Pulse Induction metal detector design.

[ATX_metalprototype]

Hey guys and gals,
I'm trying to build a metal detector that can detect foreign metal objects in otherwise non metallic material passing through a loop. Basically continuously searching and then alerting a user if it detected something metallic pass through it. My loop needs to be ~24 inches/0.6 meters in diameter, but I only need to detect objects within about 12 inches/0.3 meters away from it (in air). I'm not looking to search for an iron pipe a meter into the ground. My minimum object size is around the size of a medium sized coin and I'd like to be able to pick up stainless steel.

I built a circuit based off of the attached Spirit PI schematic, but my results leave more to be desired. I am using an 11 turn coil made out of 22AWG wire, but I'm only able to pick up giant objects like a giant stainless steel salad bowl. I can barely pick up a soda can, much less a coin. I'm only using the microcontroller for the pulse timing and looking at the waveforms on an oscilloscope. The circuit seems to work fine, it's just not sensitive enough, does anyone know which knobs I can turn to dial up the sensitivity? There doesn't seem to be much documentation out there for homebrew metal detectors. I do not need to discriminate the metal types for my application.

Thanks!

[kripton2035]

here you have a lot of metal detector schematics, where you can get ideas to increase your design sensivity.
https://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat

[T3sl4co1l]

More background than you could hope for: https://www.eevblog.com/forum/beginners/electromagnetism-questions-lc-tank-circuit-metal-detector-issues/

Tim

[ATX_metalprototype]

Great, thanks for the links! A lot to read through, funny how little the schematics... and components have changed since the '80s!

[ATX_metalprototype]

After reading through a few of the pulse induction schematics you guys linked, it looks like the biggest problem with my circuit is that I am just trying to get an on/off comparator directly off of the signal reflecting back from the coil. It looks like most of these designs have a 2-3 stage amplifier that isolates the return pulse and plumps it up to something that's workable.

[StillTrying]

It looks like most of these designs have a 2-3 stage amplifier that isolates the return pulse and plumps it up to something that's workable.

I did a few PI simulations here, the second one using analogue switches to isolate the coil switching pulses from the 1st high gain opamp.

www.eevblog.com/forum/projects/lf357-opamp-replaced-by-an-ad797-subsititude-must-work-better-but-it-dosn't!!!/msg1510033/#msg1510033
www.eevblog.com/forum/projects/lf357-opamp-replaced-by-an-ad797-subsititude-must-work-better-but-it-dosn't!!!/msg1515274/#msg1515274

Good luck.