Today we will analyze the operation of the simplest metal detector, in which the signal is processed by a smartphone. Therefore, it can be safely called a smartphone metal detector.
Parts for assembling the project
Ordinary PNP transistor – 2 pieces
Tuning potentiometer, 50 kΩ
Trimming potentiometer, 500 Ω
Capacitor 680 nF
Capacitor 220 nF
3.7V battery
Search Coil
Smartphone with app
The basic principle of these metal detectors is the LC generator, which serves as a reference, and on the other hand, the smartphone app, which is a frequency counter. When we place a metal object near the coil, the frequency changes, and the application detects the change, signaling the presence of a metal object near the search circuit.
The oscillator consists of a parallel circuit of an LC oscillator and two transistors. The frequency of this generator depends on the inductance of the coil and the capacitance of the capacitor. There are also two tuning potentiometers instead of resistors so that you can change the amplitude and partially the waveform to simplify its processing in a smartphone. It has been experimentally established that the detector has the best sensitivity when the signal has an amplitude of about 100 mV.
In terms of signal processing, this experiment uses the Neco Desarrollo team's "Smart Hunter" and "MFM3 Pinpointer" apps, which are the only Android apps of their kind. You can download free versions here: neco-desarrollo.es
This coil consists of two windings connected in series, the larger one with a diameter of 23 cm, and the smaller one with a diameter of 12 cm. Both are wound with a varnished wire with a diameter of 0.4 mm and contain 25 turns each. You can also make your own coil with an inductance of about 500 μH, and the number of turns and diameter can be easily calculated using any of the online calculators. The only important thing is that the resonant frequency of the parallel LC circuit is in the range of 4 kHz to 20 kHz so that the smartphone can process it.
By clicking the "Balance" button, the app will measure the frequency and remember it as a reference. Using the "+" and "-" buttons, you can change the sensitivity of the methyl detector. Now, if you bring a metal object close to the coil, this frequency changes, which is displayed visually in the application and can be heard on the speaker. To have an audible indication, a power amplifier is connected to the headphone jack output. The change in signal frequency is also clearly visible on the oscilloscope. The sensitivity of the detector is relatively good, of course, given the ultra-simple design.
Another interesting application is "MFM3 Pinpointer". It's also a metal detector app that can differentiate between ferrous and non-ferrous metals (kind of discrimination). When it starts, we can see the frequency of the generator circuit. Then click the "Screen" button, and now we can start scanning various materials. If we approach non-ferrous metal, green will appear, and if we approach iron, red will appear. In the first case, the frequency increases, and with glandular material, on the contrary, it decreases. Sensitivity can also be changed in the program settings.
In general, this is a very simple version of a metal detector, so it does not have any great sensitivity, but it serves well for developmental purposes, and the phone is a worthy use, except for driving Minecraft on it. However, the generator is not very stable, the frequency depends on the temperature, so you need to calibrate it quite often. In any case, try to assemble this scheme - you don't know about the treasure, but you will definitely find experience in the process of working with it!
