Introduction: Simple Arduino Metal Detector

Metal detection is a great past-time that gets you outdoors, discover new places and maybe find something interesting. Check your local regulations on how to act in case of an eventual find, in particular in case of hazardous objects, archeological relics or objects of significant economic or emotional value.

Instructions for DIY metal detectors are plenty, but this recipe is particular in the sense that it requires very few components in addition to an Arduino microcontroller: a common capacitor, resistor and diode form the core, together with a search coil that consist of circa 20 windings of electrically conducting cable. LED’s, a speaker and/or headphone are then added for signalling the presence of metal near the search coil. An additional advantage is that all can be powered from a single 5V power, for which a common 2000mAh USB power is sufficient and will last many hours.

To interpret the signals and to understand what materials and shapes the detector is sensitive to, it really helps to understand the physics. As a rule of thumb, the detector is sensitive to objects at a distance or depth up to the radius of the coil. It is most sensitive to objects in which a current can flow in the plane of the coil, and the response will correspond to the area of the current loop in that object. Thus a metal disc in the plane of the coil will give a much stronger response than the same metal disc perpendicular to the coil. The weight of the object does not matter much. A thin piece of aluminium foil oriented in the plane of a coil will give a much stronger response than a heavy metal bolt.

Step 1: Working Principle

When electricity starts flowing through a coil, it builds up a magnetic field. According to Faraday’s law of induction, a changing magnetic field will result in an electric field that opposes the change in magnetic field. Thus, a voltage will develop across the coil that opposes the increase in current. This effect is called self-inductance, and the unit of inductance is Henry, where a coil of 1 Henry develops a potential difference of 1V when the current changes by 1 Ampere per second. The inductance of a coil with N windings and a radius R is approximately 5muH x N^2 x R, with R in meters.

The presence of a metallic object near a coil will change its inductance. Depending on the type of metal, the inductance can either increase or decrease. Non-magnetic metals such as copper and aluminium near a coil reduce the inductance, because a changing magnetic field will induce eddy currents in the object that reduce the intensity of the local magnetic field. Ferromagnetic materials, such as iron, near a coil increase its inductance because the induced magnetic fields align with the external magnetic field.

The measurement of the inductance of a coil can thus reveal the presence of metals nearby. With an Arduino, a capacitor, a diode and a resistor it is possible to measure the inductance of a coil: making the coil part of a high-pass LR filter and feeding this with a block-wave, short spikes will be created at every transition. The pulse length of these spikes is proportional to the inductance of the coil. In fact, the characteristic time of an LR filter is tau=L/R. For a coil of 20 windings and a diameter of 10 cm, L ~ 5muH x 20^2 x 0.05 = 100muH. To protect the Arduino from overcurrent, the minimum resistance is 200Ohm. We thus expect pulses with a length of about 0.5 microsecond. These are difficult to measure directly with high precision, given that the clock frequency of the Arduino is 16MHz.

Instead, the rising pulse can be used to charge a capacitor, which can then be read out with the Arduino analog to digital converted (ADC). The expected charge from a 0.5 microsecond pulse of 25mA is 12.5nC, which will give 1.25V on a 10nF capacitor. The voltage drop over the diode will reduce this. If the pulse is repeated a few times, the charge on the capacitor rises to ~2V. This can be read out with the Arduino ADC using analogRead(). The capacitor can then be quickly discharged by changing the readout pin to output and setting it to 0V for a few microseconds.The whole measurement takes about 200 microseconds, 100 for the charging and resetting of the capacitor and 100 for the ADC conversion. The precision can be greatly enhanced by repeating the measurement and averaging the result: taking the average of 256 measurements takes 50ms and improves the precision by a factor 16. The 10-bit ADC achieves the precision of a 14-bit ADC this way.

This measurement obtained is highly nonlinear with the inductance of the coil and therefore not suitable to measure the absolute value of the inductance. However, for metal detection we are only interested in tiny relative changes of the coil inductance due to the presence of nearby metals, and for that this method is perfectly suitable.

The calibration of the measurement can be done automatically in software. If one can assume that most of the time there is no metal near the coil, a deviation from the average is a signal that metal has come close to the coil. Using different colours or different tones allows to discriminate between a sudden increase or a sudden decrease in the inductance.

Step 2: Required Components

Electronic core:

Arduino UNO R3 + prototype shield OR Arduino Nano with 5x7cm prototype board

10nF capacitor

Small signal diode, e.g. 1N4148

220-ohm resistor

For power:

USB power bank with cable

For visual output:

2 LEDs of different colour e.g. blue and green

2 220Ohm resistors to limit the currents

For sound output:

Passive buzzer

Microswitch to disable sound

For earphone output:

Earphone connector

1kOhm resistor

Earphones

To easily connect/disconnect the search coil:

2-pin screw terminal

For the search coil:

~5 meters of thin electric cable

Structure to hold the coil. Must be stiff but does not need to be circular.

For the structure:

1meter stick, e.g wood, plastic or selfie stick.

Step 3: The Search Coil

For the search coil, I wound ~4m of stranded wire around a cardboard cylinder with 9 cm diameter, resulting in about 18 windings. The type of cable is irrelevant, as long as the ohmic resistance is at least ten times smaller than the value of R in the RL filter, so make sure to stay below 20 Ohms. I measured 1 Ohm, so that is safe. Just taking a half-finished 10m roll of hookup wire also works!

Step 4: A Prototype Version

Given the small number of external components, it is perfectly possible to fit the circuitry on the small breadboard of a prototype shield. However, the final result is rather bulky and not very robust. Better is to use an Arduino nano and solder it with the extra components on a 5x7cm prototype board, (see next step)

Only 2 Arduino pins are used for the actual metal detection, one for providing the pulses to the LR filter and one for reading out the voltage on the capacitor. Pulsing can be done from any output pin but the readout must be done with one of the analog pins A0-A5. 3 more pins are used for 2 LEDs and for the sound output.

Here's the recipe:

  1. On the breadboard, connect the 220Ohm resistor, the diode and the 10nF capacitor in series, with the negative terminal of the diode (the black line) towards the capacitor.
  2. Connect A0 to resistor (the end not connected to the diode)
  3. Connect A1 to where the cross-point of the diode and the capacitor
  4. Connect the non-connected terminal of the capacitor to ground
  5. Connect one end of the coil to the resistor-diode cross-point
  6. Connect the other end of the coil to ground
  7. Connect one LED with its positive terminal to pin D12 and its negative terminal through a 220Ohm resistor to ground
  8. Connect the other LED with its positive terminal to pin D11 and its negative terminal through a 220Ohm resistor to ground
  9. Optionally, connect a passive buzzer headphone or speaker between pin 10 and ground. A capacitor or resistor can be added in series to reduce the volume

That's all!

Step 5: A Soldered Version

To take the metal detector outside, it will be necessary to solder it. A common 7x5 cm prototype board comfortable fits an Arduino nano and all the required components. Use the same schematics as in the previous step. I found it useful to add a switch in series with the buzzer to turn off the sound when not needed. A screw terminal allows to try out different coils without having to solder. Everything is powered through the 5V supplied to the (mini- or micro-USB) port of the Arduino Nano.

Step 6: ​The Software

The Arduino sketch used is attached here. Upload and run it. I used Arduino 1.6.12 IDE. It is recommended to run it with debug=true in the beginning, in order to tune the number of pulses per measurement. Best is to have an ADC reading between 200 and 300. Increase or decrease the number of pulses in case your coil gives drastically different readings.

The sketch does some sort of self-calibration. It is sufficient to leave the coil quiet away from metals to make it go quiet. Slow drifts in the inductance will be followed, but sudden large changes will not affect the long-term average.

Step 7: Mounting It on a Stick

Since you wouldn't want to do your treasure hunts crawling over the floor, the three board, coil and battery should be mounted on the end of a stick. A selfie-stick is ideal for this, since it is light, collapsible and adjustable. My 5000mAh powerbank happened to fit on the selfie stick. The board can then be attached with cable ties or elastics and the coil can similarly be to either the battery or the stick.

Step 8: How to Use It

To establish the reference, it is sufficient to leave the coil ~5s away from metals. Then, when the coil gets close to a metal, the green or blue LED will start flashing and beeps will be produced in the buzzer and/or headphones. Blue flashes and low-pitch beeps indicate the presence of non-ferromagnetic metals. Green flashes and high-pitch beeps indicate the presence of ferromagnetic metals. Beware that when the coil is kept for more than 5 seconds near the metal, it will take that reading as a reference, and start beeping when the detector is taken away from the metal. After a few seconds of beeping in the air, it will turn quiet again. The frequency of the flashes and the beeps indicate the strength of the signal. Happy hunting!

Comments

author
WitoldM1 (author)2017-06-29

I have built this device and it works.

I have a question
related to the analog readings by Arduino. With my coil and the npulse
variable set to 1, the min/max analog readings were in the range between
163 and 166 (a voltage around 0.78V). When I put some metal near to the
coil, then the reading were just a little bit different, which leads to
the conclusion that the voltage changes not so drastically. For my
case, the delta in analog readings is 3 (166-163) but the maximal
possible reading is 1023 (and the reference voltage is 5V by default).
The smallest possible voltage delta that could be meassured by Arduino
is then 5V * 1 / 1023 = 0.0048V. I'm using Arduino Nano, which offers to
use lower ADC reference voltage: 1.1V. If using 1.1V I think we can
detect smaller voltage changes: 1.1V * 1 / 1023 = 0.0010V. So I set up a
lower ADC voltage reference to 1.1V (in the sketch you need to call
analogReference(INTERNAL); in your setup() method). After that change the min/max readings were between 693 and 700 (so the delta is 7, which is bigger than in previous case). I had the feeling
that the device worked better (but it could be just a feeling :) ). What
do you say @rgco?

Second thing: because the voltage doesn't change so much (the delta in readings is less than 10 which is very small comparing to the maximal 1023 that can be read) I'm wondering if I could put some OpAmp between the capacitor and Arduino analog input. The OpAmp would do some additional formula on the analog signal: like 10 * (Ucap - 0.5V). This could gain the device sensitivity (but the device will be more complex). Is that a good idea?

author
rgco (author)WitoldM12017-06-29

Hi!

Using the 1.1V reference is a great idea! The sensitivity could increase by up to a factor 5! For the capacitor voltage I had to make a trade-off between large digitisation error at low voltage and low sensitivity to inductance changes at high voltage and so ended up recommending a value of 200-300 (1.0-1.5V). But going in the range 0.8-1.0 V and using the 1.1V reference should solve both problems. Thanks a lot for the idea! I'll definitely try this out for the next version: I'm working on a super-compact version based on the ATTINY13A and using a coincell.

Amplifying the difference with an opamp seems a bit of an overkill to me, the component count starts to go up, but in theory what you say should work. A diode with O(1mA) current draw might give a decent reference voltage after which the difference could be multiplied by a factor 10 or so...

Thanks for the feedback and the great idea to use the 1.1V reference!

author
GrzegorzK1 (author)rgco2017-07-25

Would voltage dropping from 5V to 1V from any arduino achieve the same result in increasing precision? Using step-down regulator for example.

I am an electronic newbie and understand may have suggested sth stupid here :)

author
rgco (author)GrzegorzK12017-07-25

An external reference can indeed also be applied, and would have a similar effect. I would think a resistor-divider would do the job as well. The difference with the internal 1.1V is that Vref from a divider would follow variations in the supply voltage and thus might give better stability...

author
Brassn (author)2017-07-02

First I'd like to thank you for sharing such a great project.

I'm builing my own version of this and now I'm wondering how one could improve the results. Based on my limited knowledge of this topic I have some questions how my changes would change the depth and sensitivity of the detection.

What if I used a N-MOSFET to send a higher voltage (e.g. 9V) through the coil and read the charge of the capacitor using a voltage-divider to protect the Arduino?

Does a longer wire / more windings on the coil make any difference? (staying under the ohm limit)

Could we use a second coil above the primary one to stretch the magnetic field and improve the penetration?

Does it matter if I use a electrolytic or a ceramic capacitor? I'm having the latter in place.

Also I don't know what happens in your video, most things (including the iron wrench) light the blue led, which should only happen on non-ferromagnetic metal.

author
Brassn (author)Brassn2017-07-02

Okay the voltage divider is probably a bad idea as it would drain the charge before it could be read, but with npluse = 1 it should stay beneath 5V whatsoever. Also the speed of the MOSFET could be a problem.

author
rgco (author)Brassn2017-07-03

Hi, glad you like it!

I did read a little about the various types of metal detectors, and I'm afraid that it's hard to improve on this type without fundamental changes. The reason is that this type of metal detection (measuring the inductance) is sensitive to the total inductance of the coil, and it only changes a tiny bit in the vicinity of metals. So you're always looking for a small change on a big number.

To overcome that, you need a pulse-induction: send a pulse, then record the response. So you get zero for no metal and something non-zero if there is metal.

I think it's possible to make that type with and Arduino as well, but it requires higher voltages, faster switching and better gating. I gave it a quick try but it didn't immediately work and I don't have a fast scope to debug or study it...

author
cshah5 (author)2017-06-24

I want to detect a 3 cm diameter metal sphere at multiple points inside a long tube of 4 cm diameter , the tube might be filled with water or other substances. Will it detect metal if the coil is around the tube ? and if possible to what accuracy i.e will it detect always at the same point?

author
rgco (author)cshah52017-06-25

Interesting!

If the tube is metal it won't work: it will effectively shield any EM effect inside.

If the tube is an isolator it should work easily: a 3cm metal sphere gives a big change in inductance of a 4- or 5 cm coil.

Make the diameter of the coils as small as possible and still fit around the tube. That way the change in inductance will be biggest.

With some changes to the code, one Arduino should be able to handle 6 coils.

Good luck!

author
ClémentG16 (author)2017-06-07

Works ! Thanks man. Can see my keys and PC at about 4 cm.

author
lamaleon (author)2017-05-02

Hi, how deep is the detection in the soil?

thanks

author
rgco (author)lamaleon2017-05-02

Hi, not very deep. For this type of metal detector (based on induction measurement), its range is about half the diameter of the coil. So a larger coil will probe deeper, but be less sensitive to small objects.

author
cesar.riojas1 (author)2017-04-21

Hi, does it need the coil cable to be 2 colors? I saw in the image a white and a red one! And if so can I join them just by sticking them?

author
rgco (author)cesar.riojas12017-04-21

Hi, well noted! No, it's actually better to have a single wire. It's just that at the the time I didn't have an 8m piece of wire, so I soldered together two pieces which happened to have two different colours, nothing more than that. Good luck!

author
cesar.riojas1 (author)rgco2017-04-21

Thank you for your quick answer. I'll get down to work right now.

author
RamU1 (author)2017-03-10

what was the value of inductance

author
rgco (author)RamU12017-03-10

In my case the search coil has R=4.5cm, N=18 gives L~40muH. But contrary to beat-frequency-oscillation (BFO) detectors, that need a very precise matching, the value of the inductance does not matter much here, and variations of a factor 2 or 3 are fine. The main thing is to have the ADC reading somewhere around 30% of the maximum, where it is most precise. In the inductance of the search coil is too low, the number of pulses per reading can be increased in the code. If the inductance is too high, the number of pulses can be decreased. Also the value of the capacitor can be adjusted.

author
R DevaD (author)2017-03-06

Hello
I have doubt pls clear it.
Difference between soldered version and prototype version
Both do same job or is there is difference between them .
Pls reply

author
rgco (author)R DevaD2017-03-06

Hi,

They do the same, only I did not put the sound on the prototype version, since I didn't intent to take it outside. Using a prototype shield is great to set things up quickly, test the idea and the software, but it's too fragile to take outside. Also, outside the sunlight is too strong to see the LEDs well, so having sound really helps. Ah also I didn't make a real search coil for the prototype, just a 6m roll of hookup wire

author
magnatron (author)2017-02-28

Please indicate where the correct source code can be found?

author
rgco (author)magnatron2017-02-28

Hi, it is attached at step 6

author
actuonix (author)2017-02-24

Wow, incredibly thorough and interesting instructable. Thanks for this!

author
rgco (author)2017-02-23

The range is similar to the radius of the coil, so 4.5cm for the 9cm diameter coil that I used. A larger coil would have a larger range, but be less sensitive to small objects, since what is measured is the relative change in inductance.

The detector 'sees' through other objects: I got clear signals through a 3cm thick wooden table when placing a metal object on top and sweeping the detector from below.

Note that a nail won't give a strong signal since it's cross-section area is small

author
rafununu (author)2017-02-20

Regulations are quite simple in France : all that is buried, even at 1 mm, even on your property, belongs to the state ! Because ground is not yours.

author
rgco (author)rafununu2017-02-20

I checked for Italy. You must immediately stop and report if it's archeological or hazardous. If it has value you have to bring it to the police, but if it's not claimed after a year it's yours. Seemed surprisingly reasonable to me...

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