A simple metal detector with a piezo filter. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

A simple metal detector with a piezo filter. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / metal detectors

Comments on the article

Purpose and opportunities

This metal detector, despite the small number of parts and ease of manufacture, has a fairly high sensitivity. It can detect large metal objects, such as a radiator, at a distance of up to 60 cm, while small ones, for example, a coin with a diameter of 25 mm, can be detected at a distance of 15 cm.

Operating principle

The principle of operation of the device is based on the change in frequency in the measuring generator under the influence of nearby metals and the separation of the difference frequency (beats) between the measuring and exemplary generator.

Since this frequency is in the audio range, it can be heard through headphones.

Schematic diagram

Schematic diagram of the metal detector is shown in fig. 2.46. In this circuit, the frequency of the reference oscillator, made on DD1.1, is stabilized using a piezoelectric element. A piezo filter (ZQ1) for an intermediate frequency (465 kHz), which is available in any household superheterodyne radio receiver, was used as a piezoelectric element.

Such elements are widespread and much cheaper than quartz resonators. The use of a piezoelectric element makes it possible to increase the frequency stability of the reference oscillator compared to conventional LC or RC oscillators, and, therefore, increase the detection range of metal objects.

Rice. 2.46. Schematic diagram of a simple metal detector with a piezo filter

The measuring generator is assembled on a logic element DD1.2 and contains a coil (L1) in the form of a frame, which is a sensor. When the coil approaches the metal, its inductance changes, which leads to a change in the frequency of the oscillator. The initial frequency of the oscillator is determined by the elements C1C2C3L1 and is adjusted using an adjustable capacitor C1, close to the frequency of the reference oscillator (slightly more or less than 465 kHz).

On the element DD1.3, the signals of the two generators are mixed. The output signal DD1.3 contains a difference harmonic, and in order to separate it from high-frequency pulses, a filter R3C5 is installed. The low-frequency signal is amplified by the field-effect transistor VT2 and fed to the sound emitter - headphones BF1 BF2.

The use of CMOS microcircuits in self-oscillators, due to their large input resistance, makes it possible to obtain a high quality factor in the oscillatory circuit of the search oscillator, which increases its frequency stability. This makes it possible to work with small beats and thus increase the sensitivity of the metal detector. The power supply of the autogenerators is stabilized using a precision KS166V zener diode. Only parametric stabilizers for a voltage of about 6 V have a voltage drift close to zero with a change in ambient temperature.

The metal detector circuit remains operational when the voltage drops to 5 V, but in this case there will be no stabilization of the supply voltage. The current consumed by the metal detector (and, therefore, the duration of operation) is highly dependent on the resistance of the headphones connected at the output. For this reason, their resistance should be as high as possible (> 100 ohms), for which the phones in the headphones are connected in series.

Resistor R7 limits the maximum current of the transistor VT2 in case of a short circuit in the headphones, and resistor R6 allows you to adjust the sound volume. For convenience, this resistor is combined with the SA1 power switch.

Headphones are connected through any standard socket.

Socket X2 is designed to connect a mains charger for the G1 battery. This will allow you to recharge the batteries without removing them from the case.

Element base

Tuning capacitor C1 can be taken from any miniature radio receiver (for example, KP-180). C2 and C3 must be with a minimum negative TKE (M47, M75), C4 and C5 from the K10 series (K10-17), C6 - K53-1 at 16 V.

The variable resistor R6 is SP3-3bM (it provides for horizontal installation on the board and has a built-in SA1 switch), a tuned resistor R5 of the SPZ-19a type, the rest will fit any small ones. A piezo resonator (piezo filter Z01) is likely to fit any of the FP1P1-61 series (-01, -02, etc.), and you can also try many other types of piezo filters from Chinese receivers that have three outputs.

Printed circuit board

The details of the device can be located on a single-sided printed circuit board made of fiberglass with a thickness of 1,5 mm and dimensions of 75 x 40 mm (Fig. 2.47, a-b).

It is desirable to place the board near the sensor coil L1. The place where the board with the elements is fixed does not need to be shielded.

Coil making

The metal detector sensor coil L1 has the form of a toroidal frame, (Fig. 2.47, c). It is wound with a copper wire PEV with a diameter of 1,2 mm, on any suitable mandrel with a diameter of 20 cm, for example, cut out of foam.

Winding should be carried out in bulk, 30 turns (the inductance is about 480 μH). After winding the coil, the frame must be wrapped with any dielectric tape (varnished cloth or electrical tape), and then with thin aluminum foil.

You can also use copper foil. At the place of the coil leads, a section of about 10 mm should not be covered with foil (a gap is left between the ends of the screen, as shown in Fig. 2.47, c).

Rice. 2.47. A simple metal detector with a piezo filter: a - printed circuit board; b - arrangement of elements on the board; c - appearance of the search coil

The screen at the coil reduces the influence of parasitic capacitances, which increases the stability of the operating frequency of the search oscillator.

The use of thick wire in the manufacture of L1 provides a higher quality factor for the coil and stiffens the frame without the use of additional fasteners.

Author: Shelestov I.

See other articles Section metal detectors.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Artificial leather for touch emulation 15.04.2024

In a modern technology world where distance is becoming increasingly commonplace, maintaining connection and a sense of closeness is important. Recent developments in artificial skin by German scientists from Saarland University represent a new era in virtual interactions. German researchers from Saarland University have developed ultra-thin films that can transmit the sensation of touch over a distance. This cutting-edge technology provides new opportunities for virtual communication, especially for those who find themselves far from their loved ones. The ultra-thin films developed by the researchers, just 50 micrometers thick, can be integrated into textiles and worn like a second skin. These films act as sensors that recognize tactile signals from mom or dad, and as actuators that transmit these movements to the baby. Parents' touch to the fabric activates sensors that react to pressure and deform the ultra-thin film. This ... >>

Petgugu Global cat litter 15.04.2024

Taking care of pets can often be a challenge, especially when it comes to keeping your home clean. A new interesting solution from the Petgugu Global startup has been presented, which will make life easier for cat owners and help them keep their home perfectly clean and tidy. Startup Petgugu Global has unveiled a unique cat toilet that can automatically flush feces, keeping your home clean and fresh. This innovative device is equipped with various smart sensors that monitor your pet's toilet activity and activate to automatically clean after use. The device connects to the sewer system and ensures efficient waste removal without the need for intervention from the owner. Additionally, the toilet has a large flushable storage capacity, making it ideal for multi-cat households. The Petgugu cat litter bowl is designed for use with water-soluble litters and offers a range of additional ... >>

The attractiveness of caring men 14.04.2024

The stereotype that women prefer "bad boys" has long been widespread. However, recent research conducted by British scientists from Monash University offers a new perspective on this issue. They looked at how women responded to men's emotional responsibility and willingness to help others. The study's findings could change our understanding of what makes men attractive to women. A study conducted by scientists from Monash University leads to new findings about men's attractiveness to women. In the experiment, women were shown photographs of men with brief stories about their behavior in various situations, including their reaction to an encounter with a homeless person. Some of the men ignored the homeless man, while others helped him, such as buying him food. A study found that men who showed empathy and kindness were more attractive to women compared to men who showed empathy and kindness. ... >>

Random news from the Archive

Desktop mobile phone 08.07.2005

The Czech electronic company "Yablotron" has launched the production of a desktop cell phone GDP-02.

It is intended for older people who want to use all the services provided by cellular communication, but do not need the portability of a telephone set. In addition, they hardly get their fingers on the tiny keys of an ordinary cell phone and have difficulty seeing the numbers and letters that appear on a miniature screen.

The GDP-02 has a large swivel screen, conventional number keys and a full set of alphabetic keys arranged in the same order as on a typewriter or computer keyboard. This makes it easy to send CMC messages. The phone has a memory for 200 numbers, it is powered by the mains, but it also has a backup battery.

The device can also be used where the telephone cable cannot reach, for example, in the country. In addition, it allows cellular operators to compete with wired communications: in Western Europe, many subscribers give up regular telephone numbers, but at home they would like to continue using the convenient desktop device.

Something similar, but with other purposes, began to be produced by the French company Epsi-Fon. This is a cellular set-top box with a jack for connecting a regular phone and with a directional antenna. The prefix is intended for residents of rural areas in developing countries where there is no conventional telephone network.

The nearest cellular telephony base station can be 32 kilometers away, the main thing is that there is a direct line of sight (this is possible in the mountains or when installing an antenna on a tower - one for the whole village). The set-top box is powered by a lead battery, which can be recharged from the mains, from solar panels or from a bicycle generator.

Other interesting news:

▪ New Kingmax memory cards record 4K2K video

▪ The first Mongolian Mozo car

▪ New battery cell is 7 times denser than lithium-ion cells

▪ Diesel from a palm tree

▪ Camera with post focus

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ site section Spectacular tricks and their clues. Article selection

▪ article by George Orwell. Famous aphorisms

▪ article Who was the youngest and who was the oldest Olympian? Detailed answer

▪ article The use of leeches. Health care

▪ article Garland switch with soft start. Encyclopedia of radio electronics and electrical engineering

▪ article Electrolysis in a glass. Chemical experience

Leave your comment on this article:

All languages of this page

Home page | Library | Articles | Website map | Site Reviews