Coil Winding Guide

[Rustybolts57]

Hi all, I am new to the forum. I have been building a simple VLF detector. "https://hackaday.io/project/171177-diy-simple-smartphone-metal-detector"
Can anyone direct me to a coil winding guide. My main questions are: how the direction of the winds should be laid out on the 2 coils and proper connection of the start and end of each coil. Shielding I will deal with later. I also plan on building a couple of Arduino based detectors, so general coil winding and polarity knowledge would be a help. PI will likely be next.
Thanks in advance

[Carl-NC]

When I design detectors I arrange the coils so that a ferrite target produces an in-phase voltage on the RX coil. I can't tell from that project what needs to be done, so just wind the coils in any fashion but before you permanently affix them test the detector. If it doesn't work right, simply flip one of the coils upside-down.

[kt315]

RB57
give a direct link. i get

Page Not Found

[Skippy]

Hopefully this link works OK: https://hackaday.io/project/171177-d...metal-detector

[Rustybolts57]

RB57
give a direct link. i get

Page Not Found

[Rustybolts57]

When I design detectors I arrange the coils so that a ferrite target produces an in-phase voltage on the RX coil. I can't tell from that project what needs to be done, so just wind the coils in any fashion but before you permanently affix them test the detector. If it doesn't work right, simply flip one of the coils upside-down.

Thank you

[Skippy]

"How the direction of the winds should be laid out on the 2 coils and proper connection of the start and end of each coil ?" The schematic diagram shows the output is completely symmetrical, so it makes no difference which way round the Receive coil is connected. But when considering the shield, connect the smartphone ground to the RX coil shield. Similarly, the Transmit circuit can be connected either way round.

[Skippy]

Also, when setting the frequency of the generator, you will need to adjust it, so that maximum resonance of the Inductor-Capacitor circuit is achieved. Capacitors have manufacturing tolerances, for example +/- 5% , and home-made coils/inductors can vary by 5% easily - for example a tightly bound coil will have more inductance than one loosely bound

[Rustybolts57]

Also, when setting the frequency of the generator, you will need to adjust it, so that maximum resonance of the Inductor-Capacitor circuit is achieved. Capacitors have manufacturing tolerances, for example +/- 5% , and home-made coils/inductors can vary by 5% easily - for example a tightly bound coil will have more inductance than one loosely bound

The circuit shown is tuned to 4.74 khz. Is that an ideal target frequency for a vlf detector? If not, what is an ideal target for general purpose metal detecting. This seems to be designed by someone who builds many things, not focused on metal detectors.

[Carl-NC]

Depends on what you are hunting for. 4.74kHz would be good for silver coins, not so good for jewelry.

[Skippy]

This metal detector is so simple, the operating frequency is almost irrelevant. More important is the audio capabilities of the cellphone 'mic' input. It's really intended for voice communication, so isn't likely to have a high sampling rate. So 5 kHz is probably chosen as the uppermost freq the phone can use, whilst giving OK 'metal detector' performance. [[ I could be wrong, and maybe the 'mic' input will sample at 'hi-fi' rates , but there's little to be gained using, say 8 kHz instead of 5kHz ]]

[Rustybolts57]

This metal detector is so simple, the operating frequency is almost irrelevant. More important is the audio capabilities of the cellphone 'mic' input. It's really intended for voice communication, so isn't likely to have a high sampling rate. So 5 kHz is probably chosen as the uppermost freq the phone can use, whilst giving OK 'metal detector' performance. [[ I could be wrong, and maybe the 'mic' input will sample at 'hi-fi' rates , but there's little to be gained using, say 8 kHz instead of 5kHz ]]

That's an interesting observation. I will have to test frequencies, I can't find much info in a Google search. I am getting the impression that I should be at a substantially higher frequency.

[Carl-NC]

The traditional telephone audio band is considered 300Hz - 3kHz, which captures the majority of human voice content. It's my impression, though, that cell phones use a normal audio codec chip for the microphone & speaker, and those tend to run at 44.1kHz and higher. That let's you listen to "Queen of the Night" in Mozart’s Die Zauberflöte, or use your BirdNet app to ID a bird by recording its chirp.

I suspect this metal detector is a simple TR-type with amplitude detection, nothing more. As such, any frequency will work up to the limit of the audio chip. But it will still probably follow the usual rules of TR/IB detectors: lower frequencies are better for high conductors (silver coins), higher frequencies are better for low conductors (small jewelry), and ground mineralization will respond to all frequencies.

[Skippy]

" I am getting the impression that I should be at a substantially higher frequency." Not really ... a decent 'all-round' operating frequency would be the 10k - 12k range; lower freqs, like 5k - 8k would favour US 1c/10c/25c/ modern dollar coins in a park-hunting scenario, for example; 15k - 20k would favour smaller targets, and those made of metals that don't conduct electricity so well - including US 5c coins, most gold finger rings, drinks can ringpulls, and here in Europe, many bronze / lead / brass artefacts and older/ancient smaller coins.

[Rustybolts57]

I can see there is lots to learn. I had to Google TR Type metal detector. This started out as a simple curiosity. I saw a diy metal detector project and thought it sounded like an interesting project. The first thing I noticed was that I needed a coil housing so I found a DIY 3d printer project. That is up and running now and the housing has been printed. I can see I will want a better detector to build after I am happy with the construction of this one. This project will be bigger than I thought.