Eco Friendly Metal Detector - Arduino

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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many thanks,

TechKiwi

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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.

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

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.

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.

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.

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.

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!!

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!