Really sensitive pinpointer for grain size objects

[Detectorist#1]

Hi Carl,
I know that TRX uses 12KHz IB principle. For the target's field is more hard to change something parameters of the coil with ferrite core.
It is interesting - the same happened with pulse detectors.

[Detectorist#1]

Hi Carl,

Thank you for this explanations. I will test variant with high inductance coil and high value of safety serial resistor.
The construction of coil's unit of TRX pointer is very interesting. I also noticed that using of ferrite needs longer delays.

[Carl-NC]

I just balanced 2 coils small black one Rx and under it is Tx coil, I didn't build LVDT, I used what I have to check the principle,

Tx coil is driven by NE555 generator , series resonance

When I designed the White's TRX I started with this approach. However, placing the RX coils on the ferrite actually reduces sensitivity. It's harder for a target field to change the field that cuts through the RX coil. Instead, I wound the TX coil on a ferrite rod and wound the RX coil with an air core. Instead of a rear RX coil for IB, I instead reverse-wound the RX wire on top of the TX coil to achieve IB. The ferrite core results in fewer counter-RX turns needed for IB. Here is an illustration of the design:

[Carl-NC]

Hi Carl,
First, thank you for suggestions.
My experience in metal detecting is very modest but I have some question to you:
Increasing of the value of the safety resistor in the input of scheme (50 times to 10K) will increase seriously the current noise. I think this create problem. To fight with this problem, you need to increase the repetition rate to 2500pps.
What is the summary profit?
Also the peek current via input resistor with 200 oHm is 50 times bigger , but discharge of current carriers in diode via 200 oHm is also 50 times faster, or not?
What is "direct sampling" - this is new idea for me.

The peak coil current is proportional to 1/L, or 1/N². The peak TX field strength is determined by the ampere-turns, so it is proportional to 1/N. The induced RX signal is proportional to area-turns, or N, so the round-trip sensitivity versus coil inductance is pretty much a wash. However, a lower inductance coil allows earlier sampling (which helps) and higher inductance coils reduce power and allow a higher rep rate (which also helps). In designing the F-Pulse I got better results with the higher inductance, but only when the clamping resistor is increased. Yes, it seems that the clamping resistor should also be a wash but I almost always find that increasing it improves early sampling, and the noise increase is hardly noticeable.

Hi Carl,
Send me please video with distance performance of F-pulse with different sub grain targets in air. This will be very useful for me. Thank you in advance!

Wish I could, this is work I did several years ago and the prototype was stolen. I was at the point of adding a GB channel which required a new PCB. I've been wanting to get back to this but no time.

[pito]

I just balanced 2 coils small black one Rx and under it is Tx coil, I didn't build LVDT, I used what I have to check the principle,

Tx coil is driven by NE555 generator , series resonance,


1mm steel ball = For now I don't have time to build a pinpointer, busy with 2 box machine.

[Detectorist#1]

Hi Pito,
4mm steel ball is very strong target for any metal detector.
Send me please video with pinpointer configuration with 1mm steel ball as target. Thank you in advance!

[Detectorist#1]

Hi Pito,
The coil's module in video seems very flat. In LVDT principle have to be used 3 coils in linear configuration. Where is the true? Or I not understand the video?

[pito]

That is a 4mm steel ball, jus testing mentioned LVDT principle, marks are at 1" and 1 1/2", Rx coil is connected directly to oscilloscope without any processing.
https://www.youtube.com/shorts/aVN0CnoN8BE

adding simple electronics will increase the sensitivity, by 2 times - expected

[Teleno]

I have done a similar setup with the Fisher F-Pulse. Here are some suggestions:[LIST][*]Increase the coil to a higher inductance. I use 1.3mH. This will decrease the TX per-pulse power consumption.

How is it possible to achieve 3us with sich a high inductance/capacitance coil? I'm using 170uH but after attaching the cable and the coil shield I'm sampling at 5us, on top of the transient with a preamp gain of just 14.

[Detectorist#1]

It is interesting that cable with 0.4mm diameter is optimal for pulse detectors. Thinner cable have more resistance by meter and not allow big currents. Wider cable is sensed as target and not allow short delays.
Litz wire maybe is the solution, but on price of hard finding and more more expensive price.

[Detectorist#1]

Exact used cable is type 25511 - 0.14mm2 and 1mm external diameter

[Detectorist#1]

PDF of this cable

[Detectorist#1]

In this document you see second, more complex variant of the coil. This variant uses wire with teflon isolation. Reducing of the capacitance of the coil allow to reduce the receiving delay to 3us instead of 4us.
This add sensitivity to sub grain targets. This is the best cable for the coil, but relatively hard to find and more expensive than magnet wire with 0.4mm external diameter.
Attached is datasheet for this teflon cable.

[Detectorist#1]

Hi All,
It is very easy to made grains pointer for rivers when electronic board is situated in front end of pinpointer. No need for special tricks for waterproof construction.
Hеre is idea...

[Detectorist#1]

Hi Carl,
Send me please video with distance performance of F-pulse with different sub grain targets in air. This will be very useful for me. Thank you in advance!