Miniature metal detectors presented in this review cannot boast of high sensitivity, but their assembly will be an exciting experience for any novice radio amateur. These devices not only allow you to gain a deeper understanding of how metal detectors work, but also provide the opportunity to experiment with settings. How a metal detector works A metal detector is a device designed to search for metal objects in soil and other environments. Its principle of operation is based on the creation of an electromagnetic field and the analysis of the response of metal objects that fall into the range of the device.
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Self-assembly of a metal detector can be difficult and costly, but compact, inexpensive kits that include all the necessary components make this process easier. This review considers three popular models that are most often found in online stores. All of them come in kits containing a PCB with clear markings for soldering.
Metal detector assembly process
Installation of all elements takes no more than 30 minutes for each device. In this case, all 3 devices worked from the first turn on. If you carefully study their diagrams, it becomes obvious that all metal detectors have identical components:
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three transistors,
four capacitors,
two resistors,
a buzzer and other components.
The only difference between one of the models is the presence of an LED with a current-limiting transistor, connected in parallel to the buzzer. However, this does not affect the sensitivity of the device in any way.
Design differences and testing
Although metal detectors have the same circuitry, their components are arranged differently, and search coils are different in shape. The question is, does this affect their sensitivity? To find out, tests were carried out under the same conditions, using a 3V supply.
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When switching on, it is necessary to adjust the tuning potentiometer so that the buzzer is on the verge of tripping. The closer the activation point, the higher the sensitivity of the device, but at the same time its stability decreases. Testing was carried out on two objects - a coin and a more massive metal object.
If you look at the board, you can see that the coil is connected to a turn that generates a phase-shift output signal (feedback). When a metal object hits the center of the coil, some of the magnetic flux is absorbed and lost. This change causes the circuit to fail, after which a voltage appears on the T3 transistor collector, activating the buzzer.
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One of the reasons for the low sensitivity of such metal detectors is the design of the printed coil. It distributes the magnetic field over a large area, which is why the flow does not focus around the detected object. However, the sensitivity can be significantly increased by replacing the built-in coil with an external one with a diameter of 10 cm and consisting of 70-80 turns of wire with a branch on the second turn. I will devote a separate material to this issue.
Circuit Test Results
The results showed that the first two metal detectors have almost the same sensitivity (7 cm per coin), which is highly dependent on the fine tuning of the tuner. The third device showed worse performance (4 cm), probably due to the elongated shape of the search coil, since the circuit and component ratings of all three devices are identical.
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Despite the low sensitivity, these compact metal detectors are an interesting project for assembling and studying the principles of operation of such devices. They give you the opportunity not only to get acquainted with the basics of electronics, but also to experiment with settings, improving the characteristics of the device. In addition, their main advantages are their availability on sale, ease of assembly and incredibly low cost. In general, for beginners - it's the best!
