Really sensitive pinpointer for grain size objects

[Qiaozhi]

It appears the TX pulse width is 25us. What is the TX frequency (pulse rate)?

On the schematic it says: 25us/20ms, so I guess the TX rate is 50pps?

[Carl-NC]

That's what I saw too, but I hope it's higher than that.

[Detectorist#1]

Hi Carl, the pulse rate is 50Hz (20ms). This allow to reduce current consumption at 16mA and 9V alkaline battery is enough for appr. 30 hours

[pito]

You may be interested using LVDT principle.


https://www.aliexpress.com/item/32968792122.html

[Carl-NC]

I have done a similar setup with the Fisher F-Pulse. Here are some suggestions:

• Increase the coil to a higher inductance. I use 1.3mH. This will decrease the TX per-pulse power consumption.
• The pulse width is fine, but increase the pulse rate as high as you can. This will increase sensitivity and increase the TX per-pulse power consumption. I use 2500-5000 pps.
• Increase the clamp resistance (R23+R27+R31) to several kΩ. I use 10k. This will significantly speed up the diode recovery time. It also increases thermal noise but you won't notice it.

With these setting I can sample at 3-4us and detect sub-grain nuggets. I use a bipolar TX which does not require the EFE sample, but I found that in severely mineralized soils it does require a GB sample. The F-Pulse runs at about 50mA (2-AA batteries, so ~40 hrs) but I have done a direct sampling version that consumes around 15mA and has almost the same sensitivity.

[Detectorist#1]

Hi Pito,
It's true, that differential schematic give very good sensitivity. But using of NE5521 from 1994 year is not good idea according me. This IC now is hard to find and the price is more than 10 USD.

[Detectorist#1]

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.

[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!

[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]

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]

PDF of this cable

[Detectorist#1]

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

[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.

[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.

[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