to give it a try,
Steve in MS

[Steve in MS]

One other question, I guess the disc turn control is a pot, if that is the case, can another pot and a toggle switch be added that would engage the extra pot and disengage the original pot kind of like the Tejon?

[Sean_Goddard]

Hi Steve, Yes, in fact this is a mod I came up with a LONG time a go, and did electronically. Yes I know Fisher did it on the 1266 but the way Tesoro do it is an exact copy of my method. Nice eh?