A simple DIY metal detector circuit
You’ve decided to delve into the creation of a metal detector and try making your experimental device to understand its working principle and test your skills, but you’re not sure where to start? Let’s figure it out together, as recreating simple electronic circuits, including making metal detectors by hand, is not as difficult as it may seem.
A metal detector allows you to find objects with specific electrical and electromagnetic properties that distinguish them from the surrounding environment. Such objects include various metals in the ground. Metal detectors are used by security services, the military, geologists, builders, and professionals in various other fields to locate metal in the soil and beyond.
Currently, metal detectors are often used by ordinary people to search for scrap metal from black metallurgy and to find non-ferrous metals, which make up various coins, jewelry, and military artifacts such as casings, tokens, and much more. Additionally, a metal detector can assist in locating items lost due to carelessness, which may be vital or have material or moral value (parts of mechanisms, keys, rings, jewelry, etc.).
The sensitivity of this device depends on the diameter of the search coil. The larger the diameter (over 30 cm), the less sensitive it is to small items but becomes more effective for larger items at greater depths. Conversely, the smaller the diameter, the better the response to smaller objects. Detailed instructions on sensitivity and the use of various coils are provided in industrial models.
In a homemade device, the dependence of its sensitivity on the coil diameter is also maintained. However, it is important to remember that a DIY device, in terms of its qualities, will be far from professional equipment due to various nuances of the physical processes involved. In a nutshell, the principle of operation of a metal detector can be described as follows: the coil sends out electromagnetic waves and simultaneously reacts to the induction currents induced in the metal, which generate their own magnetic field under the influence of the coil winding.
In reality, the physical processes are much more complex, and using professional equipment for treasure hunting (with selectivity and separation) is not as straightforward as it may seem. The abundance of debris in the soil can dampen the enthusiasm for digging when a quality device beeps every time. Therefore, the device described below cannot guarantee the discovery of unimaginable treasures but will help you understand, through your own experience, the basic principle of interaction between metal and the search coil of a metal detector.
Creating a device of this type does not pose significant difficulties; you just need to be able to understand circuits and use a soldering iron. In this case, a simple circuit using mainly the 555 integrated circuit (universal timer) will be described.
To familiarize yourself with the operation of such a timer, it is recommended to read the corresponding datasheet for this microchip.
Materials and Tools Needed:
- 555 integrated circuit (universal timer)
- 2 capacitors, each 2.2 µF
- Resistor: 47 kΩ
- Batteries: 9 V
- Wires for connections
- Cable for the battery
- Switch
- Circuit board
- Copper wire: 100 m (diameter = 0.2 mm)
- Buzzer for providing sound signals (it can be replaced with an 8 Ω resistance speaker and a 10 µF capacitor)
- Wood glue and tape
- Cardboard
- Pliers for making holes
- Tweezers
- Solder wire and soldering iron
- Ruler, sharp knife, compass, pencil
- Hot glue used for bonding
- Wires and prototype board
Metal Detector Circuit
Various DIY metal detectors might catch your attention. They are freely available; you just need to enter “metal detector schematics” in the search bar. The one presented here has been sourced in a similar manner from the vast spaces of the internet, but a buzzer has been added instead of a speaker and a switch for the battery.
Manufacturing the Coil
For ease of calculation, you can use an online calculator for an air coil. Using it, you can determine that for a coil with a diameter of 9 cm, you would need approximately 250 turns of enameled copper wire with a diameter of 0.2 mm. The inductance of this coil would be approximately 10 mH.
The smaller the diameter (d) of the coil, the more turns will be required. For a coil with a diameter of 7 cm, you would need approximately 290 turns. 
Buying a ready-made one won’t be difficult. However, if you’ve decided to assemble it yourself, before soldering the two ends of the wire, it’s necessary to clean them using sandpaper. You should strive to make all connections very carefully to eliminate the influence of poor assembly on the DIY device, which will already be sensitive to various interferences.
Testing the Coil and Creating the Printed Circuit Board
To avoid damaging the working board, it’s better to test this circuit on a prototype board.The next step is to create a schematic board. If you are not familiar with the process, it’s better to find instructions on the Internet, including video tutorials.
Now it’s time to connect all the components and test the device you’ve created. For beginners, it may be a bit challenging to understand this guide, but if you’ve tackled the creation of a metal detector before, you should catch on quickly. 
A metal detector assembled in this way is quite powerful for finding small objects at a short distance, preferably indoors to avoid malfunctions. However, it should be remembered that this DIY project will significantly lag behind in terms of sensitivity, detection quality, separation, and selectivity. It may also lack robustness against industrial equipment interference. Replicating it is more for the sake of sports interest and familiarizing oneself with the principle of metal detector operation.
