It's been a long time since we had materials about metal detectors, especially of the simplest type, without the use of expensive microcircuits and controllers. The Lords of Anchors and Chances will wince contemptuously when they look at the proposed MD, but we must also respect novice radio amateur treasure hunters, so such devices also deserve a place on the site. So, the contactless metal detector module is sold in online stores at a ridiculous price of $ 5 and let's see how it works, and what you need to assemble it yourself.
An interesting part of the design is the single rectangular coil formed by etching a single continuous track on the PCB. If you look closely at the board, you will see that the rectangular coil is made with a tap. The coil starts from the top side of the double-sided PCB and forms square loops towards the center, the via in the center of the PCB carries it to the other side of the board and the same view on the bottom side.
Next, there are several publicly available discrete components, including three bipolar transistors, several resistors, and capacitors. The audible alarm in the module is a standard active piezo buzzer, and there is also a small tuning control for fine tuning the metal detector. Here is a schematic diagram of the metal detector module. The cheap module from China did not have an operating manual and a circuit diagram, so I had to search for a long time on the Web to get its basic diagram.
The circuit sensor is a 50-turn "printed" coil (25 at the top + 25 at the bottom) with a tap on the first turn. The single-turn (1) feedback coil (L1) is routed to the base Q1 via C2, and the base Q1 is shifted to R1, while the end of the coil (50) is connected directly to the collector Q1.
The tuned circuit (the coil plus the capacitor on it) generates a sinusoidal signal.
Please note that the signal coming from L1 is not the same in phase as the L2 signal. Here, L1 supplies a signal that increases the noise produced by Q1 to create a generator with a certain amplitude. Amplitude can be controlled using the VR1 tuning control. This signal is transmitted to Q2, where it gains amplification and prevents C4 from charging through R2, thus disabling Q3 and the piezoelectric element SP1 does not make noise at rest.
On the other hand, if the metal object is in close proximity to the sensor coil, the amplitude of the magnetic waves from L2 decreases slightly along with the amplitude of the generator. This disables Q2, but creates a slight voltage on the base and emitter of Q3 to make it work slightly. The piezo emitter emits a noticeable beep signal because there is a small constant voltage on its pins.
The second version of the MD with a planar coil
Now the diagram of another version of the metal detector module, which has a round coil engraved on the PCB. In this almost identical MD, you can also see an LED metal signaling device.
The main advantage of the scheme of this type of metal detector is the simple design of both the device itself and its search sensor (coil), but the disadvantage will be low sensitivity - a short detection range compared to serious MDs.
The oscillator frequency of the circuit (at rest) is about 300 kHz. If you connect the oscilloscope probe to the Q1 collector pin, you can see the sine wave generated by the resonant circuit.
Note: There are several proven methods used to detect metals. The Run-Out Frequency Generator (BFO) is one of the simplest and most common methods for detecting metals. The Induction Balance (IB) method is another good old approach that is still in use today. Further, the detection of metal by pulse induction (PI) works on a completely different, more complex principle, although it is based on the same principle of changing the inductance of the search coil when the metal is near it. Two other popular methods are resonance (OR) and transmit/receive (T/R).
Let's now perform the testing of the Chinese metal detector module. It is designed to operate on any 3-5V DC low-voltage power supply, such as 3V battery operation (1.5V x 2 AA). According to the included manual, the current drawn by the module in standby mode is less than 3 mA (at 3 V), which reaches approximately 30 mA in the active state. The estimated detection distance is 60 mm.
Metal detection range
The experiment powered by a 3V battery showed a weak detection range, just over 1 cm when tested with a 20 mm coin.
It has also been noticed that the module often fails, especially with even small fluctuations in the input voltage level. In addition, the trimming resistor needs to be reconfigured frequently after each power on and off, which is quite inconvenient.
In general, you can try this module to create the simplest device for searching for metals, but you should not put it on a par even with a simple Pirate.
