Android Phone Based Metal Detector - FCMD

[moodz]

FCMD code ported ... full demod and phase slope implemented in code. Its just the stm32h750 chip .. no coils connected, a bit of capacitive air coupling but its working basically. LOL

[moodz]

with the help of my AI mate ... I have built a syncronous detector in the STM32H750. A python GUI talks to the chip via the inbuilt USB port. Below is the initial code extracting a transmit signal from the noise with RX amplitude and phase plotted over time.
Not bad for a $5 chip.

[Altra]

Looking good! Are you using the internal adc or external?

I'm glad I have retired. I watched a youtube video today and they were saying within in two or three years, millions of jobs will be eliminated due to AI. This will cause social disorder and a revolt against big tech.

[moodz]

The internal "16 bit" ADC1 ...oversampled 16x for 200ksps. Total time to build the Python app and the STM32 C code was about 2 hours of me telling AI what to do. Some jobs are definitely going to go .. but people with ideas but don't know how to code will be winners in the near future. As long as AI never wants to drink beer we'll be OK.

[Altra]

That seems like the ideal micro for a metal detector. Considering a 16bit 250ksps adc in small quantities are about $20 usd each.
Thanks

[moodz]

That seems like the ideal micro for a metal detector. Considering a 16bit 250ksps adc in small quantities are about $20 usd each.
Thanks

ADC prices are outrageous when you need one.

This one ( there are 3 x 16 bit ADCs on the H750 ) is not too bad for an integrated converter. The package type for the CPU can have some impact on noise as well.
AT 2.5 msps ( fast enough to sample a PI ). you get about 13 - 14 bits ENOB and that can be made better with oversampling and averaging.
Internally the oversampling is done in hardware ( upto 1024x ) so zero CPU utilisation cost. I use 16 so I can have 200KSPS which gives me a nyquist cutoff of 100Khz maximum transmit.
The only downside I have found is that the flash size is 128K ... which isnt ideal if you include a few bloatware libraries in your code.
The board is a a Weact Studio

I buy them for $10 - $20 AU from Alibaba. You cant see it below but there are two external QSPI External Flash memory chips mounted to compensate for the limited onboard flash.
They also support Boot0 so you program them without an stlink programmer.

[ivconic]

Sometimes you just don't want too advanced technology arround you:



https://youtube.com/shorts/mgp5FxAYh...F9oQO1UkiVuic_

[moodz]

LOL - gas warfare is banned by the Geneva convention.

Added bode plotting ( AKA frequency response ). Here we are sweeping a short piece of wire connecting the dac to the adc input.

For those who are following the plot ... all these functions are being added to the STM32 chip so the phone only has to do the nice display ( no DSP load ) wink wink.

big question is : Why would we want a bode plotter in a metal detector ?

[Repwoc]

Which AI do you use Moodz?

[moodz]

Which AI do you use Moodz?

https://claude.ai for most and Grok for low level cpu.

[ivconic]

Kinda reminds a bit on Carl, isn't he?

https://youtube.com/shorts/YyEGv66Ss...mS4iBiluE_l2hO

[Carl-NC]

Not me, I don't wear glasses.

[moodz]

So to answer my own question the "bode mode" sets the detector to transmit a stepped frequency signal or multipoint scan and calculate the phase and amplitude at each point.

The number of point can be 1 ( as per single frequency detector ) or 2 .... upto 50.
However because there is only one frequency at any time then the power is maximised and SNR is improved.

The fastest rate is a two frequency point signal which allows for 25 two point analysis per second. ( unless single frequency operation selected. ).

| Integration | Rate | 2 pts | 3 pts | 5 pts | 10 pts | 20 pts |
|----------------|--------|--------|--------|---------|---------|----------|
| 10ms | 100 Hz | 25.0/s | 16.7/s | 10.0/s | 5.0/s | 2.5/s |
| 50ms | 20 Hz | 5.0/s | 3.3/s | 2.0/s | 1.0/s | 0.5/s |
| 100ms | 10 Hz | 2.5/s | 1.7/s | 1.0/s | 0.5/s | 0.25/s |
| 500ms | 2 Hz | 0.5/s | 0.33/s | 0.2/s | 0.1/s | 0.05/s |

Formula: sweeps/sec = (sample_rate / integration_samples) / (2 × num_points)

- Sample rate: 200 kSPS
- 2 integrations per point (1 settle + 1 measure)

The IQ integrators can settle to 0.1 degrees and 1 millivolt within 20 milliseconds.

Below is the system running with 5 frequency points and 10 scans per second.

[moodz]

Now building the whole detector in the 750 chip. The android app just runs a GUI over bluetooth to configure the 750 chip and report status but is not necessary anymore for the detector to work. Audio comes straight from the 750 chip.

GROUND BALANCE :


TARGET TONES



TARGET INDICATION :

[moodz]

All the code to make a VLF detector that can run upto 50 frequencies between 100 Hz and 100 Khz with ground bal and disc has now been built in stm32H750 chip. There is a blue tooth interface to an android app for control and status and a USB interface for looking at the python app below for realtime data. All of the DSP is done in the chip ... the apps just send commands or display data .. no processing. The hardware could concievably fit into a matchbox size.
Auto and manual groundbalance, selectable frequencies from 1 to 50, phase slope disc, conductivity, VDI etc etc. Once you set the controls the chip will work quite happily without the apps. Target audio comes from the chip.

[moodz]

Performance on a noisy input signal ... the VDI / conductivity vs time shows the stability of the system.