Metal detectors on microcircuits with a comparison circuit. Scheme, description

Free technical library

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

Metal detectors on microcircuits with a comparison circuit. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / metal detectors

Comments on the article

Advantages and disadvantages

The principle of operation of all these devices is based on a comparison of the oscillation frequencies of two generators:

reference;
search, changing the frequency when exposed to its oscillatory circuit of a metal object.

There are other known methods:

bridge, when the unbalance of the measuring bridge is registered, in one of the arms of which the search coil is included;
phase shift method, when the phase shift of oscillations of the reference and search oscillators is measured;
the transmitter-receiver method, where radio frequency energy re-radiated by an object is recorded.

They are more efficient than the method of comparing frequency values (beat method). But it is easier to implement. Metal detectors built using it have the following advantages:

they are compact;
do not require careful tuning and measures for high frequency stabilization;
unpretentious in operation.

Therefore, they are widely used by home craftsmen and radio amateurs.

Schematic diagram of a simple metal detector

The device can detect a five-kopeck coin at a depth of up to 80 mm, and the lid of a sewer well at a depth of up to 0,8 m.

The schematic diagram of the simplest metal detector is shown in fig. 2.6, a. It is assembled on just one K176LP2 chip. One of its elements (DD1.1) is used in the exemplary generator, the other (DD1.3) - in the tunable one.

The oscillatory circuit of the reference oscillator consists of a coil L1 and capacitors C1 and C2, and the search circuit consists of a search coil L2 and a capacitor C4. The first circuit is tuned in frequency by a variable capacitor C1, and the second by a selection of capacitor C4. On the element DD1.3, a mixer of oscillations of the exemplary and variable frequencies is made.

From the load of this node - a variable resistor R5 - the difference frequency signal is fed to the input of the element DD1.4, and the audio frequency voltage amplified by it is sent to the head phones BF1.

Schematic diagram of a high sensitivity metal detector

Consider a metal detector of increased sensitivity, the circuit of which is shown in fig. 2.7, a. In it, the K118UN1D (DA1) microcircuit is used as a mixer and amplifier of the difference frequency oscillations.

Metal detectors on microcircuits with a comparison circuit

Rice. 2.6. The simplest metal detector on microcircuits with a comparison circuit: a - a circuit diagram; b - printed circuit board

Metal detectors on microcircuits with a comparison circuit

Rice. 2.7. High sensitivity metal detector on microcircuits with a comparison circuit: a - circuit diagram; b - printed circuit board

The reference and search oscillators of this device are identical in scheme. Each of them is made on two inverters (DD1.1, DD1.2 and DD2.1, DD2.2, respectively). Elements DD1.3 and DD2.3 work as a buffer, weakening the influence of the mixer on the generators.

The reference oscillator must be tuned to a given frequency with a variable capacitor C1, and the search oscillator - with a selection of capacitor C2.

Upgraded beating metal detector circuit

You can increase the sensitivity of a metal detector that uses the beat method by setting the reference oscillator to a frequency 5-10 times higher than the frequency of the search oscillator.

In this case, there are beats between the oscillations of the reference oscillator and the nearest frequency (5-10th) harmonic of the search generator. In this case, the detuning by only 10 Hz leads to an increase in the frequency of difference oscillations by 100 Hz.

It is in this way that the increased sensitivity of the metal detector is achieved, the scheme of which is shown in Fig. 2.8, a.

With the help of such a metal detector, a five-kopeck coin can be detected at a depth of up to 100 mm, and a well cover at a depth of up to 1 m.

Metal detectors on microcircuits with a comparison circuit

Rice. 2.8. The modernized scheme of the metal detector on beats: a - a circuit diagram; b - printed circuit board

The operation of the modernized metal detector circuit

The reference oscillator of the metal detector is made on two elements of the DD2 chip and is tuned to a frequency of 1 MHz. The required frequency stability is provided by the ZQ1 quartz resonator.

The search generator uses two elements of the DD1 chip. Its oscillatory circuit L1C2C3VD1 is tuned to a frequency several times lower than the reference oscillator.

To rebuild the circuit, a VD1 varicap is used, the voltage on which is regulated by a variable resistor R2. The mixer is made on the element DD1.4, elements DD1.3 and DD2.3 are used as buffers.

The search indicator is BF1 headphones.

Mounting and PCB

Each of the considered metal detectors can be mounted on a printed circuit board made of foil fiberglass with a thickness of 1,5 mm. The drawing of the board and the location of the parts are shown in fig. 2.6b, 2.7b, 2.8b, respectively.

The boards are designed for the installation of fixed resistors MJIT-OD25 (MLT-025, VS-0D25), capacitors KT-1, KM-4 or K10-7V, K50-6.

To tune the generators in frequency, variable capacitors with a solid dielectric from small-sized transistor receivers are used:

"World" in the first device;
"Planet" in the second device.

It is possible to use any other suitable capacitors in terms of dimensions and values of the minimum and maximum capacitances, including trimmers PDA-3 with a capacity of 25-150 pF.

Variable resistors R5 and R2 - small-sized any type.

Coil manufacturing

Coils L1 for metal detectors assembled according to the diagrams in fig. 2.6, a and 2.7, a, are wound on ferrite (600 NN) ring magnetic cores of size K8 x 6 x 2.

In the first metal detector, the coil contains 180 turns of PELSHO 0,14 wire, in the second - 50 turns of PELSHO 0,2.

Winding in both cases is uniform around the entire perimeter of the magnetic circuit.

In the first metal detector, the coil is glued with BF-2 glue directly to the printed circuit board, and in the second (due to lack of space) - to a small corner bent from 1,5 mm thick polystyrene sheet and glued to the board with the same glue.

The search coil of each of the considered metal detectors is wound in a ring bent from a vinyl plastic tube with an outer diameter of 15 mm and an inner diameter of 10 mm.

The outer diameter of the ring is:

for the first scheme - 250 mm (100 turns);
for the second and third - 200 mm (50 turns).

Used wire - PELSHO 0,27.

Each ring must be wrapped with aluminum foil tape for electrostatic shielding to eliminate the effect of capacitance between the coil and ground. To protect against damage, it is advisable to wrap the foil with one or two layers of insulating tape.

When winding the tape, it should be remembered that electrical contact between its ends is unacceptable (otherwise a closed loop is formed).

A view of the finished coil, made by the described method, is shown in Fig. 2.9.

Metal detectors on microcircuits with a comparison circuit

Rice. 2.9. View of the finished search coil

With a decrease in the diameter of the search coil, the "capture zone" narrows, but the device becomes more sensitive to small objects. With an increase in diameter, on the contrary, the "capture zone" expands, and the sensitivity to small objects decreases.

To indicate the search in all devices, head phones TON-2 are used. The metal detectors can be powered from one Krona battery or from two 3336 batteries or six 316, 332 cells connected in series.

Author: Sketeris R.

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

LM27761 Switched Capacitor Voltage Inverter with LDO 26.08.2016

Texas Instruments has introduced a new low voltage switched capacitor inverter. It provides very low noise at a regulated output for input voltages ranging from 2,7V to 5,5V.

Using four inexpensive capacitors, you can provide up to 250mA output current and -1,5V to -5V to power analog circuits, for example.

The LM27761 operates at 2 MHz to reduce output impedance and voltage ripple. With an operating current of 370 µA, the inverter delivers >80% efficiency and can be used in applications such as power amplifiers, DACs, and other applications requiring low output voltage noise.

Capabilities

Inverts and regulates the input supply voltage
Low output noise
Current consumption in off mode up to 7 μA (typ.)
Output current up to 250 mA
Inverter output impedance 2,5 ohm, at Vin = 5 V
Peak load regulation +- 4%
Current consumption 370 uA
Fixed frequency 2 MHz (typical) with low noise
LDO PSRR 35 dB (typical) at 2 MHz with 80 mA load current
Voltage drop across LDO 30mV, at 100mA, Vout= -5V
Current limit and thermal protection

Typical applications for LM27761:

Operational power amplifiers
Wireless communication systems
cellular
Power supplies
Portable measuring instruments
Amplifiers for Hi-Fi headphones
Data Converter Power