Homemade Metal Detector
The application is designed to connect the simple circuits of a metal detector to an iPhone through the phone’s headphone jack. The schematics of homemade metal detectors are extremely simple and can be made by anyone, even by someone who's never held a soldering iron. Metal detectors can detect ferrous and non-ferrous metals (including gold and silver) and are able to distinguish between them (with discrimination). A positive number means it's a ferrous metal, while a negative number means non-ferrous. The numbers shown on the screen are NOT "Target ID" or VDI, like they are on brand metal detectors. The first scheme is named "Very simple" metal detector (MD1), and the second is "Sensitive" metal detector (MD2).
The main part of the device is the LC circuit. The stability of its parameters is very important. They constantly fluctuate with the slightest changes in temperature. Moreover, the inductance changes with the slightest changes in the geometry of the inductance coil. These changes are constantly being adjusted by the application. The parameters of the internal iPhone circuits and the headphone jack adapter can also fluctuate, but they stabilize after approximately a minute after turning on the signal.
Frequency range is 3-14 kHz. It is possible to choose the frequency by choosing the capacity of the condenser in the LC circuit. After a change in capacity or inductance, on the first start, or when a change in air temperature occurs, you need to enable the search for the resonance frequency.
Searching using a metal detector isn't as easy as it might seem and might sometimes be deadly. Therefore, it is necessary to study this subject on specialized sites. Be sure to do some experiments with a coin placed on the ground.
FREE VERSIONS OF METAL DETECTORS ARE LIMITED BY THRESHOLD OF THE SOUND SIGNAL AND THE INDICATOR LIGHT. WHICH MEANS THAT IT'S POSSIBLE TO DETECT THE METAL BY THE NUMBERS ON THE IPHONE'S SCREEN, BUT IT CANNOT BE HEARD OR SEEN IN PERIPHERAL VISION IF NOT LOOKING DIRECTLY AT THE SCREEN.
If anything is unclear, be sure to drop me an email.
"Very simple" metal detector
This is perhaps the simplest scheme of an actual, working metal detector, for it only consists of an LC circuit and two resistors if you don't count in the iPhone :). Sensitivity is sacrificed for the sake of the scheme's simplicity. However, the sensitivity is enough to find a coin buried in the ground. To detect metal, the metal detector must be moved constantly. Metal, ferrous or non-ferrous, is determined when the coil moves from the metal.
THE PRINCIPLE OF OPERATION is very simple. The LC circuit is excited through the audio output with a signal that has a frequency equal to the resonance frequency of the LC circuit. In other words, the signal from the audio output (with a frequency equal to the resonance frequency of the LC circuit) passes through the LC circuit and generates resonant oscillations in the LC circuit. Metals which appear near the inductance coil change the signal in the LC circuit and these changes are analyzed using the microphone input.
Sensitivity in the air (see video):
To change the frequency of the metal detector, change the capacity in the LC circuit. To choose the required capacity, use a special calculator LC resonance calculator (L = 3.7 mH, for the test coil L = 0.5 mH). For example, if C = 0.15 µF and L = 3.7 mH then F = 6.76 kHz.
The value of the resistor R1 depends on the characteristics of the LC circuit. The diagram shows the value of the resistance for the LC circuit with an inductance of about 3.7 mH and the capacity of about 0.15 µF (6.76 kHz resonance frequency). When increasing capacity, for example, to lessen the resonance frequency, the resistance of the resistor R1 should be reduced (for 0.5 µF R1 should be around 6.8K Ohm). When reducing the capacity, the resistance of the resistor R1 should be increased (for 0.05 µF R1 should be around 30K Ohm). For the test coil with a 0.5 µF capacitor R1 should be about 2.2K-3K Ohm (if C = 1.0 µF then R1 = 1K-1.6K Ohm). When choosing the value of the capacitor R1 you need to rely on the value of "Max Value", which should be in the range of 0.45-0.85.
Only one headphone connects to the audio output through which the alert sound goes through.
Unfortunately, the circuits cannot be assembled on a prototyping board without soldering since the slightest shift of the parts will cause very strong noise and can even cause a loss of the resonance frequency.
"Sensitive" metal detector
The sensitivity of this scheme is comparable to the sensitivity of brand metal detectors.
THE PRINCIPLE OF OPERATION isn't much harder than that of the previous metal detector. The first operational amplifier (Op-amp1) is used as a current source (Current source). It excites the LC circuit. The signal frequency is equal to the LC circuit's resonance frequency. The frequency of the signal can be controlled through the audio output №1. The second operational amplifier (Op-amp2) subtracts from the signal from the audio output №2 (VChannel2) the signal from the output of the first operational amplifier (VOut1) and multiplies the difference by about 10 (VOut2 = VChannel2 × (1 + R4/R3) − VOut1 × R4/R3). Signal VChannel2 is made as similar as possible to the signal VOut1. With the slightest change in the signal VOut1 under the influence of metal appearing near the inductance coil, the changes are amplified and analyzed using the microphone input.
Sensitivity in the air (see video):
To change the frequency of the metal detector, change the capacity in the LC circuit. To choose the required capacity, use a special calculator LC resonance calculator (L = 3.7 mH, for the test coil L = 0.5 mH). For example, if C = 0.045 µF and L = 3.7 mH then F = 12.3 kHz.
DON'T FORGET TO CHECK WHETHER THE POWER IS ON. The power needs to be turned on after connecting the circuit to the iPhone and needs to be turned off before disconnecting it.
The resistance value of the resistor R1 should be about 1.5-3 times less than the CALCULATED resistance of the LC circuit (RLC) at the resonance frequency.
RLC = L / (C1 × r)
r is the resistance of the coil wire.
RLC = 3700 / (0.045 × 5) = 16444 Ohm
For the test coil:
RLC = 500 / (1.0 × 2.5) = 200 Ohm
When assessing the correctness of the resistance value of the resistor R1 you need to rely on the value of "Amplitude1", which shouldn't be more than the value of "Amplitude2".
You must not make the resistance R7 less because it can burn the operational amplifier.
You must not connect the operational amplifier MCP602 to supply exceeding ±3.0 V. If using a different operational amplifier, its supply shouldn't exceed ±6.0 V.
The sensitivity depends strongly on the diameter of the coil. For this scheme, the MINIMAL distance of detecting the coin in the air is equal to the diameter of the coil.
Sensor (inductance coil) and mounts
The sensor of the metal detector must be sufficiently rigid so that it won't change its geometry when moving. IF YOU PLAN TO SEARCH IN TALL GRASS, THE SENSOR MUST BE MADE MORE RIGID.
• The inner diameter of the frame: 6 1/2 inches (16.5 cm)
To calculate the inductor coil, use a special calculator Multilayer coil calculator.
The frame is glued together from cardboard with a thickness of 0.03 inches (30pt, 0.7 mm). If you want to make a more rigid frame or have cardboard that's not as thick, you would need to glue on more layers of cardboard on the frame. This way you can make a sensor of any shape.
The side of the carcass is made from one or two strips with slits. These strips are glued to a round base. Scratches are made on bending points with a knife.
The junction of the two strips is additionally reinforced.
The wire is wound tensely, but not too strong to avoid deforming the frame.
THE FOIL SCREEN IS REQUIRED.
The foil is wrapped in 50 inches (120 cm) of tinned wire. You can get such a wire from a SHIELDED twisted pair. You can use an ordinary copper wire for wrapping, but then over time the contact point of copper and aluminum will oxidize and lose conductivity.
The entire coil is wrapped in a film cut from an ordinary small bag.
It's necessary to minimize the amount of metal in the sensor's design. Therefore, the parts are joined together with duct tape. The number of duct tape layers shouldn't be less than 10.
The wires next to the sensor shouldn't hang around freely.
To increase sensitivity, you can increase the diameter of the coil. In this case, however, you must ensure that the coil's carcass is sufficiently rigid. The weight of the sensor will noticeably increase, as well.
Smartphone (or other large metallic objects) should be distanced no less than twice the coil's diameter away from the coil.
The length of the handle depends on your height.
Before making the coil, you can check the circuit using a coiled 20 ft (6 m) long cable. THE SENSITIVITY OF METAL DETECTORS WITH SUCH A SENSOR WILL BE MUCH WORSE, BUT IT CAN BE USED TO MAKE SURE THE CIRCUIT WORKS.
PLEASE NOTE THAT THE TEST COIL REQUIRES CAPACITORS WITH A MUCH LARGER CAPACITY. You can try to use any ceramic capacitors.
Capacitor for the LC circuit
It's very important that the capacitor changes its capacity as little as possible with fluctuations in temperature. The most stable ceramic capacitors are marked C0G or NP0. But such capacitors, with a capacity large enough, are very hard to buy. You can use ceramic capacitors N33, N75, polystyrene capacitors. Polypropylene film capacitors in extreme case.
It's best to buy several capacitors with a smaller capacity than one capacitor with a desired capacity. Several capacitors with a smaller capacity can be connected in parallel to obtain the desired capacity. This way, by adding or subtracting capacity, it would also be possible to choose the frequency of the metal detector.
Protection from the Sun and wind
The headphone jack adapter is the part most sensitive to solar radiation. Strong noise will appear when moving the adapter from the shadows into the sunlight, and vice versa. However, it's easy to protect the adapter from the Sun by wrapping it in several layers of 4-5 inches long (10-12 cm) cardboard glued together, and one layer of foil. From the side of the wire, you put crumpled foil. For the equator stronger protection may be needed. Such protection isn't necessary for an iPhone with an integrated headphone jack.
When unplugging the adapter from the iPhone the protection slips off it. Therefore, you can first slide the protection off and then pull the adapter out.
It's recommended to glue a casing from cardboard for the circuit and wrap it in foil. This will protect the circuit from sharp fluctuations in temperature.
HEADPHONES NEED TO BE HUNG AROUND THE NECK. An unpleasant noise can be heard when they are inserted into the ears. The "Sensitive" metal detector doesn't have as loud a sound as the "Very simple" one when metal is detected. This is due to the transfer of the signal for the headphones along with the signal to the operational amplifier.
To use headphones for the iPhone you need to buy a socket with four contacts.
Causes of non-operability
1. No power to the operational amplifier (for the "Sensitive" metal detector).
1: Start/Stop. Don't turn on the signal if the circuit isn't connected to the iPhone. If the signal is on, don't disconnect the circuit from the iPhone and don't exit the app. THE VOLUME OF THE SMARTPHONE (WHEN THE CIRCUIT IS CONNECTED) SHOULD BE MAXIMUM (1.00).
3: Search for the resonant frequency. Should be performed after any change in the LC circuit or air temperature. When searching for the frequency the metal detector should be motionless.
4: Fine frequency tuning, "Amplitude 1" and "Amplitude 2". Should start automatically after switching on (if a search for resonance frequency was performed) or after searching for resonance frequency. If the capacitor of the LC circuit is bad, this process (after turning on the signal) can take up to 10 minutes. Press it when you need to bring the figure close to zero. IF THE VALUE OF "Amplitude2" DECREASED BELOW 0.9 DUE TO PROXIMITY TO THE SENSOR OF A LARGE PIECE OF METAL, FINE-TUNING NEEDS TO BE ENABLED.
5: Disabling/Enabling of compensating changes in parameters of the LC circuit (e.g. temperature).
6: The threshold of the sound signal and the indicator light.
7: The speed of compensating changes. If the speed is too low, you will have to wait long for compensation. If the speed is too high, the sensitivity will be decreased.
8: Indicator light (Visualizer).
9: Sound alert on found metal.
10: The volume of the sound alert on found metal (only for the "Very simple" metal detector).
11: Manual search for parameters for extreme cases (only for the "Sensitive" metal detector).
eBay: "wire enameled 0.5mm"
eBay: "wire enameled 0.55mm"
eBay: "wire enameled 0.6mm"
eBay: "wire enameled 24AWG"
eBay: "wire enameled 23AWG"
eBay: "wire enameled 22AWG"
eBay: "ceramic capacitors C0G uF radial"
eBay: "polystyrene capacitors uF"
eBay: "K71-7 capacitors"
eBay: "CBB81 capacitors uF"
AliExpress: "CBB81 capacitors"
eBay: "3.5mm jack socket"
eBay: "resistor 1W kit"
eBay: "cable 3.5mm 4 pole male to male"
eBay: "battery box switch aa 2"
eBay: "prototyping board 5x7"
eBay: "8 pin dip socket round"
eBay: "MCP602 opamp"
eBay: "ceramic capacitor 1uF"
eBay: "cardboard 30pt"