Eco Friendly Metal Detector - Arduino

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About: Crazy about technology and the possibilities it can bring. I love the challenge of building unique things. My goal is to make technology fun, relevant to everyday life and help people succeed in building coo...

Metal Detecting is a lot of fun. One of the challenges is being able to narrow the exact place to dig to minimize the size of the hole left behind.

This unique metal detector has four search coils, a color touch screen to identify and pinpoint the location of your find.

Incorporating auto calibration, a USB rechargeable power pack, with four different screen modes, frequency, and pulse width adjustment which allows you to customize how you search.

Once you have pinpointed the treasure a single hole centered above each coil enables you to use a wooden skewer to push into the earth so you can start to dig a small plug from the ground reducing damage to the environment.

Each coil can pinpoint detect coins and rings at a depth of 7-10cm so is ideal for looking for lost coins and rings around parks and beaches.

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A Big Thankyou - If you pushed vote button in the top right-hand corner for the "Invention Challenge" and "Explore Science" competitions!!!

many thanks,

TechKiwi

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Step 1: The Science Behind Metal Detection

Metal Detection Design

There are multiple variations of Metal Detector designs. This particular type of metal detector is a Pulse Induction detector which uses separate transmit and receive coils.

The Arduino produces a pulse which is applied to the Transmit Coil for a very short period of time (4uS) via a transistor. This current from the pulse causes a sudden magnetic field to form around the coil, the expanding and collapsing field induces a voltage into the Receive Coil. This received signal is amplified by the receiving transistor and then turned into a clean digital pulse by a Voltage Comparator and in turn sampled by a Digital Input pin on the Arduino. The Arduino is programmed to measure the pulse width of the received pulse.

In this design, the received pulse width is determined by the receive coil inductance and a capacitor. With no objects in range, the baseline pulse width measures approximately 5000 uS. When foreign metal objects come into range of the expanding and collapsing magnetic field this causes some of the energy to be induced into the object in the form of eddy currents. ( Electromagnetic induction)

The net result is that the received pulse width is reduced, this difference in pulse width is measured by the Arduino and displayed on a TFT display in various formats.

Display Option 1: Position of Target under Detector Head

My intention was to use the 4 coils to triangulate the position of the target under the detector head. The non-linear nature of the search coils made this challenging however the animated GIF above shows the results are useful enough to show the relative position of the target under the head as well as the strength of the signal.

Display Option 2: Show Signal Trace for Each Search Coil

This enables you to track where the target object is under the head by drawing an independent signal strength trace on the screen for each search coil. This is useful to determine if you have two targets close together under the detector head and the relative strength.

Practical Uses

This approach enables you to use the first view to identify a target and the second view to pin point it to a few millimeters as shown in the video clip.

Step 2: Gather the Materials

Bill of Materials

  1. Arduino Mega 2560 (Items 1, 2 and 3 can be purchased as one bundled order)
  2. 3.2" TFT LCD Touch Screen (Ive included code for 3 supported variations)
  3. TFT 3.2 Inch Mega Shield
  4. Transistor BC548 x 8
  5. 0.047uf Greencap Capacitor x 4 (50v)
  6. 0.1uf Greencap Capacitor x 1 (50v)
  7. 1k Resistor x 4
  8. 47 Resistor x 4
  9. 10k Resistor x 4
  10. 1M Resistor x 4
  11. 2.2k Resistor x 4
  12. SPST Mini Rocker Switch
  13. Integrated Circuit LM339 Quad Differential Comparator
  14. Signal Diodes IN4148 x 4
  15. Copper WireSpool 0.3mm Diameter x 2
  16. Two Core Screened Cable - 4.0mm Diameter - 5M length
  17. USB Rechargeable Powerbank 4400mHa
  18. Piezo Buzzer
  19. Vero Board 80x100mm
  20. Plastic Case minimum 100mm Height, 55mm Depth, 160mm Width
  21. Cable Ties
  22. MDF Wood 6-8mm Thickness - 23cm x 23cm square pieces x 2
  23. Micro USB extension cable 10cm
  24. USB-A plug cable suitable to be cut down to 10cm length
  25. Headphone Audio Jack Point - Stereo
  26. Various wood and plastic spacers detector head
  27. Speed Mop Broom handle with adjustable joint (one axis movement only - see photos)
  28. One piece of A3 Paper
  29. Glue Stick
  30. Electic Jig Saw cutter
  31. A4 Sheet Cardboard 3mm thickness for creating a coil former for TX and Rx coils
  32. Duct Tape
  33. Hot Glue Gun
  34. Electric Glue
  35. 10 additional Arduino Header Pins
  36. PCB Terminal Pins x 20
  37. TwoPart Epoxy Glue - 5 min drying time
  38. Craft Knife
  39. 5mm Plastic Tube length 30mm x 4 (I used garden watering system tubing from hardware store)
  40. MDF Waterproof sealer (Ensure does not contain metal)
  41. 60cm Flexible Electrical Conduit - Grey - 25mm Diameter

Step 3: Build the Detector Head

1. Constructing Head Assembly

Note: I chose to build a rather complex mounting arrangement for the 8 copper wire coils that are used in the detector head. This involved cutting a series of holes out of two layers of MDF as can be seen in the photographs above. Now I have completed the unit I recommend using just a single cut out circle 23 cm in diameter and attaching the coils to this single layer of MDF with hot glue. This reduces the build time and also means the head is lighter.

Begin by printing out the stencil provided onto an A3 piece of paper and then glue this onto the MDF board to provide you with a guide for positioning the coils.

Using an Electric Jig Saw carefully cut out a 23cm diameter circle from the MDF.

2. Winding the Coils

Use the cardboard to create two 10cm length cylinders held together with Duct Tape. The diameter of the Transmit Coils needs to be 7cm and the Receive Coils 4cm.

Place the copper wire bobbin on a spike so that it can turn freely. Attach the start of the copper wire on the cardboard cylinder using duct tape. Wind 40 turns firmly onto the cylinder and then use Duct tape to tie off the end.

Use Hot Glue to fasten the coils together on at least 8 points around the circumference of the coils. When cooled off, use your fingers to ease the coil off and then fasten it to the Metal Detector head template using Hot Glue. Drill two holes through the MDF next to the coil and pass the ends of the coil through to the top side of the Metal Detector Head.

Repeat this exercise to build and mount 4 x Receive Coils and 4 Transmit coils. When finished there should be 8 pairs of wires protruding through the top of the metal detector head.

3. Attach the shielded cables

Cut the 5M length of shielded twin core cable into 8 lengths. Strip and solder the twin core to each transmit and receive coil leaving the shield disconnected at the Detector Head end of the cable.

Test the coils and cable connections at the other end of each cable using an Ohm Meter. Each coil will register a few Ohms and should be consistent for all Receive and Transmit coils respectively.

Once tested use the hot glue gun to fasten the 8 cables into the center of the Detector Head ready for attaching the handle and finishing the head.

My advice is to strip and tin each of the shielded cable cores at the other end in preparation for the future testing. Attach an earth wire to each cable shield as this will be connected to earth in the main unit. This stops interference between each cable.

Use a Multimeter to identify which coil is which and attach sticky labels so they can be identified easily for future assembly.

Step 4: Assemble Circuit for Testing

1. Breadboard Assembly

My recommendation is to use a breadboard to first set up and test the circuit before committing to Vero Board and an enclosure. This gives you the opportunity to adapt component values or modify the code if required for sensitivity and stability. The transmit and receive coils need to be connected so they are wound in the same direction and this is easier to test on a breadboard before labeling the wires for future connection to Vero Board.

Assemble the components as per the circuit diagram and attach the Detector Head Coils using hookup wire.

The connections to the Arduino are best made using bread board hook up wire soldered to the TFT shield. For Digital and Analogue pin connections I added a Header Pin which enabled me to avoid soldering directly to the Arduino Board. (See picture)

2. IDE Libraries

These need to be downloaded and added to the IDE (Integrated Development Environment) that runs on your computer, used to write and upload computer code to the physical board. UTFT.h and URtouch.h located in zip file below

Credit for UTFT.h and URtouch.h goes to Rinky-Dink Electronics I've included these zip files as it appears the source Website is down.

3. Testing

I have included a test program to handle the initial setup so you can deal with coil orientation issues. Load the test code into the Arduino IDE and upload to the Mega. If everything is working you should see the test screen as above. Each coil should produce a steady state value of approx 4600uS in each quadrant. If this is not the case reverse the polarity of the windings on the TX or RX coil and test again. If this does not work then I suggest you check each coil individually and work back through the circuit to troubleshoot. If you already have 2 or 3 working compare them to the coils/circuits not performing.

Note: Further testing has revealed that the 0.047uf capacitors on the RX circuit influence over all sensitivity. My advice is once you have the circuit working on a breadboard, try increasing this value and testing with a coin as I've found that this can improve sensitivity.

It is not mandatory however if you have an oscilloscope you can also observe the TX Pulse and RX Pulse to ensure the coils are connected correctly. See the comments in the pictures to confirm this.

NOTE: I have included a PDF document in this section with oscilloscope traces for each stage of the circuit to help troubleshoot any issues.

Step 5: Build the Circuit and Enclosure

Once the unit has been tested to your satisfaction you can take the next step and build the circuit board and enclosure.

1. Prepare the Enclosure

Layout the major components and position them in your case to determine how everything will fit. Cut the Vero Board to accommodate the components, however, ensure you can fit into the bottom of the enclosure. Be careful with the Rechargeable Power Pack as these can be quite bulky.

Drill holes to accommodate the back entry of the head cables, power switch, External USB port, Arduino Programming Port and stereo headphone audio jack.

In addition to this drill 4 mounting holes in the center of the front side of the case where the handle will be, These holes need to be able to pass a cable tie through them in future steps.

2. Assemble Vero Board

Follow the Circuit Diagram and the picture above to position the components on the Vero Board.

I used PCB Terminal Pins to enable easy connection of the head coil cables to the PCB. Mount the Piezo Buzzer on the PCB along with the IC and transistors. I tried to keep the TX, RX components aligned left to right and ensured that all connections to external coils were at one end of the Vero Boar. (see the layout in photos)

3. Attach the Coil Cables

Build a cable holder for the incoming shielded cables out of MDF as shown in the pictures. This consists of 8 holes drilled into MDF to enable the cables to sit aligned to PCB Terminal Pins. As you attach each coil it pays to test the circuit progressively to ensure correct coil orientation.

4. Test The Unit

Connect up the USB Power Pack, Power Switch, Audio Phone Jack and position all of the wiring and cables to ensure a snug fit in the case. Use Hot Glue to hold items in place to ensure there is nothing that can rattle around. As per the previous step, load the test code and ensure all coils are performing as expected.

Test that the USB Power Pack is Charging correctly when connected externally. Ensure there is enough clearance to attach the Arduino IDE cable.

5. Cut Out The Screen Appeture

Position screen in the center of the box and mark the edges of the LCD display on the front panel ready for cutting out an aperture. Using a craft knife and a metal ruler carefully score the case lid and cut out the aperture.

Once sanded and filed to shape carefully position lid while ensuring all components, boards, wiring, and screen are held in place with spacers and hot glue.

7. Build Sun Visor

I found an old black enclosure that I was able to cut into shape and use as a sun visor as shown in the photos above. Glue this onto the front panel using 5min two part epoxy.

Step 6: Attach Handle and Case to Detector Head

Now that the Detector Electronics and Head are built all that remains is to complete mounting the unit securely.

1. Attach the Head to the Handle

Modify the handle joint to enable you to attach this to the head using two screws. Ideally, you want to minimize the amount of metal near the coils so use small wood screws and a lot of 5minute 2 part epoxy glue to fasten to the head. See photos above.

2. Lace Up Head Wiring

Using Cable Ties carefully lace up the wiring by adding a cable tie every 10 cm along the shielded wiring. Take care to ensure you to work out the best position for the case so it's easy to see the screen, reach the controls and attach headphones/plugs.

3. Attach the Electronics to the Handle

Build a 45 Degree Mounting Block from MDF to enable you to attach the Case at an angle that means when you are sweeping the detector across the ground you can see the TFT display easily. See the picture above.

Attach the Electronics Case to the handle with Cable Ties running through the mounting block and into the case through the previously drilled mounting holes.

4. Finish off the Detector Head

The Detector Head coils need to be fixed with no movement in the wiring so this is a good time to use Hot Glue to fasten all of the coils in place thoroughly.

The Detector Head also needs to be waterproof so it is important to spray the MDF with a clear sealer (ensure sealer does not contain metal for obvious reasons).

Drill 5mm holes in the center of each coil and pass 5mm x 30mm plastic tubing through to enable you to push wooden skewers into the soil below once you have pin pointed a target. Use hot glue gun to lock into position.

I then covered the top of the head with a plastic plate and the bottom with a thick plastic book cover whilst finishing the edge with flexible electrical conduit tubing cut and Hot Glued into place.

Step 7: Final Assembly and Testing

    1. Charging

    Place a standard cell phone charger into the Micro USB port and ensure the unit is adequately charged.

    2. Upload Code

    Use the Arduino IDE to upload the enclosed code.

    3. Mute Button

    The unit defaults to being muted on power up. This denoted by a red Mute Button in the bottom LHS of the screen. To enable sound push this button and the button should go green denoting sound enabled.

    When un-muted the internal buzzer and external audio phone jack will produce sound.

    4. Calibration

    Calibration returns the trace to the bottom of the screen beneath the threshold lines. When first turned on the unit will automatically calibrate. The unit is remarkably stable however if there is a need for recalibration this can be done by touching the calibrate button on the screen which will recalibrate in less than a second.

    5.Thresholds

    If the signal on any trace exceeds the threshold line (the dotted line on the screen) and the Mute Button is off then an audio signal will be produced.

    These thresholds can be adjusted up and down by touching the screen above or below each trace line.

    6. Adjustment of PW and DLY

    The duration of the Pulse to the coil and the delay between pulses can be adjusted through the touch display.This is really in place to experiment with so various environments and treasures can be tested for best results.

    7. Display Types

    There are 4 different display types

    Display Option 1: Position of Target under Detector Head
    My intention was to use the 4 coils to triangulate the position of the target under the detector head. The nonlinear nature of the search coils made this challenging however the animated GIF above shows the results are useful enough to show the relative position of the target under the head as well as the strength of the signal.

    Display Option 2: Show Signal Trace for Each Search Coil This enables you to track where the target object is under the head by drawing an independent signal strength trace on the screen for each search coil. This is useful to determine if you have two targets close together under the detector head and the relative strength.

    Display Option 3: Same as option 2, however, with thicker line makes it easier to see.

    Display Option 4: Same as option 2, however, draws over 5 screens before deleting trace. Good for capturing signals that are faint.

    I am field testing over the next few weeks so will be publishing any treasure finds.

    Now go have some fun and find some treasure!!

    Step 8: Epilogue: Coil Variations

    There have been a lot of good, interesting questions and suggestions about coil configurations. In the development of this instructable, there were numerous experiments with various coil configurations that are worth mentioning.

    The pictures above show some of the coils I tried prior to settling on the current design. If you have further questions message me.

    Over to you to experiment further!

    Invention Challenge 2017

    First Prize in the
    Invention Challenge 2017

    Explore Science Contest 2017

    First Prize in the
    Explore Science Contest 2017

    2 People Made This Project!

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    164 Discussions

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    ellisgl

    12 days ago

    Idea: On the transmission coils, you could convert it a single one, by running the wire in a figure eight all around. On the receive code, you have it loop the receive coil readings, and shift the the starting point after each send pulse. Also you could put in a fifth smaller receive coil right in the middle.

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    SultanE3

    Question 5 weeks ago

    hello , nice job , i want to ask ; how we can increase the detection depth ?

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    PaulH250

    5 weeks ago

    I am having a problem uploading the code to my Arduino mega 2560. Is there something I need to add to the library?
    I get "exit status 1
    Error compiling for board Arduino/ Ginuino Mega or Mega 2560"
    I have all my coils ready and have the board ready to hook up to the Arduino I just can't get this code to download. Thanks.
    Oh, I think what you have done here is wonderful. Thanks for sharing it.

    6 replies
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    TechKiwiGadgetsPaulH250

    Reply 5 weeks ago

    Try just uploading the code to the Arduino Mega without anything connected to it. I suspect you have the IDE configured for the incorrect board, a library issue or cable connectivity issue. If problems persist message me with the code errors and I will attempt to help diagnose\.

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    PaulH250PaulH250

    Reply 5 weeks ago

    compilation terminated.





    Using library UTFT_SdRaw at version 1.2.4 in folder:
    C:\Users\Moroni\Documents\Arduino\libraries\UTFT_SdRaw


    exit status 1


    Error compiling for board Arduino/Genuino Mega or Mega 2560.


    Invalid library found in C:\Program Files
    (x86)\Arduino\libraries\LiveOV7670-master: C:\Program Files
    (x86)\Arduino\libraries\LiveOV7670-master


    Invalid library found in C:\Program Files
    (x86)\Arduino\libraries\MultiWiiConf: C:\Program Files
    (x86)\Arduino\libraries\MultiWiiConf


    Invalid library found in C:\Program Files
    (x86)\Arduino\libraries\UTFT: C:\Program Files (x86)\Arduino\libraries\UTFT

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    TechKiwiGadgetsPaulH250

    Reply 5 weeks ago

    It looks like the screen driver libraries UTFT are not loaded correctly. Make sure they are downloaded and added to your IDE. You can check by looking in the library menu within the IDE. The IDE also needs to be restarted after adding a library (older versions anyway) so if not already tried restart the IDE application.

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    steveg221

    5 weeks ago

    Hi, I am having a go at making one of these, but I am having trouble with the screen.

    When I compile the script in Arduino I get the following

    C:\Arduino\Eco_Friendly_Metal_Detector_V8\Eco_Friendly_Metal_Detector_V8.ino: In function 'void loop()':
    and lots of the following

    C:\Arduino\Eco_Friendly_Metal_Detector_V8\Eco_Friendly_Metal_Detector_V8.ino:177:34: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
    myGLCD.print("CAL",10,9);
    ^
    The screen is completely white.

    Hope you can help

    Steve

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    TechKiwiGadgetstava999

    Reply 5 weeks ago

    The most effective way I found was to use a Darlington configuration for the BC548 RXR circuit. Coil size and voltage to coils seem to have an overall improving effect. Let me know how you get on. I tried various coil configs as seen in Step 8.

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    GuillermoL30

    Question 4 months ago on Step 4

    your ORC photos and the DSO captures inside the PDF are not congruent, the waveforms and delays are totally different.
    ¿why is there a delay between TX pulse and collector output?, the moment the TX pulse is sent to the coil, the RX pulse will also appear and make TX transistor stop conducting and put the collector to +V, yet in your ORC captures, the collector goes up after several mS.
    ¿Is the 47nF value for the RX cap based on some time constant?, ¿what's the theory behind that value?(because the RC constant with the 1M R gives me 47mS)
    ¿why use the transistor at all when you can also use the comparator as a negative peak detector directly?.

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    GuillermoL30

    Question 4 months ago on Step 2

    Hello, i'm very interested in this project(actually modifying it heavily with a single coil and no screen), and have some questions:
    1) ¿did you chose the coil diameters based on something in particular?, ¿or can they be enlarged?
    2) ¿any reason in particular to use a NPN drive for the coil instead of a mosfet?, or directly using the arduino digital output
    3) ¿what's the point of the 2M resistor on the RX coil amplifier?
    4) ¿what's the detection depth for small objects in the ground?(~bolt sized stainless steel)

    2 answers
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    TechKiwiGadgetsGuillermoL30

    Answer 4 months ago

    Hi, you can modfy to use without a screen and one coil. I am working on arduino nano single coil version now so will publish in the next few weeks. It is for a specific application however the circuit could be modified to suit.
    1. Coils can be adjusted to suit
    2. The original design was an NPN however in recent circuits Ive used a FET as they are more efficient
    3. the 1M resistor provide forward Bias for the NPN receive transistor and gain control.
    4. This is dependent on size of object

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    GuillermoL30TechKiwiGadgets

    Reply 4 months ago

    i see that in your coil variation photos there's one which has a large diameter(the entire plate) ¿how was it's performance?.
    Also, are you triggering the receive transistor on a negative pulse of the receive coil, ¿is that correct?(i'm really rusty on my electronics!).

    i'll see if i can get a single coil/no screen version based on this one too, with a FET drive(i'll also think on using a larger Vcc for the coil drive, maybe with a dc-dc converter, but the issue is that it needs a lot of power)

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    DanielMBZ33M

    Question 4 months ago on Step 7

    Hello, I am french you will forgive me for my english. I do not know anything about Ardino and that interests me. I would like to know if you have already tried to integrate the metal conductivity index: this is what makes it possible to have on a metal detector a number that goes from 0 to 99 and which indicates which metal has been detected. I searched on the internet, but I did not find if this index is obtained by calculation or if it needs an electronic device ... My idea would be to use my disks of my real detector and to use an ardino .

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    ArtJ11

    1 year ago

    Thanks for posting this amazing project....I'm hoping to get a working rendition within a few months. Parts are on order but in the interim I've wound the coils on very thin 3D printed spindles to speed the process and hopefully give me some flexibility when it comes to mounting them in some kind of enclosure.....sort of just drop them in with a little hot glue as self contained modules and close everything up...got a few photos attached here and if anyone is interested I can supply the STL files for printing!

    IMG_20180125_151550897.jpgIMG_20180125_151739189.jpgIMG_20180125_151703215.jpg
    2 replies
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    maxdevilArtJ11

    Reply 4 months ago

    Ciao dall'Italia mi manderesti i file stl grazie