VLF metal detector "Smart Hunter"
<Step by step>
Metal detector Smart Hunter has three main parts: 1 – The coil, 2 – The app “Smart Hunter”, 3 – generator (based on arduino, internal generator or NE555 generator).The most important and hardest part to do is the coil. You have to do the coil with patience to get good results. Here I will explain what steps have to be followed to do everything right and that the final result does not disappoint us. This project is in development mode, you can try, expirement with different coils, try different generators. If you don’t understand something you can ask me here or in my email email@example.com
The main parts of metal detector:
Step 1: The generator and Rx circuit:
The first thing we have to do is the generator, because the generator will help us then adjust the coil. And of course install the Smart Hunter app if we don’t have it yet. The generator can be made with an arduino, with NE555, or using the internal generator of the app (if we use the internal generator of the app, we need an amplifier because the generator signal is weak and we have to amplify it). Here I leave the video tutorials how to make the generator with arduino and also how to use the app’s internal generator. We must check and make sure that the generator works well. For that we have to connect a speaker and if we hear a loud and uninterrupted beep then everything works fine. After checking and making sure that the generator works we can we can do the “Rx” coil circuit.
"Rx" coil circuit:
In the schematic we can see how the components are connected. We have the “Rx” coil and if it is necessary also the “Cx” coil for soft adjust (optional). In the image you can see how it connects. Then we have the capacitor “C1” which is used to adjust the resonance frequency of “Rx” coil, the value of this capacitor is 1mF if we are not going to adjust the resonance frequency in the “Rx” coil, and if we are going to adjust then the value must be calculated we’ll see that later. Then we have two diodes “D1” and “D2” to protect the smartphone from overload (if that happens but it’s almost impossible). The “C2” capacitor also serves as a protection and has a value of 1 mF. Then we have a resistance “R2” of 1.4 kOm (we can use a value of 1.2 kOm up to 2 kOm) that helps us so that the smartphone detects that a MiniJack connector is connected to the headphone jack (in some smartphones it works without this resistance). And finally we have connected an input for MiniJack to connect audio output (we can weld directly if we do not have the connector). The schematic and the sketch for arduino to download you can find here: Schematic.
Mini Jack 3.5 mm type connector:
The circuit we saw connects to the 3.5 mm MiniJack type connector. In the image you can see the connections. It is quite simple we just have to connect “GND” of the circuit to the “GND” of the MiniJack, the “MIC” to the “MIC” of the connector and the audio output we can choose one of the two “Left channel” or “Right channel”. After making the generator and the circuit for “Rx” coil we can make the coil.
Step 2: The coil:
The coil is the most difficult part to make. I use Double-D type coil but it also serves the concentric coil. The Double-D type coil is cast with two coils (Rx coil and Tx coil) when adjusting them in inductance balance “IB” the coils are converted into a Double-D type coil. When a metal object is near the center of the Double-D coil the inductance balance is lost and in this way we know that there is a metal object near the coil. To make the coil we use wire thickness 0.6 mm for Tx coil and 0.2 mm for Rx coil. In the video we can see how to make and adjust the coils. After filling the mold with the epoxy resin, the balance can get loose and there is no way to move the coils for adjust it again because they are inside the epoxy resin. In this case, if the “amp” value (which indicates the balancing adjustment) is not more than 1000, we can adjust it again with the small coil called “Cx” coil. “Cx” coil is a small coil that can have 4 to 8 turns with a diameter of 3 cm. As a mold to make the Double-D coil you can use 3D model to print on a 3D printer or foam plastic. You can see the videos with mold made with 3D model printed with 3D printer.
3D printed mold for Double-D type coil
How to use "Cx" coil if we have lost the balance:
To return the lost balance (if this happened) we can put a small coil called “Cx” coil. “Cx” coil can have from 4 to 8 turns of wire thickness 0.2 mm. We must connect this coil in series as you can see in the image, and we must place it on top of the central part (more or less) of the “Tx” coil .
To adjust the balance we must move slowly “Cx” coil looking at the “amp” value of the application. We must move in one of the directions as you can see in the image. We move in the direction where the “amp” value goes down. When “Cx” coil is at the point of balance we must fix it with glue so that it remains fixed. It is necessary to fix the coil well.
How to adjust resonance frequency:
To get the maximum performance from the coil you must adjust the resonance frequency. Resonance frequency makes a similar effect as swings, allows us to get maximum performance from the coil without spending more energy from the battery. To adjust the resonance frequency we have several options. First we have to decide what frequency of work we want for our “Double-D” coil. I use the 5.5 kHz frequency. The resonance frequency of the “Tx” coil must match the generator frequency! The option number one is to calculate the capacitor value that we will need to obtain the necessary frequency, and for this we must know at least the inductance of the coil. We must measure the inductance of the coil “Tx” with an inductance meter (if you do not have an inductance meter you can see how to make one on my website in the section “Arduino Multimeter“). Here I leave a link that will help you calculate the resonance frequency if we know the inductance: Inductance calculator. Another way is to use one of my applications called “RFinder”. It is an application that scans the coil to find the resonance frequency. We just have to make a small circuit that we will connect to the smartphone and the coil. The circuit is very simple, it has only a couple of capacitors and a resistor. You can see how to use this app on this “RFinder” page. In the same way we can adjust “Rx” coil but “Rx” coil must have the resonance frequency 100 hz less than “Tx” coil.
Step 3 - How to use the app "Smart Hunter":
To turn on the detector we must turn on generator (if we use arduino) and press this button “Play”).
Now the app is analyzing the signal it receives from the coil.
When the main button is in this position the detector is in the “Dynamic mode”, which means that we have to move the detector so that it detects the objects or moves the objects.
In “Dynamic mode” the detector is self-adjusting to the ground.
This way that you can see in the image is the “Static Mode” in this mode it is not necessary to move the objects so that the detector detects them, in this mode it works as “Pinpointer” and it no longer adjusts automatically but uses the last adjustment of “Dynamic mode”. So if the detector in this mode is mismatched we must again press “Dynamic mode” (near the ground) to be self-adjusted and then return to “Static mode”.
This button serves to increase the sensibility.
This button serves to lower the sensibility.
These are some basic knowledge we need to be able to correctly use the “Smart Hunter” metal detector.
After following the steps indicated above we have the detector done, we just have to make the case where we put all this, but we will talk about that in one of the next post. To power the detector we use 5 volts stable for this we can use a step-up inverter that will provide us with a very stable current. About supply and more tricks I will write more posts, SUBSCRIBE TO MY BLOG if you are not yet subscribed to receive your email notices when I am going to publish a new posts! If you have any questions write in the comments or to my email firstname.lastname@example.org