Almost all headsets that are designed to work with a PC have such "pathetic" characteristics that if you try to use a microphone from such a headset for sound recording or the same karaoke, you will get nothing but disappointment. There is only one reason for this - all such microphones are designed for speech transmission and have a very narrow frequency range... This not only reduces the cost of the design itself, but also contributes to the intelligibility of speech, which is the main requirement of the headset.

Attempts to connect an ordinary dynamic or electret microphone usually end in failure - the level from such a microphone is clearly not enough to "swing" the sound card. In addition, ignorance of the input circuit of sound cards affects and the incorrect connection of a dynamic microphone ends the matter. Collect a microphone amplifier and connect it "according to the mind"? It would be nice, but much easier to use the IEC-3 microphone, which was at one time widely used in wearable equipment and is still quite common. But, of course, you will have to connect "according to the mind".

This microphone is electret, has rather high characteristics (the frequency range, for example, lies in the range of 50 - 15,000 Hz) and, most importantly, it has a built-in source follower assembled on field-effect transistor, which not only matches the high impedance of the microphone with the amplifier, but also has an output level more than sufficient for any sound card. Perhaps the only drawback is that the microphone needs power. But its current consumption is so small that two finger-type batteries connected in series will last for many months of continuous operation. Let's take a look at the internal circuit of the microphone, which is located in an aluminum glass, and think about how to connect it to a computer:

The gray color indicates an aluminum cup, which is a shield and is connected to the common wire of the circuit. As I said, this microphone requires external power supply, and minus 3-5 V must be applied to the resistor (red wire), and plus to the blue one. We will shoot a useful signal from the white one.

Now let's take a look at the computer microphone input diagram:

It turns out that the signal should be applied only to the very tip of the connector, indicated in green, and to the red itself sound card supplies +5 V through a resistor. This is done to power the headset preamplifiers, if used. We will not use this voltage for two reasons: firstly, we need a different polarity, and if we just “turn” the wires, then the microphone will “hum” strongly. Secondly, the PC power supply is impulse and the noise at these five volts will be decent. The use of galvanic cells is ideal in terms of interference - pure "constant" without the slightest ripple. So, the complete diagram of connecting our microphone to a computer will look like this.

Has been hovering in my head for a long time. Gathering his strength, he began to search for amplifier circuits. Most of the circuits that I reviewed, which I did not like. I wanted to assemble it easier, better and less (for a laptop, because the built-in was done, apparently, just for show - the quality is bad). And after a short search, a phantom-powered microphone signal amplifier circuit was found and tested. Phantom power (this is when power and information transfer is carried out over one wire) is a huge plus of this scheme, because it saves us from third-party power supplies and the problems associated with them. For example: if we power the amplifier from a simple battery, then sooner or later it will run out, which will lead to the inoperability of the circuit in this moment; if we power it from a battery, then sooner or later it will have to be charged, which will also lead to some difficulties and unnecessary movements; if we power it from a power supply, then there are two minuses that, in my opinion, discard the option of using it - these are wires (for powering our PA) and interference. You can get rid of interference in many ways (put a stabilizer, all sorts of filters, etc.), then it is not so easy to get rid of the wires (you can, however, make the transmission of energy at a distance, but why fence a whole complex of devices for powering some the microphone amplifier?) and it also reduces the practicality of the device. Let's move on to the diagram:

A variant of the amplifier circuit for a dynamic microphone

The circuit is distinguished by its super-simplicity and mega-repeatability, in the circuit there are two resistors (R1, 2), two capacitors (C2, 3), a 3.5 plug (J1), one electret microphone and a transistor. Condenser C3 acts as a microphone filter. The capacity of C2 should not be neglected, that is, it is not necessary to put either more or less of the nominal value indicated in the diagram, otherwise it will entail a lot of interference. We put the T1 transistor domestic kt3102 ... To reduce the size of the device, I used an SMD transistor marked "1Ks". If you don’t know how to solder at all - go to the forum.

When replacing T1, there were no significant changes in quality. All other parts are also in SMD cases, including the capacitor C3. The whole board turned out to be quite small, although you can make it even smaller using the technology for manufacturing LUT printed circuit boards. But he got along with a simple half-millimeter permanent marker. I etched the board in ferric chloride in 5 minutes. The result is such a microphone amplifier board, which is attached to the 3.5 plug.

All this fits well inside the casing from the plug. If you also do this, then I advise you to make the board as small as possible, since in my case it deformed the casing and changed its shape. It is advisable to rinse the board with a solvent or acetone. As a result, we got such a useful device with good sensitivity:

Before connecting the microphone to the computer, check all the contacts and whether there is a + 5v power supply at the microphone input (and it should be), in order to avoid comments like: "I assembled exactly as in the circuit, but it does not work!". This can be done like this: connect a new plug to the microphone connector and measure the voltage with a voltmeter between ground (large tap) and two short soldering taps. Just in case, try not to short-circuit the plug leads between each other when you measure the voltage. I don’t know what will happen then, and I don’t want to check. My microphone amplifier has been working for 3 months, I am completely satisfied with the quality and sensitivity. Collect and unsubscribe on the forum about your results, questions, and maybe even about the modifications of the case, circuitry and methods of their manufacture. I was with you BFG5000, good luck!

Discuss the article ELECTRIC MICROPHONE AMPLIFIER

Microphones (electrodynamic, electromagnetic, electret, carbon) - the main parameters, marking and inclusion in electronic circuits.

In radio electronics, a microphone is widely used - a device that converts sound vibrations into electrical ones. A microphone is usually understood as an electrical device used to detect and amplify weak sounds.

Basic parameters of microphones

The performance of a microphone is characterized by several standard technical parameters:

• sensitivity,
• nominal frequency range,
• frequency response,
• orientation,
• dynamic range,
• impedance module,
• rated load resistance
• and etc.

Marking

The microphone brand is usually printed on its body and consists of letters and numbers. The letters indicate the type of microphone:

• MD - reel (or "dynamic"),
• MDM - dynamic small-sized,
• MM - miniature electrodynamic,
• ML - tape,
• MK - capacitor,
• FEM - electret,
• MPE - piezoelectric.

The numbers indicate the serial number of the development. After the numbers there are letters A, T and B, denoting that the microphone is made in export version - A, T - tropical, and B - is intended for consumer electronic equipment (CEA).

The marking of the MM-5 microphone reflects its design features and consists of six symbols:

• first and second ............... MM - miniature microphone;
• third ................................ 5 - fifth design;
• fourth and fifth ........... two numbers indicating the size;
• sixth ............................... a letter that characterizes the shape of the acoustic input (O - round hole, C - nozzle, B - combined).

In the practice of radio amateurs, several basic types of microphones are used: carbon, electrodynamic, electromagnetic, condenser, electret and piezoelectric.

Electrodynamic microphones

The name of this type of microphone is considered obsolete and is now called coil microphones.

Microphones of this type are very often used by recording enthusiasts, due to their relatively high sensitivity and practical insensitivity to atmospheric influences, in particular, to the action of wind.

They are also shock resistant, easy to use, and have the ability to withstand high signal levels without damage. The positive qualities of these microphones outweigh their disadvantage: average sound recording quality.

Currently, small-sized dynamic microphones produced by the domestic industry are of great interest for radio amateurs, which are used for sound recording, sound transmission, sound amplification and various communication systems.

Microphones of four complexity groups are produced - 0, 1, 2 and 3. Microphones small-sized groups of complexity 0, 1 and 2 are used for sound transmission, sound recording and sound reinforcement of music and speech, and groups 3 - for sound transmission, sound recording and speech sound reinforcement.

The microphone symbol consists of three letters and numbers. For example, MDM-1, a dynamic small-sized microphone of the first design.

Of particular interest are the electrodynamic miniature microphones of the MM-5 series, which can be soldered directly into the amplifier board or used as a built-in element of electronic equipment.

Microphones belong to the fourth generation of components that are designed for electronic equipment on transistors and integrated circuits.

The MM-5 microphone is produced in one type in two versions: high-resistance (600 Ohm) and low-resistance (300 Ohm), as well as thirty-eight standard sizes, which differ only in the DC winding resistance, the location of the acoustic input and its type.

Basic electroacoustic parameters and specifications microphones of the MM-5 series are given in table. one.

Table 1.

Microphone type	MM-5
Execution option	low impedance	high resistance
Nominal range working frequencies, Hz	500...5000
Complete module electric resistance winding, Ohm	135115	900 ± 100
Sensitivity on frequency 1000 Hz, μV / Pa, not less (load resistance)	300 (600 Ohm)	600 (300 Ohm)
Average sensitivity in range 500 ... 5000 Hz, μV / Pa, not less (load resistance)	600 (600 Ohm)	1200 (3000 Ohm)
Frequency irregularity sensitivity characteristics in the nominal range frequencies, dB, not more	24
Weight, g, no more	900 ± 100
Service life, year, not less	5
Dimensions, mm	9.6x9.6x4

Fig. one. Schematic diagram inclusion at the input of the ultrasonic speaker as a microphone.

In the absence of a dynamic microphone, radio amateurs often use a conventional electrodynamic loudspeaker instead (Fig. 1).

Electromagnetic microphones

For low-frequency amplifiers assembled on transistors and having a low input impedance, electromagnetic microphones are usually used.

Electromagnetic microphones are reversible, which means they can also be used as telephones. The so-called differential microphone of the DEMSh-1 type and its modification DEMSh-1A are widespread.

Quite good results are obtained when instead of electromagnetic microphones DEMSH-1 and DEM-4M, ordinary electromagnetic headphones from TON-1, TON-2, TA-56 and others are used (Fig. 2-4).

Fig. 2. A schematic diagram of the inclusion of an electromagnetic earphone at the input of the ultrasonic frequency band as a microphone.

Fig. 3. Schematic diagram of the inclusion of an electromagnetic microphone at the input of the ultrasonic frequency converter on transistors.

Fig. 4. Schematic diagram of the inclusion of an electromagnetic microphone at the input of the ultrasonic frequency converter on an operational amplifier.

Electret microphones

Recently, electret condenser microphones have been used in household tape recorders. Electret microphones have the widest frequency range - 30 ... 20,000 Hz.

Microphones of this type produce an electrical signal twice as large as conventional carbon microphones.

The industry produces electret microphones MKE-82 and MKE-01 in dimensions similar to the carbon MK-59 and the like, which can be installed in ordinary handsets instead of coal ones without any alteration of the telephone set.

This type of microphone is significantly cheaper than conventional condenser microphones and therefore more readily available to radio amateurs.

The domestic industry produces a wide range of electret microphones, among them MKE-2 of one-way directionality for reel-to-reel tape recorders of the 1st class and for embedding in electronic equipment - MKE-3, MKE-332 and MKE-333.

For radio amateurs, the most interesting is the MKE-3 condenser electret microphone, which has a microminiature design.

The microphone is used as a built-in device in domestic tape recorders, radio and radio tape recorders, such as "Sigma-VEF-260", "Tom-303", "Romantic-306", etc.

The MKE-3 microphone is manufactured in a plastic case with a flange for mounting on the front panel of a radio device from the inside. The microphone is omnidirectional and has a circle pattern.

The microphone is resistant to shocks and strong shaking. Table 2 shows the main technical parameters of some brands of miniature condenser electret microphones.

Table 2.

Microphone type	FEM-3	MKE-332	MKE-333	MKE-84
Nominal range working frequencies, Hz	50...16000	50... 15000	50... 15000	300...3400
Sensitivity to free field on frequency 1000 Hz, μV / Pa	no more than 3	not less than 3	not less than 3	A - 6 ... 12 B - 10 ... 20
Unevenness frequency response sensitivity in range 50 ... 16000 Hz, dB, not less	10	-	-	-
Complete module electrical resistance at 1000 Hz, Ohm, no more	250	600 ± 120	600 ± 120	-
Equivalent level sound pressure, due to its own microphone noise, dB, no more	25	-	-	-
Average level difference sensitivity "Front - rear", dB	-	not less than 12	no more than 3	-
Operating conditions: temperature, С relative humidity air, no more	5...30 85% at 20 "C	-10...+50 95 ± 3% at 25 "C	10...+50 95 ± 3% at 25 "C	0...+45 93% at 25 "C
Supply voltage, V	-	1,5...9	1,5...9	1,3...4,5
Weight, g	8	1	1	8
dimensions (diameter x length), mm	14x22	10.5 x 6.5	10.5 x 6.5	22.4x9.7

In fig. 5 shows a diagram of the inclusion of an electret microphone of the MKE-3 type, which is widespread in amateur radio designs.

Fig. 5. Schematic diagram of turning on a microphone of the MKE-3 type at the input of a transistor ultrasonic frequency converter.

Fig. 6. Photo and internal schematic diagram of the MKE-3 microphone, arrangement of colored conductors.

Carbon microphones

Despite the fact that carbon microphones are gradually being replaced by other types of microphones, but due to the simplicity of design and sufficiently high sensitivity, they still find their place in different devices communication.

The most widely used are carbon microphones, the so-called telephone capsules, in particular, MK-10, MK-16, MK-59, etc.

The simplest scheme for turning on a carbon microphone is shown in Fig. 7. In this circuit, the transformer must be a step-up and for a carbon microphone with a resistance R = 300 ... 400 Ohm it can be wound on an E-shaped iron core with a cross section of 1 ... 1.5 cm2.

The primary winding (I) contains 200 turns of PEV-1 wire with a diameter of 0.2 mm, and the secondary (II) - 400 turns of PEV-1 with a diameter of 0.08 ... 0.1 mm.

Carbon microphones, depending on their dynamic resistance, are divided into 3 groups:

1. low-impedance (about 50 Ohm) with a supply current of up to 80 mA;
2. medium-resistance (70 ... 150 Ohm) with a supply current of no more than 50 mA;
3. high-resistance (150 ... 300 Ohm) with a supply current of no more than 25 mA.

From this it follows that in the carbon microphone circuit it is necessary to set the current corresponding to the type of microphone. Otherwise, when the current is high, the carbon powder will start to bake and the microphone will deteriorate.

In this case, nonlinear distortions appear. At a very low current, the sensitivity of the microphone decreases sharply. Carbon capsules can also operate at reduced power supply current, particularly in tube and transistor amplifiers.

The decrease in sensitivity with reduced microphone power is compensated for by simply increasing the gain of the audio amplifier.

In this case, the frequency response is improved, the noise level is significantly reduced, and the stability and reliability of operation are increased.

Fig. 7. Schematic diagram of the inclusion of a carbon microphone using a transformer.

A variant of including a carbon microphone in an amplifier stage on a transistor is shown in Fig. 8.

Option to turn on a carbon microphone in combination with a transistor at the input tube amplifier audio frequency according to the scheme in Fig. 9 allows a high voltage gain to be obtained.

Fig. 8. Schematic diagram of the inclusion of a carbon microphone at the input of a transistor ultrasonic frequency converter.

Fig. 9. Schematic diagram of turning on a carbon microphone at the input of a hybrid ultrasonic frequency converter, assembled on a transistor and an electronic tube.

Literature: V.M. Pestrikov - Encyclopedia of radio amateurs.

This article was written based on the experience of manufacturing more than two hundred of these adapters. The diagram was taken as a basis from an article from the magazine "Radiodesign" No. 18, p. 52 (Fig. 1).

The elements indicated in the diagram are not very critical, the RFC choke can be omitted. Initially, this circuit was made in a small neat box, which constantly clung to something and soon became very annoying. Then the idea of ​​making an adapter arose, which did not differ much from the proprietary connector. Many different options have been tried. The final version is presented to your attention.

We take a standard 8-pin connector and disassemble it as shown in Fig. 2.

All conclusions, with the exception of No. 7, are shortened as much as possible. The remaining pin is the common microphone lead for all transceiver models. We increase the diameter of the hole at the end of the connector to 7.2 mm with a file (I grind it on a lathe, but I also get it pretty quickly with a file).

Next, we take the connector for the 3.5 mm audio plug, cut it off, as shown in Fig. 3 (the length of the remaining screw-on part is 10-11 mm). To improve contact, the center conductors are bent and soldered together, and the terminal box is shortened. We solder the conductors to the conclusions, preferably in fluoroplastic insulation.

For insulation, a heat-shrinkable tube is put on and heated in any way. We screw on the remnant of the case, cut off by 11 mm. We make and put on a gasket with a cut-out hole, depending on the diameter of the case with a heat-shrinkable tube from any insulating material - fluoroplastic, textolite, but the simplest option is from a polyethylene bottle.

And this is how the finished board looks like (Fig. 4). Two conductors pass through the hole in the center, and a notch along the edge of the board serves for soldering to the remaining pin 7 on the microphone plug.

The build process is as follows:

1. Unscrew the half-ring at the end of the microphone plug, insert the audio connector, clamp the bracket and cut the insulating gasket around the perimeter. Thus, the two parts of the manufactured adapter are insulated and the braid of the microphone cable will not have contact with the transceiver body;

2. We take two fluoroplastic wires 1 cm long and solder them -

ICOM - to pins 1 and 2

KENWOOD - to pins 1 and 5

YAESU - to pins 2 and 8 (before making an adapter to a transceiver of this model, check the presence of voltage at pin 2).

We pass the conductors through the holes in the board and solder them to the corresponding points, and the board itself is attached to pin 7 (microphone body) in the place of the notch;

3. We solder the conductors from the audio connector to the board;

4. A heat-shrinkable tube is put on the board and heats up to prevent accidental contacts of the board with the wall of the connector housing;

5. We assemble the connector, tighten the fastening screw.

The assembly steps can be seen in Figure 5.

This is how the adapter looks like on my ICOM 756 PRO 3 (Fig. 6).

Any constructive wishes and suggestions are greatly appreciated.

My address: [email protected] Tel. 8-067-167-34-50 or 8-05662-2-22-23

Yuri Primak, UT7EL

Microphones are used to convert the energy of sound vibrations into alternating electrical voltage. According to the classification, acoustic microphones are divided into two large groups:

High-resistance (capacitor, electret, piezoelectric);

Low-resistance (electrodynamic, electromagnetic, coal).

Microphones of the first group can be conventionally represented as equivalent

variable capacitors, and microphones of the second group - in the form of inductors with moving magnets or in the form of variable resistors.

Among the high-impedance microphones, electret microphones are more affordable. Their parameters are normalized in the standard range of audio frequencies, which has the popular name "two to twenty" (20 Hz ... 20 kHz). Other features: high sensitivity, wide bandwidth, narrow radiation pattern, low distortion, low noise.

There are two- and three-lead electret microphones (Fig. 3.37, a, b). To make it easier to identify the wires coming out of the microphone, they are deliberately made multi-colored, for example, white, red, blue.

Rice, 3.37. Internal circuits of electret microphones: a) two communication wires; b) three communication wires.

Despite the transistors inside the microphone, it is short-sighted to send the signal from it directly to the input of the MK. We need an audio pre-amplifier. In this case, it does not matter whether the amplifier is built into the MK ADC channel or it is a separate external unit assembled on transistors or microcircuits.

Electret microphones are similar to vibration sensors, but unlike the latter, they have a linear transmission and a wider frequency response... This makes it possible to process audio signals of human speech without distortion, which, in fact, is the direct purpose of the microphone.

If we sort electret microphones made in the CIS countries in order to improve their parameters, we get the following row: MD-38, MD-59,

MK-5A, MKE-3, MKE-5B, MKE-19, MK-120, KMK-51. The operating frequency range is from 20 ... 50 Hz to 15 ... 20 kHz, the non-uniformity of the amplitude-frequency characteristic is 4 ... 12 dB, the sensitivity at a frequency of 1 kHz is 0.63 ... 10 mV / Pa.

In Fig. 3.38, a, b show diagrams of direct connection of electret microphones to MK. 3.39, a ... k shows circuits with transistor amplifiers, and in Fig. 3.40, a ... p - with amplifiers on microcircuits.

Fig. 3.38. Diagrams for direct connection of electret microphones to MK:

a) direct connection of the microphone VM1 to MK is possible if the ADC channel has an internal signal amplifier with a coefficient of at least 100. Filter R2, C / reduces the low-frequency background from the ripple of the supply voltage +5 V;

b) connecting a stereo microphone BMI to a two-channel ADC MK, which has an internal amplifier. Resistors R3 limit the current through the MK diodes in case of strong impacts on the microphone body or on the piezoplate itself.

c) the VTI transistor should have the highest possible gain (coefficient hjy ^) ’,

d) the resistor R3 selects the voltage on the collector of the transistor VT1, close to half of the power supply (for symmetric limitation of the signal from the microphone VM 1) \

e) the chain /? /, C1 reduces the amplitude of the mains ripples from the +5 V power supply, and therefore the unwanted "rumble" with a frequency of 50/100 Hz decreases. Hereinafter, the letters "c", "b", "k" will denote the color of the microphone wires "blue", "white", "red";

f) simplified connection of the BMI three-pin microphone. The absence of a resistor in the emitter of the VTI transistor reduces the input impedance of the stage;

g) a remote "two-pole microphone" with phantom power supply of VTI, VT2 transistors through a resistor R5. Resistor R1 selects voltage + 2.4 ... + 2.6 V at the emitter of transistor VT2. Analog comparator MK captures the moments when the signal from the microphone is greater than a certain threshold, which is set by the resistor R7 \ 0

h) the transistor operates in the cut-off mode, in connection with which the sinusoidal sound signals from the BMI microphone become rectangular pulses;

i) connection of a three-pin BMI microphone using a two-wire circuit. Microphone BM1 and resistor R1 can be swapped. Resistor R2 selects the voltage at the input of the MK, close to half of the power supply;

j) the resistor selects the voltage at the input of the MC, close to +1.5 V.

a) the transformer gap allows you to take out the elements BM1, DAI, GBJ, T1 over a long distance, while the input of the MK should be protected with Schottky diodes. The current consumption of the DA microcircuit / is extremely low, which allows you not to put the switch in the battery circuit GB1 \

Fig. 3.40. Wiring diagrams for electret microphones to M K through amplifiers on

microcircuits (continued):

b) amplifier for microphone "light music". Resistor R4 sets the triggering threshold of the analog comparator MK in the range of 0 ... + 3 V;

c) "electronic sound level meter". A smoothed voltage proportional to the average signal level from the BM1 microphone is supplied to the positive terminal of the MK analog comparator. On the negative output of the analog comparator, a "saw" is formed in software;

d) resistor R3 regulates the symmetry of the signal, and resistor R5 regulates the amplification factor of the op-amp DAL. The detected signal (elements VDI, VD2, C3, C4) is fed to the input of the MC. The average sound level is measured by an internal ADC;

e) non-standard use of the "LED" microcircuit Z) / l / from Panasonic. Possible replacements are LB1423N, LB1433N (Sanyo), BA6137 (ROHM). The ZL1 switch sets the sensitivity in five gradations on a logarithmic scale: -10; -five; 0; +3; +6 dBu;

f) the gain of the stage on the op-amp Z) / 4 / depends on the ratio of the resistances of the resistors R4, R5. Frequency response in the area low frequencies determined by the capacitor C /;

g) the gain of the stage on the op-amp Z) / l / is set by the ratio of the resistances of the resistors R5, R6. The symmetry of the signal limiting depends on the ratio of the resistors R3, R7 \

h) microphone amplifier with smooth control of the sound level by resistor R5 \

i) a two-stage amplifier with a distributed transmission coefficient: Ku = 100 (DAI.I), Ku = 5 (DAI.2). The divider on resistors R4, /? 5 sets the offset, which is slightly less than half of the supply. This is because the DA / op amp is not rail-to-rail;

Fig. 3.40. Diagrams for connecting electret microphones to MK through amplifiers on

microcircuits (continued):

j) the capacity of the capacitor C4b in some circuits is increased to 10 ... 47 microfarad (improvement of parameters is verified experimentally);

k) The "left" half of the DAI op-amp amplifies the signal, and the "right" half is switched on according to the voltage follower circuit. This solution is usually used when the MC is located at a considerable distance from the amplifier or it is required to branch the signal into several directions;

m) resistors R2, R4 transfer the inverters of the logic chip DDI to the amplifying mode. Resistor R3 can be replaced with a 0.15 μF capacitor;

m) a specialized microcircuit DA1 (Motorola) reacts only to the sound signals of a person's voice;

o) the plug inserted into the XS1 socket automatically breaks the connection between the capacitors C / and C2, while the internal microphone BM1 is turned off, and the external sound signal is fed to the DAL / input. Both amplifiers on Z) / l / have rail-to-rail output levels;

n) the resistor sets the symmetry of the signal limitation at pin 1 of the DA 1 microcircuit. The VTI transistor, together with the R5, SZ elements, performs the function of a detector. ^

3.5.2. Electrodynamic microphones

The main elements of the design of electrodynamic microphones are the inductance coil, diaphragm and magnet.The diaphragm of the microphone, under the influence of sound vibrations, brings the magnet closer / further away from the coil, in connection with which the latter arises AC voltage... Everything is like in school experiments in physics.

The signal from the electrodynamic microphone is too weak, so an amplifier is usually installed to interface with the MC. Its input impedance can be low. The connecting wires from the microphone to the input amplifier must be shielded or reduced in length to 10 ... 15 cm. To eliminate false alarms, it is recommended to wrap the capsule with foam rubber and do not screw the microphone tightly to the housing wall.

Typical parameters of electrodynamic microphones: winding resistance 680… 2200 Ohm, maximum operating voltage 1.5… 2 V, operating current 0.5 mA. An important practical consequence - electrodynamic microphones

easy to distinguish from electret (capacitor, piezoceramic) by the presence of ohmic resistance between the leads. An exception to the rule is industrial microphone modules containing a transistor or integrated amplifier inside the housing.

The electrodynamic microphone can be replaced with an electret one through the adapter shown in Fig. 3.41. Capacitor C2 adjusts the high frequency response. The divider on the resistors R1 creates an operating voltage for the BML microphone. C1 capacitor serves as a power supply filter.

Fig. 3.43. Wiring diagrams for connecting dynamic speakers to the input MK:

a) transistor amplifier of the shock sensor using a BAI loudspeaker. The sensitivity is regulated by resistors RI, R2. Capacitor C2 smoothes signal peaks. Capacitor C / is necessary so that the base of the transistor VT1 is not connected to the common wire through the low impedance of the BAI loudspeaker;

b) the VTI transistor is an amplifier with common base... Its peculiarity is its low input impedance, which matches well with the parameters of the BAI loudspeaker. The RI resistor sets the operating point of the VTI (collector voltage) of the transistor to obtain symmetrical or asymmetric clipping of the signal. Resistor R3 adjusts the threshold (sensitivity, gain);

c) BAI headset performs the microphone function. It has a higher winding impedance than a low impedance speaker, which increases sensitivity and makes it easier to connect to the MCU. The RI resistor adjusts the signal amplitude;

In Fig. 3.43, a ... d shows the diagrams of connecting dynamic loudspeakers to the MK input as microphones.

d) part of the intercom circuit, in which the BAI loudspeaker alternately performs the function of a microphone and a speaker. MK determines the state "Receive / Transmit" by LOW / HIGH level on the input line (HIGH level from resistor R4, and LOW - from and BAI). If the MC has an ADC with an internal amplifier, then you can "listen" to the conversation in the path. In addition, if the MK line is switched to the output mode, then it can be used to generate various sound signals in the ULF (through R3, VD1, R2, C2).