EEVblog #714 – Metal Detector Reverse Engineering - Page 1
Forum section: electronics / general

author: Stonent
author_rank: Super Contributor
timestamp: February 13, 2015, 03:05:03 pm
content: I wonder if there's another version that takes advantage of the missing parts on the board?

author: bitwelder
author_rank: Super Contributor
timestamp: February 13, 2015, 06:46:39 pm
content: "This video is unlisted"On purpose or by accident?

author: SeanB
author_rank: Super Contributor
timestamp: February 13, 2015, 07:01:26 pm
content: On purpose, for the Patreon feed. It will be on the regular feed later on during the weekend. Dave does this for the patreon feed for a lot of the videos as an incentive.Patreon is not expensive in any case, there are tiered options.

author: max666
author_rank: Frequent Contributor
timestamp: February 13, 2015, 07:45:51 pm
content: Quote from: Stonent on February 13, 2015, 03:05:03 pmThe only interesting parts not populated, that I could see, is the "MOTOR" connection, which is in series with the unpopulated resistor R14.And if you look at the bottom side you can see that the buzzer "BZ" and "MOTOR" traces just run up to the 6 holes labelled "SW", perhaps an unpopulated switch, but they are just using that as a switchboard to select either the buzzer or the motor. Whatever the hell "MOTOR" would be in this case ... maybe a vibration motor?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 13, 2015, 10:45:26 pm
content: Does it detect nonmetallic conductors, such as graphite?Also, what does "firfy" (10:09) mean?

author: Dave Turner
author_rank: Frequent Contributor
timestamp: February 13, 2015, 11:18:15 pm
content: What interests me is two fold:-a) the 'wand', how the inductors are wound, their size and the interaction with the coupling metal.b) how would one discriminate between different metals?

author: xrunner
author_rank: Super Contributor
timestamp: February 14, 2015, 12:15:31 am
content: Well ... thanks Dave! I sent that thing to him - gosh - a year ago? I totally forgot about it. I haven't watched it yet but will later this evening when I can give it complete attention.I hope it can be of some use, or perhaps Sagan can find things in the yard with it.

author: BeagleBoy
author_rank: Newbie
timestamp: February 14, 2015, 03:01:00 am
content: Here in the US (I have no idea about the rest of the world) it's becoming much more common to use "reclaimed wood" for woodworking projects. If one of these cheap-ass detectors saves a saw blade by detecting a hidden nail, it's more than paid for itself.That said, I'd personally spend a little more. But for its purpose, it probably suffices.

author: Rory
author_rank: Frequent Contributor
timestamp: February 14, 2015, 03:04:57 am
content: Quote from: Dave Turner on February 13, 2015, 11:18:15 pmMy two cents:a) Not critical. You could use a ferrite rod from a loopstick antenna from an AM BCB receiver with similar results. It looks to me like a Hartley oscillator circuit, you use enough windings in the tank circuit to produce the frequency you desire, and the other winding is a feedback circuit that creates the oscillation. The number of turns in the "tickler" winding depends on how much positive feedback is required. Dave was not specific on the operating frequency but you can determine that from the measured inductance and the parallel capacitance of the tank circuit. This is not a field balancing coil so you should be able to wind the tickler winding directly over the tank.This circuit operates similarly to a dip meter or even a regenerative receiver - the presence of the metal detunes the oscillator, reduces the Q of the circuit and absorbs a portion of the generated RF. The detector output decreases and once it goes below the opamp comparator's 2.5v threshold it energizes the piezo buzzer.b) This circuit only detects the amplitude of the oscillator output. To detect the metal type, the frequency of the oscillator must be monitored with a frequency discriminator, hence the need for a second oscillator to act as a beat frequency oscillator. The different types of metals have different effects on the tuned circuit. Ferrous metals cause the frequency to decrease (increased permeability of the inductor) while nonferrous metals like copper, silver and gold have the effect of decreasing permeability which reduces the inductance of the LC circuit and causes the frequency to increase.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2015, 05:36:20 am
content: I will guess the sensor is a standard LW/MW ferrite bar antenna, with the primary winding being the LW coil and the MW winding being the feedback coil. Inductances measured by Dave and the 10n tuning capacitor are the right ballpark for one of these antennas, so if you want to build one you just gut a dead cheap AM/FM transistor radio and use the small ferrite bar antenna as the sensor. donor radio should be the cheapest smallest AM radio that you can find, as those will have the smallest ferrite bars inside.

author: Stonent
author_rank: Super Contributor
timestamp: February 14, 2015, 05:39:55 am
content: I seem to remember on my old 200 in 1 kit they had a project in there for a metal detector that used the ferrite AM antenna. Perhaps Dave could look it up in his book since he still has his.

author: nowlan
author_rank: Frequent Contributor
timestamp: February 14, 2015, 06:46:56 am
content: via google image search.

author: EEVblog
author_rank: Unknown
timestamp: February 14, 2015, 08:07:46 am
content: Quote from: Stonent on February 14, 2015, 05:39:55 amYes, I remember that too!

author: EEVblog
author_rank: Unknown
timestamp: February 14, 2015, 08:10:44 am
content: Quote from: nowlan on February 14, 2015, 06:46:56 amAh, didn't think to look!That is different to mine though. In mine the sensitivity adjust definitely connects to the Q1/Q2 bases via R4 in my circuit.

author: jancumps
author_rank: Unknown
timestamp: February 14, 2015, 08:19:42 am
content: Quote from: Stonent on February 14, 2015, 05:39:55 amI did a quick check in the 200-in-1 and 65-in-1 kits. I could not find a metal detector in them.The 60-in1 has it:The 65-in-1 has a photo of Tandy's Metal Locator KIt:

author: xrunner
author_rank: Super Contributor
timestamp: February 14, 2015, 05:30:35 pm
content: Thanks Dave - good reverse engineering job. I think it would be interesting (though I doubt it's easy) to actually find the person or little group that comes up with these designs, in China or wherever. It would be of interest to hear what pressures they have: to come up with the design that (sorta) works, yet at the dirt-cheapest price they can muster. Must be a whole set of horror stories waiting to be revealed. The bean counters over there must be absolutely overbearing.Now I have to come up with an idea for the next shipment ...

author: R_Gtx
author_rank: Regular Contributor
timestamp: February 14, 2015, 10:13:48 pm
content: Those wanting an in-depth understanding of modern metal detector design will find the book "Inside the Metal Detector" published by Geotech Press invaluable.http://www.amazon.com/Inside-Metal-Detector--depth-Technology/dp/098583420X/ref=sr_1_1?ie=UTF8&qid=1423950390&sr=8-1&keywords=inside+the+metal+detectororhttp://www.geotech1.com/forums/content.php?125-inside-the-metal-detectorThere is also a wealth of information on theGeotech website.The first metal detector that I built, was the Practical Wireless "Sandbanks" Pulse Induction Metal Detector with PCB coil, circa 1980, non discriminatory (voracious appetite for batteries!!), this was followed up a year later by the dual pcb (boards were sold through Maplins in the UK) "Magnum" metal detector, published in Practical Electronics. These I later replaced with commercial "Motion Detection" Induction Balance detectors, Arado and later Tesoro machines.

author: orolo
author_rank: Frequent Contributor
timestamp: February 16, 2015, 07:01:10 am
content: Quote from: Dave Turner on February 13, 2015, 11:18:15 pmFrom the online schematic, the turns ratio is sqrt(1.8e-3/15e-6) = 10.95, so there are 11 turns of the primary for each of the secondary. My guess is both inductors are tightly wound in different, but close, places of the rod. If the rod is similar to type 61 material (AL = 55), to get 15uH in the secondary you would need about 16 turns, then about 176 in the primary. Just a guess.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: February 20, 2015, 08:53:37 pm
content: And the winner is, the metal detector Granddaddy of them all, the VT Radio Proximity Fuse invented during WW2.Same principal as the cheapo HF device, but 1M times more reliable.Look them up on Google and see the 10 pounds of electronics stuffed in a 2 pound bag.The whirly gig on the bottom left in the schematic is a wind generator to power the thing. An oscillator - receiver combo, followed by a tube version of an SCR to fire the fuse. BOOM!Youtube:Schematic of a typical fuse:http://www.operatorchan.org/stem/src/138955717197.jpg

author: ConKbot
author_rank: Super Contributor
timestamp: February 20, 2015, 10:09:53 pm
content: Quote from: max666 on February 13, 2015, 07:45:51 pmA vibration motor makes the most sense. If youre using it to screen people at a loud place (i.e. a night club, stadium event etc) it would be easier to feel the vibrations than to listen for the feeble little beeper. Or if just trying to be discreet while using it.

author: michi42
author_rank: Newbie
timestamp: February 21, 2015, 08:29:48 pm
content: Hi is there anyone who could explain how that pulse-induction type of detectors work?They are allways talking about the "reflected" part of the coil signal or back-emf from eddy currents when the exciting field is collapsed.I don't think there are any eddys when the exciting field is gone (i-e dPsi/dt = 0 -> ieddy = 0).So it is all about energy loss in a resitive secondary like in a transformer.

author: Icchan
author_rank: Regular Contributor
timestamp: February 27, 2015, 06:09:59 pm
content: I think this one quote from Dave was priceless :D