Turn your iPhone into a metal detector

  

The used app is designed to connect simple metal detector circuits to the iPhone via the headphone jack. This homemade metal detector circuits are very simple and can be made by anyone who has never held a soldering iron in their hands. Metal detectors can detect and discriminate between ferrous and non-ferrous (particularly gold and silver) metals ( discriminating against metals ). Positive number means ferrous metal, negative number means non-ferrous metal. The numbers shown on the screen are NOT the "Target ID" or VDI of a branded metal detector. The first scheme (WITHOUT MICROCIRCUITS) has the name "Very simple" metal detector, and the second "Sensitive" metal detector .

 The main element of the circuits is an oscillatory circuit ( Oscillatory circuit ). The stability of its parameters is very important. They constantly fluctuate with the slightest change in temperature, in addition, the inductance changes with the slightest change in the geometry of the coil. These changes are constantly updated by the application. The parameters of the internal circuits of the iPhone and the jack adapter may also fluctuate, but they will stabilize approximately 2-3 minutes after the signal is turned on (therefore, the metal detector must work continuously during the search).

The frequency range is 3-14 kHz. That is, you can select the frequency by choosing the value  of the capacitor in the oscillator circuit. After each change in capacitance or inductance, or at the first start-up, or when the air temperature changes (for example, when you went out into the open air), you need to turn on the search for the finding the new resonant frequency.

The schematic diagram

This detector is very simple  metal detector works without microcircuits

MAYBE THIS IS THE SIMPLE SCHEME OF A REALLY WORKING METAL DETECTOR, BECAUSE IT CONSISTS ONLY OF A VIBRATORY CIRCUIT AND TWO RESISTORS, IF YOU DO NOT COUNT THE IPHONE :). For the simplicity of the circuit, you have to sacrifice sensitivity. However, the sensitivity is sufficient to find a coin buried in the ground. To detect metal, the metal detector must be constantly moved. Metal (ferrous or non-ferrous) is detected by moving the coil away from the metal.

THE PRINCIPLE OF OPERATION is extremely simple. The oscillating circuit is excited through the audio output with a signal having a frequency equal to the resonance frequency of the oscillating circuit. In other words, the signal from the audio output (with a frequency equal to the resonance frequency of the oscillating circuit) passes through the oscillating circuit and generates resonant oscillations. Metals appearing near the search coil change the signal in the oscillatory circuit and these changes are analyzed through the microphone input of the headphones.

Sensitivity in air:
• Coin: 10 cm
• Small scoop: 16 cm

To change the frequency of the metal detector, you need to change the capacitance in the oscillatory circuit. To select the required capacitance, you need to use a special calculator Calculation of the resonance frequency  (Example : 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 resistance value of the resistor R1 depends on the characteristics of the oscillating circuit. The diagram shows the resistance value for an oscillatory circuit having an inductance of about 3.7 mH and a capacitance of about 0.15 µF (The resonant frequency is 6.76 kHz). When increasing the capacitance, for example, to reduce the resonant frequency, the resistance of the resistor R1 must be reduced (for 0.5µF R1 should be about 6.8K Ohm), and when the capacitance is reduced, the resistance of the resistor R1 must be increased (for 0.05 µF R1 it should be about 30K Ohm). For a test coil with a 0.5μF capacitor, R1 should be about 2.2K-3K Ohm (if C = 1.0μF then R1= 1K-1.6K Ohm). When choosing the value of the resistor R1, you need to be guided by the "Max Value" value, which should be in the range of 0.45-0.85.

The metal detector assembling

Only one earphone is connected to the audio output, which carries the sound alert signal.

Such a simple circuit can be easily assembled without a soldering iron. In this case, manufacturing a metal detector will require more time and labor, but you will not have to buy a soldering iron, solder, flux, cellulose sponge to clean the soldering iron tip (if there are none) and you do not need to learn the basics of soldering.

To avoid soldering, wire lugs (with an inner diameter of 3-4 mm, for example 1.25-4S), screws and a wooden plank were used. Thin metal ring lugs and an audio output cable with terminals can be used to make the job easier, but a more complex option will be shown here. In addition, in this version, a simplified version of the coil (WITHOUT SHIELDING) is used from a conventional wire for electrical wiring. You can also use enameled (winding) wire (with a diameter of 0.5 mm or more), but remember to remove the transparent insulation before putting on the terminals.

A DRY pine plank was used. The photo shows that the resistance between the screwed in screws is more than 20M Ohm. Do not over tighten the screws. This will reduce the amount of sawdust generated, which can then be trapped between the terminals.

Now you can connect the circuit to your smartphone for verification. But no headphones yet.

Application Download:

https://apps.apple.com/app/id1404284012

Testing the Metal Detector