Beat metal detector with Schmidt trigger. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Beat metal detector with Schmidt trigger. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Schematic diagram metal detector is shown in fig. 2.12. The 32768 Hz reference oscillator is assembled on a DD1.1 logic element and a ZQ1 quartz resonator.

The search generator is made on the element DD2.1 and the coil L1, which is a metal sensor. In addition, the generator includes frequency setting circuits - a tuning capacitor C3 and an electronic frequency tuning unit on a zener diode VD1, which plays the role of a varicap.

Elements DD1.2 and DD2.2 - buffer. The DD1.3 element performs the functions of an adder, its load is the L2C8 filter, which suppresses high-frequency oscillations, but passes a low beat frequency.

On transistors VT1, VT2, a Schmidt trigger is assembled, which forms rectangular pulses from a signal close to a sinusoidal shape. The amplifier on the transistor VT3 increases the amplitude of the pulses to a level equal to the supply voltage. Element DD3.1 completes the formation of rectangular pulses and inverts them.

Beat metal detector with Schmidt trigger
Fig. 2.12. Schematic diagram

These pulses are fed to one input of the coincidence element DD2.3, and rectangular pulses with a frequency of 2 kHz are connected to the other input from the output of the generator on inverters DD3.2, DD3.3. Elements DD3.4-DD3.6 play the role of an output amplifier for the piezoceramic sound emitter HA1.

The metal detector is powered by a GB1 battery.

Setting up the metal detector

Before starting to search for hidden metal objects, it is necessary to achieve zero beats at the output of the adder DD1.3, or, in other words, to establish the exact equality of the generator frequency values.

To do this, the device sensor - coil L1 - you need:

place in a place away from the ground and metal objects at a distance of at least 1 m;
turn on the device;
move the variable resistor R7 to the middle position;
with a tuned capacitor C3, set such a frequency of the search generator, at which the sound emitter HA1 reproduces rare short tonal signals or stops altogether;
to achieve by turning the knob of the variable resistor R7 within a very small range of the complete cessation of sound.

Now the sensor needs to be lowered to the ground and slowly begin to drive over its surface, listening to the sound of the device. As the sensor approaches a metal object, infrequent short tones appear, which gradually become more frequent and finally merge into an almost continuous hum.

Concept work

At zero beats, the difference frequency of the generators is zero, there are no voltage fluctuations at the input of the Schmidt trigger, so it does not switch. The transistor VT3 is closed, the output of the inverter DD3.1 is low, so the element DD2.3 does not pass to the output amplifier of the generator oscillations DD3.2, DD3.3.

As soon as the L1 coil approaches a metal object, its inductance will change, and hence the frequency of the search generator. An alternating voltage will appear at the output of the L2C8 filter. The larger the object and the closer the sensor is to it, the higher the beat frequency.

This leads to more frequent opening of the DD2.3 element and an increase in the frequency of repetition of sound tones. Generator frequency on the elements DD3.2. and DD3.3 (2 kHz) is chosen close to the natural frequency of the ZP-5 piezoelectric emitter in order to ensure the maximum volume of the signal.

Element base and replacements

The metal detector uses a clock-type quartz resonator, but other resonators with frequencies up to 80-100 kHz are also suitable. However, this will have to adjust the number of turns L1 and the values of the capacitors in the generators. Capacitors C1, C2, C4, C5 should be selected with a minimum TKE. The variable resistor R7 must be of group A.

Transistors KT361B can be replaced with KT3107K. KT3107L, and KT315B - on KT3102GM, KT3102EM. Instead of ZP-5, other piezo emitters can also be used. Power source - battery "Korund" or accumulator.

Metal detector design

It is advisable to place the board with the parts in a small, durable plastic box, to which the L1 sensor coil will be attached. The coil should contain 450 turns of PEV-2 0,18. It must be wound on a round boss with a diameter of 210 mm, then removed and wrapped with adhesive PVC tape. Coil resistance - about 200 ohms.

The coil must be placed in a rigid screen, which is an open ring bent from a soft duralumin tube. Along the tube it is necessary to cut a groove with a width sufficient for laying inside the coil, which is insulated with tape. Wrap the screen with the stacked coil tightly with adhesive PVC tape and attach it to the box with the board with two screws.

Unused pins of microcircuits DD1, DD2 should be connected to the positive power wire.

Zero beat setting

Sometimes it is not possible to immediately establish zero beats. The reasons for this may be:

discharged battery;
malfunction of the variable resistor R7;
generator frequency instability.

It is advisable to supply power to each of the microcircuits through an RC filter. The filter capacitor with a capacity of 0,01 uF should be soldered directly to the pins of the microcircuit, and it is better to choose a resistor experimentally.

Author: Kompanenko L.

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