Android Phone Based Metal Detector - FCMD

[Marchel]

I would not bother with an external ADC unless it was 24 bit or more. The ADC implemented on the H7 series is a "proper" 16 bit SAR ADC.
On the H743 there are 3 of these onchip ADCs ..
Below is the demodulated ( 0 - 25 hertz ) noise floor of the current firmware running on the chip with the coil connected and the TX running in a noisy lab environment.

we are interested in around the 5 hertz point ( target sweep speeds ) ... the adc is better than -120 db in noise performance. This rivals alot of so called 24 bit converters.
The real benefit is the tight integration of the ADC to the CPU ... and the H743 is a cheap chip ..
you will be hard pressed to build a better combo unit using an external ADC that is anywhere near this price point and performance.

Yes, I also use these STM32H7 boards and they are really great, the STM32WB board is good for wireless applications for transmission via Bluetooth or the LLD method which works similarly to the NRF24L01, the only disadvantage of these chips is that they do not have a 16bit ADC like the STM32H7 has.

[Aziz]

Hi Paul,

yes, the STM32 platform will allow true real-time DSP processing. Some very special real-time algorithms can be made possible (latency much lesser than 1 ms). For instance a real-time software PLL implementation for demodulating the RX channel via the TXref channel.

As you have 3 ADC channels, you could use one for TX-ref and one for RX channel. With a relative High-Q LC tank in IB-configuration (Q around 10 is enough), you can process the target response on both channels. And a big plus: Active EMI noise cancellation as the TX and RX coils will pick up EMI too, which can be detected and eliminated.

I think such a complex IB design processing has not been done yet.
The STM32 platform will make it easily possible with less problems which I am facing with the sound card solution at the moment.
Cheers

[moodz]

I started testing this STM32 wireless board and it is also very good, just need at least 16bit external ADC. For a metal detector that will work with a mobile phone via Bluetooth, it is a good solution.

https://www.aliexpress.com/item/1005...origin_prod%3A

I would not bother with an external ADC unless it was 24 bit or more. The ADC implemented on the H7 series is a "proper" 16 bit SAR ADC.
On the H743 there are 3 of these onchip ADCs ..
Below is the demodulated ( 0 - 25 hertz ) noise floor of the current firmware running on the chip with the coil connected and the TX running in a noisy lab environment.

we are interested in around the 5 hertz point ( target sweep speeds ) ... the adc is better than -120 db in noise performance. This rivals alot of so called 24 bit converters.
The real benefit is the tight integration of the ADC to the CPU ... and the H743 is a cheap chip ..
you will be hard pressed to build a better combo unit using an external ADC that is anywhere near this price point and performance.

[moodz]

Hi Paul,

maybe I should buy a few STM32H743VIT6 boards to be able to use your ultimate detector soon.
The STM32 boards are really quite interesting.
Cheers

The sound card is interesting for playing with the math .. but you need low level access to timings and data to achieve real results and the STM32 boards allow that for low cost.

[Marchel]

I started testing this STM32 wireless board and it is also very good, just need at least 16bit external ADC. For a metal detector that will work with a mobile phone via Bluetooth, it is a good solution.

https://www.aliexpress.com/item/1005...origin_prod%3A

[Aziz]

Hi Paul,

maybe I should buy a few STM32H743VIT6 boards to be able to use your ultimate detector soon.
The STM32 boards are really quite interesting.
Cheers

[moodz]

hi mr moodz.
thank you for your nice & exciting work.
congulation.
i'd like to make it, but unfortunately i cant find your app.
would you please out it here ???????
please help me.
so so thanks.
best regards

... thankyou for your comments .. this is still a work in progress.
I am just reporting where I am up to in the project.
It is not ready for release yet.

moodz

[syavash]

Here is the FCMD app video .... the video was made by the app itself ( no editing ).
This makes detector videos super easy ..

hi mr moodz.
thank you for your nice & exciting work.
congulation.
i'd like to make it, but unfortunately i cant find your app.
would you please out it here ???????
please help me.
so so thanks.
best regards

[moodz]

Here is the FCMD app video .... the video was made by the app itself ( no editing ).
This makes detector videos super easy ..

[moodz]

The HC05 is just a stop gap measure to send some data to the app for testing ... the software is the priority. Wired headphones are fine for now.

[JoyJo]

But in the very idea of using Bluetooth, aren't you confused by the delay in sound transmission? At the moment I know only 1 fairly good chip with support for fast codecs and at a reasonable price: csr8670. By the way, it is installed on equinox. However, there is another chtp CC8530 on the equinox. They work on one antenna and switch with a high-frequency radio switch. But that's not the point. And the essence is the delay of sound transmission that Bluetooth has. Nrf24l01 has a very low delay even in comparison with csr8670.

[moodz]

I see that you are using the HC-05 Bluetooth module to transfer data to your phone. I think it would be worth trying to use the nrf24l01 for data transfer because it should also be able to communicate with the phone via BLE and its power consumption is lower, I think.

https://www.youtube.com/watch?v=2ZlZVE3XjOw

That is a possibility...but also looking at simultaneous audio and data over BT using BT401 or similar.

[Marchel]

I see that you are using the HC-05 Bluetooth module to transfer data to your phone. I think it would be worth trying to use the nrf24l01 for data transfer because it should also be able to communicate with the phone via BLE and its power consumption is lower, I think.

https://www.youtube.com/watch?v=2ZlZVE3XjOw

[moodz]

The good news .. the ground balance test in the field worked better than expected.
Easily balanced a range of goldfield grounds except beach ( to do ).

The bad news ... trying to video one phone (running the detector app) with another phone and holding the detector on a hot day with severe shading and reflections just did not work.

...So I have changed the app so it can do all the video work.

It records the detector app screen in real time and overlays the front or back cam on the screen.
1 tap toggles the video to full size vs PIP ( picture in picture ).
2 taps toggles the video back / front cam.
The PIP may be dragged to any position on the screen.
The detector app controls are fully functional and will be captured if you are recording.
You can stop or start recording obviously and sound may be captured or muted.
Video is captured as MP4
The default is PIP ( recording off ).

Below is a screen shot from a short video shot just now in the lab ... .... might add timestamp and GPS coordinates ( or not ).

This should make detector videos in the field really easy ... I hope.

[Marchel]

Good work...what is your transmit scheme.

The RX part is based on Minelab coils and the TX circuit is based on the XP Deus 2.