Do-it-yourself simple radio microphones diagram description. Homemade wiretapping from a karaoke radio microphone. Transmitter specifications
Simple radio microphone
If you and your friend each have a pocket radio with an FM band, adding two simple radio microphones to them, you can organize a good radio connection, with a range of up to 100 meters. Of course, 100 meters is not very much (you can shout at such a distance), but in some cases such a range can be useful. For example, you can organize a connection between two apartments or rooms (through a wall) or between cars driving one after another at a short distance.
circuit diagram
radio microphone is shown in the figure. There is only one transistor, an electret microphone and a few details. The microphone is powered by a three-volt battery (composed of two 1.5V AA cells).
Works radio microphone at a frequency near the middle of the range 88-108 MHz.
All parts, except for the antenna and power supply, are located on printed circuit board, the wiring diagram of which is in the figure.
Coils L1 and L2 are wound with a thick winding wire, for example, PEV -0.61. The inner diameter of the coil L1 is 3 mm, and it contains 8 turns. Coil L2 is wound on the surface of L1, it contains 3 turns. The coils are frameless, in order to give them a decent shape, it is desirable to make the initial winding on some mandrel with a diameter of about 3 mm, for example, on the shank of a drill of this diameter. First, the coil L1 is wound, its leads are formed and cut for holes in the board, and then, on the surface of L1, approximately in the middle, L2 is wound (see figure).
After winding both coils, forming and cutting their conclusions (the winding wire is covered with varnish insulation, which needs to be cleaned off only at the soldering points), the coils are installed on the board.
The electret microphone (M1) can be any electret microphone from a portable tape recorder, voice recorder, electronic telephone. For example, the SZN-15 microphone or another. The microphone has two outputs, one of which is marked with a “+” sign, this must be taken into account during installation (it will not work when turned on again).
Trimmer capacitors C1 and C2 are ceramic.
Antenna- a piece of mounting wire about a meter long.
Before setting up, find on the scale of the receiver operating in the FM band a place free from radio stations. Then, placing the receiver at a distance of 1-2 meters from the radio microphone antenna, sequentially adjust C1 and C2 until the signal is received by the receiver (in this case, you can talk in front of the microphone, and the assistant can listen to the receiver on headphones).
Then, gradually increasing the distance between the receiver and the radio microphone, adjust C1 and C2 more precisely so that the maximum communication range is obtained.
I bring to your attention a spy radio microphone with extremely low power consumption. This is perhaps the longest-playing bug of all that I have collected.
Of course, you have to pay for low power consumption with a small range, but for many purposes this is quite enough.
The radio microphone confidently breaks through two reinforced concrete walls, and in open space the range will be from 50 to 200 m (depending on the steepness of your receiver).
The bug circuit is incredibly simple and contains only 6 radio components, not counting the batteries:
Coil L1 - 4 turns with wire 0.5 mm on a mandrel Ø2mm. Choke - 100 nH for surface mounting. BFR93A transistor (the main thing is not to confuse it with the BFR93 p-n-p transistor).
and etched in ferric chloride: 
All this took about 20 minutes. Then I irradiated the finished board and cut off the excess: 
The most hemorrhoidal business is to connect the battery. I had an old (!!!) CR2032 lithium battery at my disposal (which is usually found in motherboards to power the BIOS chip).
To avoid unnecessary wires, I simply glued a strip of tin from tin can(this will be the negative contact): 
The rest of the piece of tin came in handy as a positive terminal: 
It is necessary that the battery is tightly inserted into the resulting slot, like this: 
It remains only to solder all the parts to the board according to the scheme: 
I'm sure it can be made even smaller. Replace the microphone, arrange the parts closer to each other, take small watch batteries and you're done. It will be possible to shove the entire circuit, for example, into the housing from the marker.
I used a wire 6 cm long as an antenna. The inductor was made by winding a thin enameled wire on a piece of toothpick (80 turns). 
The microphone, of course, is too big for such a scheme, but I didn’t have another one. In general, any electret with a diameter of 3-10 mm is suitable. Usually they are taken out of any telephone or intercom handsets.
By the way, the circuit does not work without a microphone - power goes through it. It also acts as a current stabilizer.
It is important not to confuse the polarity of the microphone: the negative terminal should ring on the body (for this reason, I put it in heat shrink so that, God forbid, nothing shorted out). 
The frequency is adjusted by compressing/stretching the turns of the coil. In my case, the bug was caught at a frequency of 424.175 MHz. The signal level at such a distance, of course, rolls over: .jpg)
If you wind 11 turns on a 2 mm mandrel, then the frequency will be approximately 150 MHz. In general, this bug works up to 1GHz. I didn’t try further, because. nothing to catch.
To test the range, he went outside and walked around the house. Amazingly, in the room where the bug was left, every rustle is perfectly audible.
P.S. This tiny bug worked on a half-dead battery for almost 2 weeks! It's scary to imagine how long it would last on a new one, because the current consumed is only 300 μA. 
Answer
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Do-it-yourself radio microphone 150m
I present to your attention a circuit of a simple transmitter powered by a 1.5V galvanic cell. The current consumption of the circuit is about 2 mA and the duration of operation is more than 24 hours. The range of the bug, depending on the conditions, can be up to 150m.
Device Diagram:
About work:
The master oscillator is assembled on a KT368 transistor, its operating mode is according to direct current are set by the resistor R1-47k. The oscillation frequency is set by a circuit in the base circuit of the transistor. This circuit includes coil L1, capacitor C3-15pf and the capacitance of the base-emitter circuit of the transistor, the collector circuit of which includes a circuit consisting of coil L2 and capacitors C6 and C7. Capacitor C5-3.3pf allows you to adjust the level of excitation of the generator.
Customization:
When setting up the device, they achieve the maximum high-frequency signal by changing the inductance (squeezing - stretching) of the coils L1 and L2. The finished bug scheme is placed in a small plastic case. If the dimensions are not too tight, put a mini-finger or finger-type battery to power the bug. In this case, the scheme will work much longer, up to several months. A miniature power switch can be installed for convenient operation.
If you cannot find the MKE-3, you can put any button microphone from a radiotelephone or mobile phone. It may be necessary to add a ULF cascade, but the increase in sensitivity will be significant.
I propose a scheme for a very stable radio microphone. The creation of this circuit was prompted by the need for a high-quality beetle, with a stable frequency that does not go away when a person approaches or the device moves. As a result, it was developed and assembled this scheme. Even if you turn the device in your hands, twist and unwind the antenna, the frequency does not go away at all. How to achieve stability will be discussed below.
So, the distinctive qualities of this radio microphone:
- adjustable sound sensitivity
- extremely stable performance
- adjustable power
Characteristics:
Power: 30-300mW
Supply voltage: 3-15V
Range: 70-140MHz
Description of the scheme
Through R1, power is supplied to the electret capsule, then with the help of C1, the useful signal is separated from the constant component of the power supply and enters the VT1 base. On VT1, an ultrasonic frequency converter is assembled, which is necessary to pre-amplify the signal from the microphone. An ordinary cascade with a common emitter, in which R3 sets the base offset, and R2 is the load. R4 limits the current of the stage, which is necessary to adjust the gain of the stage, and C4 shunts it with alternating current, that is, passing only the useful signal. R5 limits the current of the low-frequency part, and together with C2 acts as a G-filter that protects the circuit from self-excitation. Through C3, the signal enters the VT2 base, on which the MHF is performed. R6 and R7 set the base offset, R8 limits the stage current. C5 shunts the base to a common terminal, for which such a cascade is called a cascade with a common base. C7 creates a feedback, and C8 shunts R8, allowing the RF signal to pass freely. A parallel oscillatory circuit is assembled on L1 and C6, on which the generation frequency depends. Through C9, the already generated VT2 HF signal, and modulated by the LF signal from VT1, it enters the VT3 base, on which the UHF is assembled. R9 and R10 set the offset based on VT3. R11 limits the current of the stage and allows you to change the output power of the device. L2 and C10 form an oscillatory circuit similar and resonant to the GHF circuit. Capacitor C11 is separating, between the UHF and the antenna. C12 shunts the RF circuit, which prevents self-excitation at high frequencies.
Used elements and interchangeability
VT1-9014; VT2, VT3-9018.
L1, L2 - 6 turns with 0.5mm wire, on a frame with a diameter of 3mm.
Antenna - a piece of wire 20-60cm.
All resistors are 0.125-0.5W. Capacitors C1, C2, C3 and C4 are electrolytic, the rest are ceramic.
Power source: any voltage 3-15V, in my case 2 lithium tablets of size CR2032.
VT1 can be replaced by a KT315, BC33740 transistor or almost any low-power NPN transistor with a sufficient gain. VT2, VT3 can be replaced with a KT368 transistor, or any other low-power ones with a cutoff frequency of at least 200 MHz.
Setting
The setup comes down to setting the microphone sensitivity, setting the frequency and setting the UHF circuit to resonance.
Using R4, it is necessary to adjust the sensitivity of the ULF cascade so that a close conversation does not cause overload, and the sensitivity is still sufficient to hear it within a room or apartment.
With the help of C6, a rough choice of frequency is made; for more precise adjustment, it is necessary to change the geometry of L1 by stretching the turns. Using C10, the UHF circuit must be tuned to resonance with the carrier. The output power depends on the value of R11.
Assembly
In my assembly version, the device was assembled on a double-sided foil fiberglass. On one side, there is a surface-mounted circuit directly, on the second, pads for 2 lithium batteries of CR2032 tablets were organized. One of features - use key as a power switch. In order to activate the device, you need to insert the key into the connector, this was done for convenient and reliable switching on.
The photo shows a beetle assembled and covered with a thermotube, as well as a key. A piece of tin was soldered to the end of the antenna, for the possibility of more convenient fastening of the end of the antenna.
You can download the printed circuit board in the format below
Methods for Improving the Stability of Radio Microphones
Many novice radio amateurs who decide to try simple and interesting "bug" circuits often fail to set up the circuit after assembly. And when faced with a problem, at best, they bother on the forums, at worst, they give up this idea. One of the most common problems in such designs is instability and drift.
First of all, consider the factors affecting the operation of the MHF, on which the stability of the carrier depends. Most of the "bugs" are created using a three-point type MHF on a single transistor. Let us consider several factors affecting the generation stability.
1. The case in which the antenna clings directly to the MHF and the influence of the antenna.
An antenna connected directly to the GHF through a capacitor or inductive coupling, in fact, becomes a receiver, and not just a transmitter, because. its capacitance, as well as its location in space and the extraneous RF currents induced into it, are transmitted to the MHF circuit and have a great effect on its operation. It's like connecting a source of interference to the GHF.
The solution to this problem is a simple UHF cascade, or repeater, that is, UHF with virtually no gain, only necessary to limit the UHF from feedback from the antenna. An example of a simple low-power UHF is shown below.
2. Oscillatory circuit.
The influence of the quality of the coil of the oscillatory circuit on the stability of work also takes place. A coil made of too thin wire, which does not have a body and is not filled with anything, will change its geometry when the device is physically affected, that is, during movements and other vibrations. A change in geometry will cause a change in inductance, which in turn will cause a frequency drop.
The solution to this problem is sizing the coils, winding them on the frame, winding the coils with a thicker wire.
3. Nutrition.
The operation of the device in general always depends on the power supply. Batteries over the course of their work will change the voltage quite significantly, which will also be expressed by a gradual departure of the frequency.
The solution is to use stabilizers, and circuit solutions that do not have a strong dependence on the power source.
4. Screening.
When approaching metal or other objects with electrical conductivity, they affect the inductive and capacitive environment of the circuit. For example, a metal shield passing next to the oscillatory circuit will affect its inductance, increasing it and lowering the frequency. Permanent shielding with an unchanging geometry that has a permanent effect is not a problem, on the contrary, it encloses the device from external influences. Otherwise, when the device is placed on a metal base, it may affect the operation. The solution is to use shielding, using a thick plastic case that limits the minimum possible distance to the board.
List of radio elements
| Designation | Type | Denomination | Quantity | Note | Score | My notepad |
|---|---|---|---|---|---|---|
| VT1 | bipolar transistor | 9014 | 1 | KT315, BC33740 | To notepad | |
| VT2, VT3 | bipolar transistor | 9018 | 2 | KT368 | To notepad | |
| C1 | 0.47uF | 1 | To notepad | |||
| C2, C4 | electrolytic capacitor | 10 uF | 2 | To notepad | ||
| C3 | electrolytic capacitor | 1 uF | 1 | To notepad | ||
| C5 | Capacitor | 100 nF | 1 | To notepad | ||
| C6, C9-C11 | Trimmer Capacitor | 35 pF | 4 | To notepad | ||
| C7 | Capacitor | 15 pF | 1 | To notepad | ||
| C8, C12 | Capacitor | 470 pF | 3 | To notepad | ||
| R1, R2, R5, R6, R9 | Resistor | 9.1 kOhm | 5 | To notepad | ||
| R3 | Resistor | 470 kOhm | 1 | To notepad | ||
| R4 | Trimmer resistor | 3 kOhm | 1 | To notepad | ||
| R7, R10 | Resistor | 3 kOhm | 2 | To notepad | ||
| R8 | Resistor |
Simple radio microphone
Here is a diagram of a radio microphone operating at a frequency of 100 MHz. If desired, the transmission frequency can be changed by changing the number of turns of the L1 circuit. The antenna is spiral and contains 25 turns copper wire with a diameter of 1-1.2 mm, wound on a mandrel of 8 mm with a pitch of 1.2 mm. L1-contains 5 turns of wire with a diameter of 0.8 mm, an inner diameter of 4 mm with a pitch of 1.2 mm. Ceramic capacitors should be used in frequency-setting circuits .Capacitors C1 and C7 must be located near the transistors.
Radio microphone on the AL2602 chip
Wireless microphone LIEN
The radio microphone LIEN (translated from French - communication) is designed for one-way communication in the VHF band, as well as for sounding discos and other events.
The radio microphone (PM) LIEN operates at a frequency of 70 MHz (VHF1 band) and is a micropower transmitter with frequency modulation. The PM circuit (Fig. 1) is highly economical and, operating from a 9-volt Korund battery, consumes a current of 6 ... 15 mA. Since the maximum allowable discharge current of Corundum is 20 mA, an LED power-on indicator HL1 is introduced into the PM circuit. With a small current consumed by it (3 mA), it does not overload the battery, but significantly increases the usability of the RM 
Fig.1. Schematic diagram of a radio microphone
The microphone amplifier, which is part of the MKE-3 electret microphone, is powered by an unstabilized voltage through an L-shaped RC link (R1-C3) and provides an AF voltage of up to 30 mV at the output. This signal is fed through the coupling capacitor C2 to the input of the amplifier on the transistor VT1. To improve the temperature stability of the cascade, the bias voltage to the base VT1 is supplied from the collector through R2, and R5 is introduced into the emitter circuit. Capacitor C5 is a blocking capacitor and cuts off the RF components penetrating the ultrasonic frequency circuit from the generator to VT2.
The cascade on the transistor VT2 is a capacitive three-point. Resistive divider R7-R8 determines the bias voltage (Ucm) based on VT2, which operates in cutoff mode (class C). Therefore, Ucm based on VT2 can be selected within +0.8 ... +1.2 V. Parallel to the tuning resistor R8, two silicon diodes are connected, which stabilize Ucm and minimize the generator frequency drift when the battery is discharged.
The frequency modulator is assembled on the elements R6, VD3, C5. When the AF voltage is applied from the output of the UZCH through the resistor R6, the VD3 varicap changes its capacitance. From the anode VD3 through C5, the modulating voltage is applied to the tap (4th turn from the top) of the coil L1. This is done to reduce the modulation depth. In a simplified (non-retractable) version of L1, the right (according to the diagram) output C5 can be connected to the lower output L1. You can also reduce the modulation depth by reducing the capacitance C5 or using a varicap as VD3 with a lower capacitance overlap coefficient. In practice, when overmodulation occurs (deviation is more than 150 ... 250 kHz), the capacitance C5 should first of all be reduced.
The RF signal, modulated by the AF voltage, is fed through the coupling coil L2 to the WA1 antenna, made of a PEL 0.96 single-core copper wire. WA1 - type Short whip (short pin) has a length of 184 ... 206 mm, which is selected experimentally when setting up. An important factor to ensure the stable operation of the RM is the mechanical strength (immobility) constituent parts oscillatory circuit and especially the antenna.
Before turning on the radio microphone, carefully check the installation. Then it is recommended to check the resistance between the power contacts. The resistance of the measured circuit should not be zero and should change when the polarity of the tester connection changes.
Further, a DC milliammeter with the shortest possible length of connecting conductors is included in the PM power supply circuit. The current consumed by the radio microphone should not exceed 20...25 mA. Otherwise, check the installation again and eliminate possible short circuits. With Iп = 3...18 mA, you can start setting up the PM for direct current:
*set the voltage on the microphone +1.2...+3 V by selecting R1;
* set the voltage to 0.5Up on the collector VT1;
*set U=+0.8...1.2 V based on VT2.
Now you can start setting up the generator:
* put a VHF receiver tuned to the desired range (70 MHz) at a distance of at least 2 m from the radio microphone;
* turn on the power supply of the RM and achieve the appearance of generation by rotating the slot of the trimmer capacitor C8 with a dielectric screwdriver. The occurrence of generation can be controlled by ear by the characteristic frequency capture (disappearance of the hiss of the receiver). To avoid tuning the receiver to the harmonic, do not place the receiver closer to the RM;
* tune the oscillatory circuit in the VT2 collector circuit with a brass or ferrite core to the resonance frequency (70 MHz) according to the maximum capture width of the broadcasting range between two stations (tuning is possible to another frequency from the edge of the range or on any free section of the broadcasting range, equidistant from two neighboring stations ).
In case of unsatisfactory results, you should change the capacitance C7 and repeat the setting. To reduce the tuning time, it is recommended to replace the capacitor C7 with a tuning capacitance of 6 ... 30 pF. If the tuning results are satisfactory, you can try to further increase the resonance amplitude by changing the number of turns of the coil L1 by 5 ... 10%.
The oscillation amplitude will be maximum when the elements of the oscillatory circuit are in balance, that is, when the reactances L1 and C1 are equal. Coarse tuning of the L1-C7 circuit is carried out by selecting the number of turns L1 and (or) changing the capacitance C7, and smooth tuning is carried out by a tuning core. The presence of resonance can also be controlled by the minimum Ip. To control Ip, in order to avoid a noticeable frequency drift, you should use a milliammeter with a minimum length of connecting conductors.
It is better to repeat the setting several times with a successive change in the parameters C8, L1, C7, focusing on the minimum current consumed when the oscillatory circuit enters resonance and the maximum bandwidth of the VHF receiver. Therefore, it is more convenient to use a receiver with an arrow setting indicator. And as the power emitted by the radio microphone increases, the distance between the receiver and the RM should be increased.
You can specify the depth of deviation (the magnitude of the change in the frequency of the FM signal) by selecting the capacitance of the coupling capacitor C5 (C5 \u003d 1.2 ... 10 pF). With an increase in C5, the depth of deviation increases. The capacitance of this capacitor should be such that even in the loudness peaks when the receiver is operated from the RM, there are no crackles, distortions, and even more so, excitation and disruption of radio reception. This type of excitation should not be confused with the characteristic whistle that appears when the RM is close to the receiver tuned to its wave. In this case, to remove the excitation (acoustic feedback), it is enough to reduce the volume of the receiver.
Next, the Lien radio microphone is connected to a battery pack (for example, two 3336L batteries), its frequency is adjusted and the range is checked. After tuning, the core of the inductor L1 is filled with paraffin, and the rotors of the trimmer capacitors are stopped with nitro paint.
The tuned Lien radio microphone was tested in operation with the Ishim-003 broadcasting receiver and had a range of up to 500 m (with line of sight).
You can speed up the process of adjusting a roughly tuned RM using a wavemeter (Fig. 2). The wavemeter consists of a parallel oscillatory circuit C1-C2-L1, a diode detector VD1 and a low-pass filter SZ. The parameters of the wavemeter circuit are similar to the parameters of the parallel circuit of the radio microphone. A tester (multimeter) is connected to the sockets XS1, XS2 of the wavemeter in the mode of a DC voltmeter (measurement range - 12 V)
Measurement of the strength of the alternating magnetic field in the antenna PM produced as follows. RM included. The antenna WA1 of the radio microphone (evenly, along its entire length) is wrapped around two or three turns of flexible stranded wire in isolation and pull this wire from the PM antenna in the direction of the arrow (Fig. 2), while simultaneously measuring the voltmeter readings. The maximum readings of the wavemeter are achieved by adjusting the RM contour and the length of its antenna. You can start a similar procedure when using a quarter-wave pin as an antenna. The wavelength L for a given resonance frequency can be calculated using the formula:
L = C/f
where L is the wavelength, m; C is the speed of light (300,000 km/s); f is the frequency in megahertz.
The wavelength L for a frequency of 70 MHz is 4.2857 m, and the quarter-wave pin (L / 4) has a length 4 times less - about 107 cm.
In the RM circuit, resistors of the OMLT, VS and similar small-sized resistors with a dissipation power of 0.125 W can be used. Trimmer resistor R8 - type SPZ-22. Capacitors SZ, C10 - K50-6, K50-16, K50-35 or similar oxide; C1, C2, C4 ... C7, C9 - type KM4, KM5, K10-7 or any other ceramic (non-inductive). Trimmer capacitor C8 - type KT4-23. It is permissible to replace the VD3 D902 varicap with almost any silicon or germanium diode with a capacitance Cd of more than 1 ... 3 pF. You can find a replacement for VD3 using the table.
Transistor VT1 can be replaced by transistors KT315B, G, and VT2 - KT368B. Diodes VD1, VD2 - any silicon with a direct voltage drop of at least 0.7 V. The value of the resistor R6 can be any in the range from 10 to 100 kOhm.
The inductor L1 is wound on a frame with a diameter of 6.3 mm with a PEV wire ø0.5 ... 0.55 mm with a winding pitch of 1.5 mm. L1 contains 5 turns and has a tap from the 4th (top of the diagram) turn. A coil made of silver-plated copper wire has a high quality factor and is easier to enter the generation mode. You can silver the wire in a spent photo fixer (sodium hyposulfite). But the best results are obtained by using ready-made coils from VHF receivers with a resonance frequency of about 70 MHz, for example, from the VHF-2-01E unit from the Ilga-301 radio.
Structurally, the RM is made on a board of fiberglass foiled on both sides with a thickness of 1.5 ... 2.5 mm. One side of the board is a screen, and the other side, cut into 8x4 mm cells, is being assembled. Board size - 110x27 mm.
Microphone for toastmaster
For servicing collective events in enclosed spaces, ordinary home-made radio microphones turn out to be of little use.
First, when designing such devices, the authors mainly pay attention to achieving high sensitivity to weak audio signals and eliminating nonlinear distortions of loud signals by introducing AGC in the modulator. But collective events are always accompanied by background noise, sometimes reaching a significant level. Influencing the sound amplification installation through a constantly on sensitive microphone, this background in the pauses of performances further multiplies the overall rumble in the room. Specialized microcircuits with a compressor and a noise suppressor used in modulators make it possible to find a compromise between the sensitivity of the microphone to weak sounds and the general background noise, however, they are not available to all radio amateurs, and the devices require complex adjustment.
Secondly, all simple radio microphones have another drawback - the uncertain reception of their signals. This happens either due to the "departure" (instability) of the operating frequency, or due to insufficient radiation power. We are not talking about different sensitivity of receiving devices: higher sensitivity of the receiver - more confident reception. High-frequency signals in such radio microphones enter the antenna through the P-loop from the output of the master oscillator. Such a generator, assembled on a single transistor, operates in the limiting mode for direct current and behaves unstably. In addition, the P-circuit connected between the antenna and the generator transistor collector does not eliminate the effect on the generator frequency.
objects located near the antenna. It is possible to significantly weaken the extraneous influence on the generation frequency only with a buffer amplifier loosely coupled to the master oscillator. The antenna and objects located near it only affect the parameters of the buffer (output) power amplifier.
Thirdly, in the VHF-2 broadcasting range, the standard frequency deviation value of 75 kHz is adopted. Of course, such a large deviation is typical only for music programs; when transmitting voice messages, it is usually less. But its too low value in homemade radio microphones leads to a quiet booming and poorly recognizable sound. It is possible to increase the deviation in the transmission of speech signals by fully including the varicap in the oscillatory circuit of the master oscillator, and in order to reduce the distortion caused by the dependence of the capacitance of the varicap on the high-frequency voltage applied to it, use a varicap matrix or, in extreme cases, two
efficient varicaps by switching them on at a high frequency of encounters, but sequentially. As you know, to reduce the noise level when using frequency modulation, the modulating signal is predistorted (raising its high-frequency components) during transmission and their compensation (blockage of these components) during reception. Pre-distortion compensation circuits are indispensable in all industrial FM receivers. For this reason, the signals of homemade radio microphones, where pre-emphasis is not introduced, are received with a noticeable blockage of high frequencies. When designing a radio microphone, this must be taken into account by applying an audio signal to the varicap array through a frequency-dependent circuit.
These factors are taken into account in the radio microphone, the scheme of which is shown in the figure. It consists of a microphone amplifier (DA2), a master oscillator (VT5) with a bias voltage stabilizer (VT2, HL1) and a frequency-modulated VD2 varicap matrix, a power amplifier (VT6), a supply voltage regulator (DA1) and a transmitter voice control unit (VT1 , VT3, VT4).
The author has repeatedly experimented with the K157XA2 chip and chose it for a microphone amplifier due to the large gain, efficient system AGC, a small number of attachments.
Considering the high sensitivity of the microcircuit, the signal to its input (pin 1) is supplied from the BM1 microphone through the resistor R2. To improve the characteristics in the pre-amplifier through the resistors of the microcircuit, the AC feedback is used (pin 2 is not used). Capacitor C2 attenuates high frequency components sound signal manifested as knocks and rustles.
The supply voltage for the BM1 microphone comes from the output of the AGC system (pin 13) through the resistor R1. During the adjustment, in the absence of a voice signal, by selecting this resistor, we
the voltage between the microphone outputs is set in the range of 1 ... 2.5 V. When the AGC system is triggered, the supply voltage of both the microcircuit preamplifier and the microphone decreases, which contributes to greater regulation efficiency. The amplified signal through the capacitor C4 is fed to the input of the main amplifier (pin 5).
The time characteristics of the AGC system depend on the capacitance of the capacitor C8 and the resistors built into the microcircuit. At low capacitance values, the AGC works too quickly, "croaking" sounds appear. At very large capacity(100 uF or more) AGC does not have time to operate at the peaks of the audio signal, which leads to its distortion. The voltage from the output of the amplitude detector available in the microcircuit (pin 9) is used to operate the voice control system.
When pronouncing words in front of the BM1 microphone, voltage surges of up to 1.2 V are formed at pin 9 of DA2, which charge the capacitor C7 through the VD1 diode. When the voltage across this capacitor reaches approximately 0.6 V, transistor VT1 opens, charging capacitor C9. As a result, transistors VT3 and VT4 open and the power amplifier of the radio microphone, assembled on transistor VT6, receives a supply voltage. The transfer starts.
If a voice pause occurs, then after approximately 20 ... 30 s determined by the time constant of the R5C9 circuit, the transistor VT4 closes and turns off the power amplifier. With uniform constant noise, even very loud, there are no voltage surges at pin 9 of the DA2 microcircuit, the VT4 transistor remains closed, and the radio microphone is in standby mode. The current consumption in this case is 4 ... 4.5 mA, during transmission it increases to 25 ... 30 mA. Diode VD1 prevents the discharge of capacitor C7 through the output of the DA2 chip.
Thus, being in constant readiness for operation, the radio microphone does not broadcast general noise, but only reacts to a voice of medium volume from a distance of 10 ... 15 cm. comfortable to work without failures in the broadcast. Switch SA1 selects the option of working with a microphone: when its contacts are open, the voice control system operates, when closed, the transmitter is always on.
The supply voltage of 3 V is supplied to the DA2 chip from the DA1 integrated stabilizer. Although the recommended supply voltage for the K157XA2 microcircuit is 3.6 ... 6 V, experiments have shown that it works quite satisfactorily even at this voltage. The performance of the entire radio microphone is maintained when the voltage of the primary power source is reduced to 4.5 V.
Capacitors SU and C12 are separating. Capacitor C11, together with the introduced part of the resistor R4, is a frequency-dependent pre-distortion circuit of the modulating signal. The L1C13 filter prevents the carrier frequency from entering the microphone amplifier.
The radio microphone master oscillator is assembled on a high-frequency (cutoff frequency - at least 900 MHz) VT5 transistor according to an inductive three-point circuit. Such an oscillator is a little more complicated than one assembled according to the capacitive three-point circuit (requires a tap from the loop coil), but it has better frequency stability and contains fewer capacitors. The capacitance of the coupling capacitor C15 is chosen to be minimal, at which the generator is confidently excited. Under these conditions, the influence of the VT5 transistor on the L2VD2 circuit is insignificant, the losses are minimized and the high quality factor of the circuit is maintained. The stability of the operating point of the transistor VT5 is achieved under-
by connecting the resistor R8 to the bias voltage regulator assembled on the HL1 LED, the current through which is set by the field effect transistor VT2.
The LED simultaneously serves as an indicator of the inclusion of the radio microphone. The voltage of the same stabilizer through the resistor R6 is supplied to the vari-cap matrix VD2, setting its operating point.
The requirements for the accuracy of maintaining the mode of the VT6 transistor in the power amplifier are not so high, so no special measures have been taken to stabilize it. Due to the low capacitance of the isolation capacitor C17, the connection with the master oscillator is weak and the change in the load of the amplifier has practically no effect on the generated frequency. Capacitor C20 eliminates the high-frequency negative feedback created by resistor R11, which increases the gain of transistor VT6. The amplified signal through a matching high-frequency transformer T1, a filter C21L3C22C24 and an isolation capacitor C23 enters the antenna WA1.
The integral stabilizer ZR78L03 (DA1) can be replaced with KR1170ENZ. When choosing a replacement for the diode D311 (VD1), one condition must be met - the minimum forward voltage drop. A D310 diode and a low-power Schottky diode, for example, 1N5817 or similar, will do. Transistors VT1, VT3 are selected with the highest base current transfer ratio. We will replace the KPZOZE transistor (VT2) with any of the KPZOZ series. The criterion for replacing the transistor KP501A (VT4) is the threshold voltage of not more than 2 V. The LED is any low-power one. Matrix KVS111A can be replaced by KVS111B. Ceramic capacitors C15, C17, C21, C24 must have a minimum TKE. Trimmer capacitor C22 - KT4-23 or KPKM, oxide - imported analogues of K50-35. The blocking capacitor C16 is installed near the output of the collector of the transistor VT5, and C19 is the output of the transformer T1, which goes to the power line. Both capacitors are ceramic KM, K10-17. Fixed resistors - S2-23, MLT, tuning resistors - SPZ-38a, SPZ-19a.
Inductor L1 and transformer T1 are wound on ring magnetic cores K7xZ, 5x2 made of 50VN ferrite. It is acceptable to replace it with a magnetic core of size K7x4x2 made of ZOVN ferrite. Choke L1 contains 40 turns of wire PELSHO 0.15. Transformer T1 is wound with two twisted wires PELSHO 0.15. The number of turns is 25. The middle output is obtained by connecting the end of one winding wire to the beginning of another. Coil L2 contains 4 turns (with a tap from the 1.25th turn from the end connected to the common wire), and L3 - 6 turns of silver-plated wire with a diameter of 0.5 mm. Both of them are wound on frames with a diameter of 6 mm from the TV channel selector. Frame length - 16 mm, winding pitch - 1 mm. The coils are arranged mutually perpendicular. Trimmers SS 2.8x12, shortened to 4 mm, are screwed inside the frames. You can use frames and trim
nicknames of other sizes. Formulas for calculating the number of turns can be found in the reference literature.
The establishment of a radio microphone begins with checking the voltage on the capacitors C1 and C14. When the supply voltage changes from 4.5 to 9 V on the capacitor C1, it should remain approximately 3 V, and on the capacitor C14 - 2 V. Having turned off the BM1 microphone, the tuning resistor R3 sets a voltage close to 0.25 at pin 9 of the DA2 microcircuit B. Having closed the terminals of the L2 coil, with the SA1 switch closed, the collector current of the transistors VT5 and VT6 is measured. It should be within the limits of 4.5 ... 5 and 15 ... 18 mA, respectively. If necessary, the current is set by a selection of resistors R8 and R9. After removing the jumper from the coil, a frequency meter is connected to the antenna contact and, by rotating the L2 coil trimmer, the RF master oscillator circuit is tuned, achieving a frequency meter reading of 87.9 MHz, after which the frequency meter is turned off.
Further adjustment is carried out with a connected antenna and an existing VHF receiver. Within the premises, it is sufficient to use as an antenna a piece of a mounting wire about 80 cm long, coiled in a spiral in the body of the radio microphone. You can tune the master oscillator circuit without a frequency meter using a VHF receiver, controlling the reception by ear and counting the frequency on its scale (preferably digital).
After tuning the master oscillator circuit, gradually removing the radio microphone from the receiver and rotating the L3 coil trimmer and the C22 capacitor rotor, the signal is received at the maximum range. This operation is best done with an assistant, and in order to avoid acoustic communication with the radio microphone, it is better to receive during tuning on the head phone, turning off the receiver's loudspeaker.
The frequency deviation is also adjusted with an assistant. The volume control in the receiver is set to the middle position. Having removed the radio microphone from the receiver by 10 ... 15 m (the farther, the better), speak or hum into it in an undertone. According to the instructions of the assistant, one should find such a position of the engine of the tuning resistor R4, at which the voice in the receiver sounds at the highest volume, but without noticeable distortion.
If a blockage or an excessive rise in high frequencies is felt in the received signal, capacitor C11 is selected. Sometimes, if the BM1 microphone has an increased response at high audio frequencies, this capacitor can be omitted at all.
The next step is to check the operation of the AGC. Both soft and loud sounds spoken in front of the radio microphone must be heard in the receiver without distortion. If loud sounds are distorted, you should change the capacitance of capacitor C8 or install a resistor in series with capacitor C4, the resistance of which is selected experimentally.
The voice control system does not require adjustment. It should only be noted that the turn-on delay is proportional to the capacitance of the capacitor C7. It is not advisable to install a capacitor with a capacitance of less than 10 uF here, since the radio microphone begins to behave unpredictably. The turn-off delay is corrected by selecting capacitor C9. The voice control system can, of course, be excluded and switch SA1 replaced by a jumper. There is no need to install transistors VT1, VT3, VT4, diode VD1, capacitors C7, C9 and resistors R5, R7, but capacitor C5 remains in this case. The device turns into a conventional radio microphone capable of broadcasting weak audio signals.
To increase the receiving range, the capacitance of the capacitor C23 should be increased to 33 pF, and when transmitting signals over a distance of 100 m or more, you can try the option proposed in. However, stable reception can only be guaranteed by high-quality VHF-2 receivers. Unlike cheap or simple home-made ones, in combination with good sound fidelity and high sensitivity, they also provide noise suppression in the pauses of the radio microphone. There is no need to keep his transmitter constantly on, wasting power. With such receivers, the advantages of the voice control system of this radio microphone will be fully realized.
LITERATURE
1. Naumov A. Radio microphone. - Radio, 2004, No. 8, p. 19.20.
2. Kuznetsov E. Microphone without wires. - Radio, 2001, No. 3, p. 15 17.
3. Markov V. Musical synthesizers. - Radio, 2004, No. 12, p. 52, 53.
4. Markov V. Signaling device on the K157XA2 chip. - Radio, 2004, No. 8, p. 60.
5. Ivashchenko Yu., Kerekesner I., Kondratiev N. Integrated circuits of the 157 series. - Radio, 1976, No. 3, p. 57, 58