Really sensitive pinpointer for grain size objects

[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 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.

[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.

[pito]

You may be interested using LVDT principle.


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

[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

[Carl-NC]

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

[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]

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

[Detectorist#1]

Many good projects was nulling in attempt to modify as universal - many examples in history

[Detectorist#1]

Quick lock isn't deep lock. First, using of uP chip need to program this part - not easy task for wide DIY peoples. And microprocessor add noise in schematics - when you target minimal noise in receiving time, this will be a problem. U1 and U5 enable to use whole voltage of the battery in transmit mode - max power tor transmit short pulse. Q10 and Q11 only give the possibility to generate additional +3V supply. Yes, It's true - using of microprocessor give more options to use different transmit power, delay time, sample time, processing of receiving signals. I have variant of scheme with PIC16F18323 but not tested yet.

[pito]

It was in the air, but the detector was so sensitive that small pieces at the ends of the laces give a signal when walking, so I had to check my shoes before hunting.
Quick look at schematic tells me that using stm32f103 will made circuit much simpler. Pulse 25us, strobe1, strobe 2, U1, U5, Q10, Q11 , that is just some components which can be replaced.

[Detectorist#1]

That is OK. This result is in air or in heavy ground? Also you move target or you move coil? When I made my first metal detector project, I recognized that magnetic pole of the earth is very strong and I have to reduce ten times sensitivity of my metal detector to use it real word.

[pito]

That's ok, my regular IB was able to detect 1 mm steel ball from 5 cm.

[Detectorist#1]

My aim was to overcome White's TRX pinpointer possibilities to find one grain objects from 1/2 inch and I work 2 years for this. Now my pinpointer have possibilities to recognize one grain object from one inch without problem. I know for Falcon 20 pinpointer with better possibilities but I newer viewing real video in real groud test for this pinpointer.

[Detectorist#1]

Pito Sorry! I have not possibility to make video. But I made more than 10 pieces of this project and all of them give the same result - recovering of one grain pieces from one inch distance without problem in air and in ground and salt water. First variant of this project was with only one probe of receiving signal after 4uS and have problem with salt water.