An improved version of the beat detector. Scheme, description

Free technical library

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

An improved version of the beat detector. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / metal detectors

Comments on the article

Metal detector features

The sensitivity of this metal detector is increased by using the dependence of the duration of the probing pulse on the intensity of the parcels themselves. Automatic frequency adjustment has been introduced into the search generator. No additional measures for voltage stabilization and temperature compensation of electronic units are required.

Schematic diagram

The schematic diagram of the device is shown in fig. 2.30.

Rice. 2.30. Schematic diagram of an improved version of the beating metal detector (click to enlarge)

The master oscillator is made on the element DD1.1. Its frequency is stabilized by a ZQ1 quartz resonator included in a positive feedback circuit. To ensure the excitation of the generator when the power is turned on, resistor R1 is used. The buffer element DD1.2 unloads the generator, and also generates a signal with digital levels. Resistor R2 determines the degree of load and the maximum power dissipated in the quartz resonator.

This generator can work with almost any resonator at a current consumption of 500-800 μA. The frequency divider that follows it by two (element DD2.1) generates a signal with a symmetrical meander, which is necessary for the normal operation of the mixer. The measuring generator is assembled according to the scheme of an asymmetric multivibrator (transistors VT1 and VT2). The output to the self-excitation mode provides a positive feedback circuit on the capacitor C7.

Frequency-setting elements are capacitors C3-C5, varicap VD1 and search coil-sensor L1. Generation is carried out in the range from 500 kHz to 700 kHz, depending on the available quartz resonator. The frequency drift of this generator for the first 10 seconds immediately after turning on the power is no more than 0,7 Hz (and every 30 minutes - up to 20 Hz).

For normal operation of the device, a frequency drift of 1 Hz per 1 min (without AFC) is considered acceptable. The sinusoidal signal produced by the measuring generator, having an amplitude of 1-1,2 V, enters through the separating capacitor C9 to the elements DD3.1, DD3.2. These elements form rectangular pulses with digital levels and a duty cycle of 2. Resistors R5R6 form a divider necessary for the normal operation of this section of the circuit, and the DD3.3 element acts as a buffer stage. The signal from it is fed to the trigger DD2.2. The signal from the divider of the reference oscillator also arrives there.

The peculiarity of the operation of the DD2.2 trigger is such that if two pulse sequences close in frequency come to the inputs C and D of this logic element, then a difference frequency signal with a strictly symmetrical meander is formed at the outputs.

Direct, as well as delayed, and at the same time inverted (thanks to the R8C11 circuit and the DD4.2 element), the signals are summed up on the DD5.1 key, which acts as an AND / OR logic element. In this case, short positive write pulses are generated for the operation of an analog storage device (DD5.2. C13, VT3). The signal taken from the DD4.2 output comes to the integrator, made according to the classical scheme using elements VD2, R10-R11, DA1, C12.

Resistor R11 limits the recharge current of the capacitor C12, unloading the output of the element DD4.2. The integrated signal through the key DD5.2, which is controlled by pulses from DD5.1, is fed to the storage capacitance C13. On this capacitor, a voltage equal to the peak value of what comes from the integrator is formed and held with high accuracy until a new recording cycle. Capacitor C14 smooths out the step-type effect that can occur with a sharp change in beat frequencies.

From the source follower on the transistor VT3, the signal arrives:

to comparator DD4.3;
to a voltage controlled generator;
into the AFC loop circuit.

The divider R21R22, together with the feedback resistors R23 and R24, narrow the control voltage range to an amplitude of 1,2 V.

Operational amplifier DA2 compares the received voltage with that given by the divider R26R29, and generates a varicap control voltage VD1.

Adjustment of the metal detector

With resistor R26, you can set the starting point of the AFC capture (SENSITIVITY) roughly, and with resistor R27 - more accurately.

When moving the R26 slider towards the extreme (upper or lower according to the scheme) position, you can easily leave the AFC capture zone (± 300 Hz), implementing the one-to-one operation mode with a beat frequency, which makes working with the device more flexible.

In fact, the AFC has two time constants (depending on which direction the beat frequency changes). The special design of the sensor coil practically eliminates the effect of the ferromagnetic properties of the objects being detected. Therefore, there is no effect on increasing the frequency of the search oscillator. Therefore, the AFC and the device as a whole work very correctly in all modes.

VCO operation

The VCO on the elements DD4.4, R18, C15 converts the voltage, which changes with the beat frequency, into an audio frequency. The DD16 comparator configured using the divider R17R4.3 allows him to do this in the zone of maximum sensitivity, when the beat frequency lies in the range of 0-70 Hz.

The signal from the VCO is fed to the input "A" of the mixer (key DD5.4). The difference beat frequency comes to the input "CO" from the logic element DD4.1. As a result, the output of the mixer is:

or a beat-frequency modulated VCO signal;
or just the beat frequency.

Moreover, the scheme performs the transition from one mode to another automatically.

The variable resistor R30 serves as a load and volume control, and SA1 combined with it serves as a power switch. The use of microcircuits of the CMOS series and operational amplifiers operating in the microcurrent mode made it possible to reduce the current consumption of the circuit to the level of 6 mA, making it acceptable to use the Krona battery as a power source.

The location of the elements on the board is shown in fig. 2.31.

An improved version of the beat detector
Rice. 2.31. Location of elements on the board

Mounting the frame-sensor of the metal detector

The technology and carefulness of manufacturing the frame-sensor greatly affect the quality of the entire device. As a basis, it is recommended to use a bundle made up of eleven pieces of wire PEV-2 1,2 mm 1100 mm long. It must be tightly wrapped with a layer of electrical tape and squeezed into an aluminum tube having an inner diameter of 10 mm and a length of 960 mm. The resulting workpiece must be shaped into a rectangular frame 300 x 200 mm with rounded corners.

The end of the first of the wires, placed in an aluminum case - an electrostatic screen, is sequentially soldered to the beginning of the second wire, and so on, until a kind of 11-turn inductor is formed. The solder joints must be insulated from each other with paper tape and filled with epoxy resin, while excluding the appearance of a short-circuited coil due to the tube itself bent into the frame.

It is advisable to provide here any closed high-frequency connector and a suitable (not metal) mount for the handle, which can be used as one or two sections from a collapsible rod. The cable connecting the frame to the block is better to use coaxial, television, for example, PK75.

Almost the entire metal detector can be mounted on a printed circuit board (Fig. 2.32) made of one-sided foil fiberglass.

An improved version of the beat detector
Rice. 2.32. PCB

The search generator is recommended to be placed in a shielding box made of tin.

Element base

Choke L2 search generator has 150 turns of wire PEL-1 0,01. Winding must be carried out in bulk on a frame with a diameter of 4 mm and a length of 15 mm with a ferromagnetic tuned core 600НН. The inductance of such a choke is 1-1,2 mH.

The device uses capacitors KSO or KTK (C3, C4, C5), KLS or KM (C1, C2, C6-C13, C15), K50-6 or K53-1 (C14, C16. C17). Resistors - MLT 0,125, tuned R26, R27 fit SP5-2 or SP-3.

As transistors VT1 and VT2, for example, KP303B (F) is suitable. In place of VT3, KP303 or KP305 with any letter is acceptable, KT3102G (VT4) will be replaced by KT3102E. Quartz - at 1,0-1,4 MHz. Varicap D901 can be replaced by D902.

Author: Stafiychuk Yu.

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

sky fish 15.09.2010

Swiss inventors have created a prototype airship without motors and propellers. Inside this fish-shaped, eight-meter-long inflatable balloon, "artificial muscles" are stretched along their entire length, which contract when electrical voltage is applied to them.

Curving like a fish, the airship floats across the sky at a speed of half a meter per second.

Other interesting news:

▪ Why does a person continue to eat, although already full

▪ Wrist camcorder

▪ The robot performed before the British House of Lords

▪ Hybrid TV remote control and portable speaker

▪ talking fingers

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Dosimeters. Selection of articles

▪ article Life has become better, life has become more fun. Popular expression

▪ article What invention of Edison is used every day? Detailed answer

▪ article Work on string casting typesetting machines. Standard instruction on labor protection

▪ article Greenhouse on biofuel. Encyclopedia of radio electronics and electrical engineering

▪ article Three phases - from one. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

Comments on the article:

Gringo
Not so complicated, but it will work clearly.

leonid
DD4 - should be K561LA7, DD5 - should be K561KT3. The rest are correct. Leonid, Kyiv.

a guest
whether the pin numbering of k561tl1 is correctly indicated on the diagram does not correspond to the reference data

All languages of this page

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