December 2024 - metal detectors
Metal Detector
I am interested in making a relatively simple metal detector, similar to the ones used in airports to scan people for metal, more of a gimmick thing. I was thinking that having it work with an Arduino, would make it more flexible, so that or an ESP32 would be a plus. I have seen some crazy metal detectors used to find meteorites deep underground (6 feet) and that also does intrigue me a little, so off I went on the internets to do some background reading.
Background reading
Looking at my first go to magazine - Silicon Chip, it turned up a couple of recent artilces on metal detectors
- this feature https://www.siliconchip.com.au/Issue/1998/May/A+Detector+For+Metal+Objects
- And a mention in Notes & Errata of June 2017 issue that the March 2017 issue had an ATmega-based Metal Detector with stepped frequency indication
- Taking a look at SC March 2017 issue on page 86 I get a greyed out preview only bocker https://www.siliconchip.com.au/Issue/2017/March
I am a little surprised that Silicon chip does not provide previewes for things like reader submitted circuits
But presumably that person or someone else would have published such a thing online, so I searched
“atmega based metal detector with stepped frequency indication”
A bunch of promising “images” came up including
- https://www.instructables.com/Minimal-Arduino-Metal-Detector/
- https://www.instructables.com/Simple-Arduino-Metal-Detector/
- https://simplemetaldetector.com/pulse-induction-metal-detectors/metal-detector-avr-microcontroller/
- https://circuitcellar.com/research-design-hub/projects/advanced-metal-detector/
- https://forum.arduino.cc/t/pulse-induction-metal-detector-rx-methods/683722
- https://github.com/microchip-pic-avr-examples/pic18f56q71-opamp-metal-detector-mplab-mcc
- https://www.electroschematics.com/metal-detector/
- https://circuitdigest.com/microcontroller-projects/arduino-metal-detector-circuit-code
- https://www.electronicshub.org/metal-detector-circuit/
All of which are probably worth reading up on to get some background information and different circuit ideas.
Finally this one stuck out
- https://simplemetaldetector.com/pulse-induction-metal-detectors/arduino-based-pulse-induction-detector/
- also on instructables https://www.instructables.com/Arduino-Based-Pulse-Induction-Detector/ The idea was to do away with most signal processing components. The Arduino runs at 16MHz and has 0.0625µS reslution on clock speed which is not enough. Instead comparing voltage drop over time to a fixed reference voltage and using the D6 - D7 internal comparator to trigger an interrupt is what this design uses. The idea comes from TPMID - Tiny Pulse Induction Metal Detector home page - www.miymd.com/index.php/projects/tpimd/ now available here webArchive: - www.miymd.com tpimd It sounds like these circuits usually have 2 voltages, to get around that here, the circuit is triggered and then a MOSFET is used to isolate the coil and bleed it via a 220Ω resistor. A voltage divider sets a voltage of around 0.04V and this is used in the comparator to trigger when it is reached. If metal is near the coil, the decay curve lasts longer and the interrupt gets longer. The detector works but is of limited capability, still it uses Arduino so might be worth checking out.
It had a relatively simple Arduino and minimal external components but also details of the limitations on Arduino’s being able to detect the frequency changes quickly enough and some work arounds as well as references to older and even non existant sites.
one of these is this article
- https://www.basic4mcu.com/bbs/board.php?bo_table=k1&wr_id=17&page=3
- webArchive: - www.miymd.com tpimd
Which lead to a few more reads from the miymd.com site
- Metal detector types
- webArchive: miymd mdtypes
- referencing radioShack
- TPIMD - Tiny Pulse Induction Metal Detector
- XPI
- Technology behind Pulse Induction Metal Detecors, Tramsmitter TX and Receiver RX
from some of the comments this lead me to
Gary’s Pulse Induction Metal Detector
- http://chemelec.com/Projects/Metal-1a/Metal.htm (not sure how long this site will be around as it says the author is closing it down)
- a bunch of projects here but not all have schematics, some simple ones below
- http://chemelec.com/Projects/Metal-BFO-1/BFO-1.htm
- http://chemelec.com/Projects/Metal-BFO-2/BFO-2.htm
As well as some other random but interesting work on mini Maximite
which lead to this
https://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat
- Metal Detector Projects
- Frequency-shift Detectors
- Induction Balance Detectors
- ETI549 (Aus) (ETI, Australian version) Basic IB Detector
- Matchless Metal Locator (Thomas Scarborough, Silicon Chip) - World’s
simplest (?) IB design
- https://www.geotech1.com/pages/metdet/projects/matchless/matchless300c.pdf Full SC article in the link above. It covers off a simpler “induction balance” IB design using 2 coils and 2 555 timers. One transmits a signal and the other one detects it, as the signal is in the audible range, it powers some headphones using your ears to detect the presence of a coin at 15cm’s in air. Also a fair bit of detailed construction using PVC tubing. Certainly a simple desing that could translate to Arduino with some square waves and ADC of sound?
- Shadow IB Detector (ETI)
- Pulse Induction Detectors
- EE Treasure Hunter - Mark Stuart, Everyday Electronics
- Gary’s Pulse Induction Metal Detector (from Gary’s Chemelec web site)
- with the original domain, now only available on the way back machine, but still no circuit diagram
- now http://chemelec.com/Projects/Metal-1a/Metal.htm
- Twin Loop Treasure Seeker - Robert & David Crone ETI
- Misc
- Coil winder, coplanar search coils and probe detectors