Easy Adding of Iron Recognition in PI MD

[Detectorist#1]

Hi all,
Now, I propose an solution for easy adding of iron recognition to many of PI simple detectors. The solution uses ready DD coil for PI detectors. For right work, the inductivity of the two coils have to be very close. This is not the case with DD coils for VLF detectors. In "Fe" mode, the detector works in passive mode - without TX pules. "Fe" targets induces signals under influence of the Earth's magnetic field in the two coils and this signals are amplified by the input differential amplifier. Differential amplifying eliminates influence of near electromagnetic fields. The coil from DETECH company type "15" x 12" SEF coil" is used in experiments. For the results - beer cap is recognized from 25cm and 25c USA from 30cm. These results aren't too big but the advantage of this solution is 100% sure Fe recognition of the targets.
The scheme how to add this solution is attached.

[GeoMax]

Hi Detectorist#1

Interesting approach. Where do you connect the signals of "DISABLE PULSES" and "U3 Pin 6"?

[Detectorist#1]

Hi GeoMax,
Usually, it is enough to stopping of the generation of the main oscillator (NE555 or CD40106) for stopping of all pulses. The output of U3 have to be connected to SAT parts and further to gain and threshold parts before sound generation. The idea for DD coils arrangement for MD is genius!!!

[Detectorist#1]

Sorry,
I found one mistake in the scheme concerned with SW1-1 function. The new one is attached.

[eduardo1979]

Hello Detectorista1. Your approach is very interesting. I'd like to apply this schematic to a Surf PI. Obviously, I'd have to modify a few things, since the Surf PI works with a P-channel MOSFET. I'd like to make a small contribution to your schematic; I would add a 1K resistor between the gate and source of the MOSFET, so that when we disconnect the pulses, the MOSFET doesn't burn out. I'd also like you to answer a question: do you think hot rocks would be detected as iron objects?

[Detectorist#1]

Hello eduardo1979,
First, thank you that you find this approach as interesting. For the small contributions: I assume the control circuit for the GATE of MOSFET haven't tri-state output and when the TX pulses will be disabled (not disconnect), the GATE will be not in float state. For hot rocks - because of symmetrical and differential shape of the two coils in DD configuration, in "Fe" mode the detector will be ground balanced by default. In "ALL" mode, the detector will needs additional "ground balance" channel. Also NE5534 is very old design and OPA1611AID gives two times reduced noise in experiments - but this IC is more expensive part and needs SO-8 to DIP-8 adapter.

[Detectorist#1]

Hi eduardo1979,
The hot rocks most probably will be recognized as Fe targets. The general search haves to be made in "ALL" mode. After recognizing of some target, the mode have to be switched to "FE" mode and have to be repeated the searching on the same place. Have two reasons for this order of searching: 1. The sensitivity of the detector is higher in "ALL" mode. 2. The work in "FE" mode haves to be more careful - the detector is very sensitive to weаk
magnetic fields and the tilt of the search coil in left-right direction is not allowed especially for the soils with big magnetization - this tilt will produces false signals.

[eduardo1979]

Hello Detectorist 1.
Thank you very much for the detailed explanation.

[Detectorist#1]

Hi eduardo1979,
For my experiments on this idea, now I use "DETECH 15" x 12" SEF DD coil" sold on market for PI detectors of MINELAB company. In this coil, the inductivity of the two coils are not very close - 300uH for TX coil and 380uH for RX coil. As a result, the suppression of 50Hz electromagnetic field in the room experiments is not so good as will be, if the two coils in DD sensor was be more identical. Nevertheless, this sensor is good enough for the practical use. It is interesting for me if on the market is sold DD coil with equal inductivity of the two legs. Maybe the German company LORENZ uses 26cm and 35cm fully symmetrical PI DD coils with equal inductivity but I haven't information for free sold of these products on the market.

[eduardo1979]

Hello detectorist1.
I have three original Minelab coils. I have an 11-inch DD coil, an 8-inch mono coil, and an 11-inch mono coil. I also have two original Whites TDI coils. One dual field and one 6-inch mono coil, if I remember correctly. I really enjoy tinkering with metal detectors, as they've been my passion since I was little. I love PI detectors, especially those for gold nuggets. I've been working for a while on a PI detector with HI-LOW conductivity discrimination, based on the GS5 and TDI, but controlled by an ESP32 microprocessor and with a 256x128 pixel LCD display. I made a prototype about a year ago, but now I'm improving certain features that I missed at the time.

[Orbit]

Eduardo you say you have the original double coil for the TDI, does it heat the irf with it or only with Comander coils?In essence, the transistor on the tdi sl heats up a bit within the limits of the normal value, I have never measured the temperature, but it does not overheat if you use Comander, I think that you still need a series resistor of 1r - 2r in order for the Irf to work without load.

[eduardo1979]

Hi Orbit. I don't have the original Whites DD coil for the TDI. For the TDI, I have the 12.5" Dual Field coil, which comes standard with the detector, and a 6" Mono coil. I have a DD coil for Minelab, and if I connect it to the TDI, the IRF740S gets much hotter than with the original coil. Also, the TDI doesn't work well with Minelab DD coils. The ground balance doesn't work correctly. Ground balance works fine with Minelab Mono coils, but the IRF740S gets much hotter than with the original coil. I hope I understood your question correctly.

[Orbit]

Let me understand Eduardo, the original double field coil and mono original for TDI has a higher resistance so it doesn't overheat.. I have written somewhere exactly how much the resistance is.. Otherwise the DD Commander is not very well balanced. tried because I don't have it at the moment. But you can try the Explorer Etrac coil, if you have one, I think the TDI will work better or simply make the coil one more time, note DD tested, not repeating the balance correctly, I'm sure of that. Eric added 2R to the original coil on the gs 4, although the original coil is on the gs 4 and gs 5, I think the resistance is around 3.5R or 4 R, I can measure it since I have the original coils. I remembered now that the Comander DD has the same resistance as the mono Comander 0.8R, my multimeter says so! Which of course warms Irf!

[Detectorist#1]

Hi eduardo1979,
Please measure the DC resistance and inductance on both sides of the MINELAB 11" DD coil. For good Fe/non-Fe separation, MINELAB recommends using Commander type DD coils and include a size of DD 15"x12" semi-elliptical and DD 18" round. I am sure that these coils should have equal resistance and inductance on both sides.

[eduardo1979]

Hi Detectorist1.
Sorry for the delay.
My coil is an 11" round DD for the GP series, which comes with a large blue sticker. The specifications are as follows:
TX=470uH..............0,4 Ohms
RX=800uH............16,6 Ohms

PIN1 = -RX
PIN2 = +RX
PIN3 and PIN4 = +TX
PIN5 = -TX