Hobby Hybrid MD1

[Qiaozhi]

Hi pechkata,
Now, the tests on the air points very good sensitivity and discrimination Fe/non-Fe. I hope, soon I will make tests on the real terrain. For now, I await comments on the schematic solution from serious designers of PI MD.

Air tests can initially be useful, but also misleading.
The design is not really worth anything until you get it off the bench and test it in the field.

A good example is a Viking 5, which is a simple non-motion VLF detector. It tests superbly on the bench with excellent ferrous rejection. But try it out in the field, and it's basically unusable.

[algan]

Hello, Detectorist#1, in your opinion, what is the most suitable rectangular pulse to use for a pi detector, with what time and frequency, to be the best for detecting gold and silver. For iron, it is clear that any can be used.

[Detectorist#1]

Hi algan,
I'm not big specialist in MD area, but my answer is - first you have to know this MD will be for small nuggets or coins searching. For small nuggets you need very small time delay before first RX reading. For these small delays is very important good Ground Balance and good electrostatic screening of the case of MD -this is recommendation from Eric Foster - the guru in PI metal detecting. For searching of the coins in 20-30cm deepness, good Ground Balance is not so essential as Ivconic mention. For small object the TX pulse of 20-100us if enough. For TX frequency: For low power consumption is possible to use 100(120)Hz, but in this case the noise of the first amplifier have to be very low - this OpAmp will be relatively expensive (up to 10-15 USD). Also in this case the influence of electromagnetic fields with 50(60)Hz indoor and outdoor will be notable. To avoid this influence, the working frequency have to be 2-5KHz. In this case, supply current will increase seriously. TX MOSFET will need heat-sink and damping resistor for TX coil have to be for 2-3 Wats. Also the serially connected safety resistor in the input of the first RX stage have to be with power of 2-3 Wats. The good news when using of high TX frequency is that in this case you have possibility to use in first stage of RX channel OpAmp with not so strong requirements for low noise (more cheap). Also, on the output of RX preamplifier have to be used high-pass RC filter with 500-800Hz cut-off frequency. This reduces the influence of fields with 50(60)Hz frequency. Further reduction of this influence is possible with low RC time-constant in the first integrator (sample-hold) stage. This time-constant have to be 3 times lower than time of sample pulse.

[Ivont]

Hello Detectorist#1. Can you explain, the output of Q16, what function it performs.

[Detectorist#1]

The transistor Q16 is controlled from ALL channel. When some target is detected, this transistor enables the work of the final stage of the DISCRIMINATION channel to show the type of the target - Fe/non-Fe. For Fe target, The "Fe" LED lights up first and after that "non-Fe" LED lights. For non-Fe target this order is reverse.

[Ivont]

I understood today after reviewing it in detail. The scheme is not bad. In my opinion, on mineralized soil it will give wrong signals.