Simple Arduino Metal Detector




Posted in TechnologyArduino

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


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!



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    8 Questions

    Man, can you tell where is a code about 5 seconds?

    You wrote: "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."

    I want to change from 5 to 6, but can't find where to change anything. Where is that code..?

    hello how can i increase sensitivity and distance for metal detection?

    bonjour comment je peux augmenter la sensibilité et la distance pour la detection de metal ?


    hi guys can this be made to an 1m depth metal detector? im in need of help and I chose this as a project but I cant seem to make it reach 1m depth altho it works fine please I am in need of help

    Hi, I'm going to try this project but I don't understand why there is two strands of differennt colour on your coil??


    I didn't have a long enough strand, so connected two together, that's all!

    Hello sir. I'm doing this for a project. The metal detector seems to not work all the time. We're using arduino UNO and protoshield.



    Hi, I've made this project with arduino UNO and it works perfectly, i advice you to verify the circuit, also verify that you uploaded the program correctly



    Hi. I want to have an LCD with something like a loading bar to show how far is the metal. Is there any way to get a value so i can display it as a bar?


    The value of the variable 'diff' quantifies the strength of the signal so that could be used.

    you mean the 1om is resistan of L


    Nice ! Mine works, I have a mute button for my speaker... It's an annoying sound ^.^

    here's mine using arduino uno , not detecting smaller objects XD


    More question sir, is it okay to use two coil, fo example 1st coil has smaller diameter, and second got 10cm , is it possible?

    3 replies

    You mean in series? yes should be OK, also in parallel. The inductance changes in a similar way as for a single coil...Although I wouldn't see what's the advantage?

    Im just thinking of what I read in the comment , "The higher the diameter the deeper it range, but the smaller chance to detect smaller metals" .. so I want to make two , one with smaller diameter and the other one is wider haha. Thank you for your comment

    I built a breadboarded version on the bench. It worked the first try. I adjusted the count up and down from the 3 counts in the original sketch. It did change the capacitor reading, I left it at 3, which gave a value of 273. Changes in the count didn't seem to affect the sensitivity. One problem I have is that I used an unknown generic .001uF (10 nF) capacitor from my junk box. I suspect it's a Y7R. It is *very* sensitive to temperature changes! air currents in the room change the value enough that the detector goes off with no metal around it. Putting an upside down cardboard box over the circuit pretty much stops that, but is impractical. I've ordered some better caps to solve that problem. With a 5 1/2 inch (14 cm) coil, it will detect a medium size piece of metal at about 3 inches. (7.6 cm) A US quarter gas to be about an inch (2.54 cm) from the coil for detection. That's not good enough for my purposes, I'm going to try some modifications. The first will be using the internal voltage ref of 1.1V [ analogReference(INTERNAL); ] To do this I'll have to make some circuit changes as the capacitor voltage hits 1.3V which is above the 1.1V internal reference. If I can get the sensitivity up, I'm going to 3D print some mechanical parts and build a permanent version. Thanks for a great instructable!

    13 replies

    Hi sir , I noticed you have made another and upgraded version of this nice metal detector, do you have test video about your final circuit sir? , Thanks.

    Not yet, it's been put on th back burner for now, ubt it will get done some time. I'll probably write an Instructable about it when I buiild the complete version.

    Wow, Thanks sir, we will wait :D

    I just tried the 1.1V internal reference. I had to raise the resistor to 330 ohms to drop the capacitor voltage to about 0.9V, under the new reference voltage. The sensitivity was definitly improved, but not as much as I had hoped. About 25% better on a US quarter. The increased sensitivity caused some problems though. It's even *more* sensitive to capacitor temperature, and it seems to be triggerd by electrical noise, causing random clicks and blinks. I'm going to have to think about this more...

    No real improvement on sensitivity yet. I'm going to try different coil sizes soon, I was having trouble with instability, the detector going off for no reason. It turned out to be my fault, I made two mistakes. The first was the capacitor, as I noted above, it was an unknown ceramic. I replaced it with a polystyrene capacitor, no more sensitivity to air currents. I can blow on it, or even hold it between my fingers without the detector going off. There was still some random chirping going on though, both with a 5V reference and the 1.1V reference, I realized that it might be an unstable supply voltage, I was running it off of the USB port of the computer that I was programming with. I substituted a bench supply set to 9V and connected through the Arduino's barrel jack. The chirping totally went away with the 5V reference, it was still there with the 1.1V reference. The 1.1V reference is regulated on the chip, not provided by the power source, so it should have been stable even with the so-so supply voltage. I'll try that smaller coil next, and see if I can find any other ways to improve the circuit.

    Thanks so much Jim for your great add on and follow thru comments. I am just beginning my own quest to immerse myself and try my hand at DIY a PI detector which when using with a large coil, and greater power, will be able to search at depth range of 3-15 feet. Was thinking of using landscape semi-rigid tubing for a larger coil frame of either 4' to 8' diameter and using a table saw to notch out either top or outer perimeter for inserting coil wire/s. Will add non-metallic wheels directly to frame and dragging behind an ATV | UTV | SUV.

    I look forward to your continued efforts and posts. Thanks again.

    That sounds like a cool project. If you want, you can let me know how it goes at Anyone else who wants to discuss this circuit may contact me as well.

    Even with the smaller coil, I didn't get the sensitivity that I wanted on smaller objects, and the distance at which I could detect larger objects was reduced. I tried a 40 turn coil instead of the 18 I was using for the 4 inch coil, but other than being able to raise npulse from 3 to 7 (for what that is worth), there was no appreciable difference.

    I had the idea to measure the pulse width from the 40 turn coil with pulseIn() directly instead of looking at the voltage on a capacitor. A little research showed that this would not work, the pulse was too narrow for pulseIn() I then had the idea to "ring" an LC circuit and measure the width of the first half cycle with pulseIn().

    I Googled this idea and found:

    He's using a very similar method to measure the inductance of assorted coils but does mention that it might be used as a metal detector.

    I built part of his circuit up and am measuring the pulse width of the first half of the first cycle of the damped sine wave using a 'scope. Big objects make a huge timing change, smaller objects almost immeasurably so with the 'scope. I'll build the full circuit (probably without the diode, it seems to serve no purpose and the results I get are the same without it) and see whether there is enough time to do the job. I am getting what will be pulse widths of about 25uS with 2 0.47uF caps, well within pulseIn() capabilities. The amplitude of the first ring is about 250mV using a 150 ohm resistor, not enough to read directly with the Arduino. The comparator will be needed to get it up to logic levels. There is so much noise and 5V random spikes that I fear that the pulse output will not be clean. I'll have to look into a Schmitt trigger.


    Thanks for all your feedback Jim and please keep up updated!

    The method in your link does not look very promising to me.

    Metal detection by inductance change has severe limitations regarding depth and size of the object. As a rule of thumb, the change in inductance at zero depth corresponds to the area of the object divided by the area of the coil. At nonzero depth, divide that fraction by the 3rd power of the ratio of the distance to the radius.

    So for a 10cm coil, a 2cm coin at 10cm distance, gives an inductance change of (2/10)**2 / (10/5)**3 = 0.04 / 8 =0.5%.

    So staying with this technique, the most important is to get good sensitivity to very small changes.

    The Arduino ADC can easily get to 10^-3, especially with a well-chosen reference and averaging multiple measurements. But 10^-4 would be hard if not impossible.

    In fact, the classical BFO metal detector (see e.g. )

    can be sensitive to 1Hz on a base of 10^5Hz, so 10^-5 sensitivity.

    I think the following might work: if the coil is made part of an LC circuit (with a good stable capacitor), the Arduino could measure the frequency with very good precision, e.g. by recording time-stamps of the internal timers whenever the oscillating signal goes over threshold (and fires an interrupt). This way the super-high sensitivity of a BFO is achieved, but with much fewer components, and all the flexibility of the Arduino, such as autocalibration, noise filters, and a wide range of output modes.

    Thanks for the idea, I'll try that maybe after the pulsIn() idea. I don't think it will work either, I can hardly see a change on the 'scope when I move a smaller object close.


    Time is something that the Arduino can measure with very high precision: it has a pretty stable crystal and the timers can be set up to measure with a resolution of a single clock cycle (1/16th of a microsecond). The length of a single LC pulse won't be very accurate (10^-3 for a single pulse of a 16kHz signal). But keeping track of multiple transitions for even 10ms should give 10^-5 precision.

    An LC oscillator like the one in (left side of the schematic) uses just 1 transistor, 2 capacitors and 1 resistor (plus the coil). There may not even be a need for an external comparator since the Arduino has a built-in comparator, which can be used for interrupts (if I recall well...)

    OK sorry, just thinking aloud. Could be a project for the christmas holidays! Cheers

    I just built the oscillator on the left side of that schematic. It works well, and did on the first try. Just looking at the frequency measurment on my 'scope, it seems a lot more sensative than either of the inductance measurement methods

    Sorry I'm taking so long to reply, I have a lot of other stuff going on...

    This seems promising! I would like to try this out and experiment with the coil size on sensitivity, but quick question. Which node from the left side of that circuit would feed into the Arduino to read the frequency? Would it be the node leaving the 10k resistor at the bottom that would go to the 3rd transistor if it was being used? Also, did you make any modifications to the circuit to make it readable?

    I just looked at my comment and it looks like I said that I thought the frequency method wouldn't work. Sorry, I didn't proofread, it actually sounds like a very good idea...