Induction Balance Metal detector

[goldsniffer]

I am trying to build a detector based on a design from Silicon Chip May 1994. What I have built does work and I think it will be very good for what I want but there is a problem. The circuit board is extremely sensitive to me being close to it. Trying to adjust the controls is very difficult because of that. The unit gets loud just bringing my hand close to the control knobs. Has anyone had this problem and what did you do to overcome it?

Thanks for any advice.

[Skippy]

For the benefit of everyone, this is the project, by John Clarke: https://www.geotech1.com/cgi-bin/pag...arke/index.dat

[Mr.Jaick]

I am trying to build a detector based on a design from Silicon Chip May 1994. What I have built does work and I think it will be very good for what I want but there is a problem. The circuit board is extremely sensitive to me being close to it. Trying to adjust the controls is very difficult because of that. The unit gets loud just bringing my hand close to the control knobs. Has anyone had this problem and what did you do to overcome it?

Thanks for any advice.

This is called capacitive coupling:
your body acts as a plate of a capacitor while the DUT(device under test) another plate.

that means any environmental noise couples through you to the DUT.
solutions:
Shielding is the answer.
Approaches to Shielding:
1) connect the body of the Pots to the AGND(Analog Ground) node of your circuit, you might just get away with this.
2) use a metal box for the project and connect it to the AGND, or if you insist on plastic boxes then conductive tape or thick graphite paint to the walls and you know where to connect them.
option 1 is still recommended if the pot body does not electrically connect to the chassis.
now there are other things to do but probably not an option for you right now.
like designing a 4-layer PCB with proper stack-up so you can contain the fields inside and out, but still there will be susceptibilities cause you run RX wires around.(there are solutions to this as well)

some more theory if you like...
basically there are three plates for this capacitor:
A) Noise source, like your house wiring and AC adaptors, etc.
B) Your body
C) conductive areas of the circuit board

the reason why A does not harm C as much without B being in the middle is that C has very small surface area and they are "Far away" so A to C forms a very small capacitor so only supper high frequencies can couple, but on the other hand your circuit doesn't work at those high frequencies and filters it out, but when your body B comes in between, things change, now there is a massive plate to couple those far coming E-fields to the DUT and it's "Much Closer" to circuit's sensitive traces.

[goldsniffer]

Thanks Mr. Jaick. I actually did try suggestion one but it did not help. I am using a plastic case but I do have an old metal (aluminum) case and will try that. It is not suitable for a finished product but as a test it should be fine.

[waltr]

I can believe this circuit is sensitive to proximity of your hand.
Did you do the PBC layout the same as in the article?

Shielding is a good solution. If a trimmer needs access drill a hole in the metal box to reach the trimmer. A small hole will not affect the shielding.
Then a piece of tape over the hole to keep dirt out.
Good luck and have fun with this vintage detector.

[goldsniffer]

I did not do a PCB. I used a perforated circuit board and wire. It could be my layout of components.

It is difficult to get the controls set correctly but when I have it seems to work quite well. I have the coils set on a plastic stand. It can detect a small rock, smaller than my thumb, containing pyrrohtite from about 18 inches. It even detects a really small rock, maybe 1/2 inch at the most in any direction. It reacts well to a loonie suspended from a string in order to keep my hand well away from the coils.

I think we will move on to a pulse detection design after I get this working.

Thanks for the suggestions.

[Skippy]

It's not the kind of circuit that works OK on perf-board. It runs at 80 kHz - meaning capacitive coupling becomes more significant, eg. between tracks, to your body, to the target and the ground it's buried in. Also, 4046 PLL chips are notoriously difficult ... sensitive to power supply variations and noise .. and this design has a switching regulator for a PSU, whish is a bad idea for a sensitive high-gain analog circuit. The regulator itself doesn't suit perf-board, as it is a high-freq power-switching circuit. To make testing it's response easier, blu-tack your coin etc to a 30cm plastic ruler, then you can wave it about without your hand coming too close.

[GeoMax]

I am just curious. Would be the circuit suitable to make out of it a two-box detector?

[goldsniffer]

Thanks for the info Skippy. I did not use the power supply in the schematic. I just powered it with 6 AA batteries. The voltage going to the ICs and transistors is 7.15 volts and not the 7 in the schematic. I did buy a power supply unit that is a similar design. But powering it with 2 AA batteries would not give the voltage required and I could not buy (locally) a C cell holder that the design requires. The unit I bought is capable of well over the 8.8 volt input on the schematic but the AA just will not do the job. If I want to use it I wil have to order the C cell holder. Of course none of that goes back to the perforated CB. If I have a fundamental flaw that has to be cured.

When I built a magnetometer I did make a couple PCBs but I found it easier to just use perforated board as I improved things. No issues at all there. The mag did not use the 4046 PLL. I know that Scintrex did use them in their early MP2 mags. I used a mega162 as the microprocessor clocked at 16 mhz to run the program.

I wondered about the same thing as Geomax. If I separate the transmit circuit completely away from the receive if it would make a difference.

Thanks again for all the advice. Always appreciated.

[Carl-NC]

I am just curious. Would be the circuit suitable to make out of it a two-box detector?

Yes, since there is no synchronous demodulation you can separate the TX from the RX with no problem. Probably won't help with the proximity effect, which I don't understand. This design doesn't look to be especially conducive to proximity, it's just a peak detector. Sometimes that problem is indicative of something not working right. Try holding a long metal probe and see if you can better pinpoint where the sensitive area is.

[leejohn63]

The best solution is to shield. Make a notch in the shipping container if accessibility to a trimmer is required. The shielding won't be affected by a little hole.
After that, tape the opening to prevent dirt from getting in. Enjoy and success with this antique detector. you can visit the Tyler the Creator Merch Shop for better understading

[goldsniffer]

As several suggested it appears that shielding was the answer. I replaced the cables I had with some heavier coax cable. So 4 new cables coming from the sensor to the board. I lined the enclosure with foil. Not sure if that made any difference or not. The change in cables appears to have made a major difference. I can now touch the controls with virtually no change at all. Now we can move on the assembling everything and when the snow goes I can try it. Maybe 3-4 weeks.
To make the box remveable I need some connectors for the coax cable to the box. The box has simply 22 gauge braided wire. I want to go from that to coax. I have looked and I do not see a connector that does that but they do exist. Anyone out there know the right name or maybe someone has their own connection design.

Thanks for all the advice.....always appreciated.

[waltr]

Good news with the shielded cable (coax)..

Exactly which coax did you use? This is very important to obtain the correct connector.

Various types of coax connectors are:
BNC
UHF
'F' type

These are available as Coax ends and panel mount (for the enclosure)

[goldsniffer]

The cable is RG6 18 gauge conductor (High performance 3.0 GHZ Digital Cable)

Checked out the types and 'F' type would be the one. Pretty certain the cable ends are not an issue and I have access to a good crimper....much better than the cheap one I bought years ago. It was the end on the box that I need. Not really sure why I did not use the term "panel" in my initial search but I didn't. Anyway I found something on amazon that appears to be what I need. But if you have a suggestion pass it along.

Thanks

[gerson]

I DON'T KNOW IF IT ACCORDS WITH THE ORIGINAL SCHEMATIC. BUT IF SOMEONE WANTS TO CORRECT IT. https://drive.google.com/file/d/1l0N...usp=drive_link https://drive.google.com/file/d/1_d0...usp=drive_link https://drive.google.com/file/d/1NvX...usp=drive_link