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Experimenting with metal detecting

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ppTRN:
Hi, I want to understand more about metal detectors and possibly design one of my own. It will be uC based, but the problem I am having difficoulties to overcome is how to amplify a signal to feed to the searching coil (some 20mH, 1Ohm resistance). I literally have no idea about which circuit to use. Keep in mind that i do not want it to amplify just a sine wave, but i want to be able to feed my own signal to the coil, possibly generated by the uC, and I want the signal to be as strong as possible.

Edit: bandwith of the signal Is no more than 50kHz

If it wasn't for the low resistence of the winding i would even have used a power amp like TDA2030 or something like that.
You guys have any suggestions?

Infraviolet:
"Metal detector" describes a whole family of devices which operate by a variety of different principles.

The simplest trick, though with perhaps the shortest ranges is to have an inductor set up as part of an LC oscillator, when metal comes near the inductor eddy currents induced in to it have an effect where the inductance of the inductor gets changed by upto a few %. The LC oscillation the changes frequency.

Your plans sound like you'd rather use an emitting coil and receiving coil instead and look for signals in the receiving coil "reflected" off the metal. If you simply need to amplify a signal from the MCU, probably a digital signal anyway as most microcontrollers have ADCs but not DACs (some will let you use PWM to fake an analog voltage, but that is only for things like motor speed or LED brightness where a fast flicker looks like a continuous medium level on human timescales), how about just a transistor acting as a switch to control larger signals? NPN as a low side driver?

zrq:
I had some fun experimenting a computer sound card as DAC-> Power Amp (headphone amp) -> Pre Amp (for mic) -> ADC for induction balance metal detection, and managed to achieve reasonably good result with trivial DSP. I simply changed the winding turns to make it have around 6 ohm impedance, so can be comfortably driven with audio amplifiers. BJT input stage headphone amplifiers also have nice performance at low input impedance.

ppTRN:

--- Quote from: Infraviolet on June 12, 2023, 07:16:08 pm ---"Metal detector" describes a whole family of devices which operate by a variety of different principles.
[....]
Your plans sound like you'd rather use an emitting coil and receiving coil instead and look for signals in the receiving coil "reflected" off the metal.

--- End quote ---

That is correct, i want to develope a metal detector with a double D coil


--- Quote from: Infraviolet on June 12, 2023, 07:16:08 pm ---If you simply need to amplify a signal from the MCU, probably a digital signal anyway as most microcontrollers have ADCs but not DACs

--- End quote ---

I will use a Teensy4.1, so a really powerful device, that even if it does not have a DAC, it will be fairly easy to use it with an audio IC to reproduce sound stored in a uSD. Said sounds will be the waveform i want to feed the TX coil.


--- Quote from: zrq on June 12, 2023, 07:59:32 pm ---I simply changed the winding turns to make it have around 6 ohm impedance, so can be comfortably driven with audio amplifiers. BJT input stage headphone amplifiers also have nice performance at low input impedance.

--- End quote ---

I guess i'll try to increase the resistance of the coil and use an audio amp, but this will decrease the bandwith of the signal. Not that big of a problem, but i was really hoping to reach 50kHz

jmelson:
I had a Vietnam-era US Army mine detector.  It used vacuum tubes, but the way it worked (I think) was there was a square transmitting coil with 4 round coils inside the square.  The 4 coils were connected in a plus-minus configuration so their signal would cancel out when no metal was near.  Any metal nearby would disturb the field and the transmit signal would not cancel out.  The signal would then be hetrodyned down to the audio range (or maybe the main oscillator was actually IN the audio range.  Detection was with headphones.  Anyway, the thing was totally amazing!  It would detect a sewing PIN at over a foot.  There would be a null in the signal when the object was exactly in the center of the 4 coils.
Jon

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