DIY wireless microphone. DIY radio microphones
But in terms of its ease of setup, stability (when the power changes from 2 to 12V, the frequency changes by only 0.1 MHz) and operating range (200 m for a conventional Chinese receiver), there is no better than this radio microphone circuit. It is her assembly that we will consider.
Radio microphone - diagram and description
The first stage on the transistor VT1 - KT3102 amplifies the signal from the condenser "button" microphone, and also sets the generator DC mode on the transistor VT2. As it can be used KT368, as the most stable in operation.
The VT3 transistor amplifier operates in class C with high efficiency. When the supply battery is discharged below 5V, VT3 closes and the signal from the generator to the antenna goes through the base-collector pass-through capacitance.
These ratings of the radio elements were repeated many times, so the setting consists only in stretching and compressing the L1 coil to select the desired frequency. It will be useful to provide the circuit with an LED signaling the inclusion and sufficient supply voltage. A small increase in current consumption (approximately 2 mA) is compensated by the convenience of control.
The circuit is powered by a krona battery and consumes a current of about 15–18 mA.
- Read also how to
Coil L1 contains 8 turns of PEL 0.8 wire with a tap from the middle, wound on a mandrel with a diameter of 4 mm. Some wound on 4.5, it's not scary. In this case, 9 turns of wire 0.5–0.8 mm were obtained, 4 turns to the side to the conclusions. On the middle turn, you need to make a tap with a soft thin wire.
The Dr1 inductor is wound on a K7x4x2 ferrite ring and contains 5–10 turns of PEL 0.2 wire. For the antenna, 80 cm of wire with a diameter of 1–1.5 mm is taken and wound evenly on an AA battery.
The whole design fits perfectly into a pack of cigarettes, the beetle can be picked up and there is practically no frequency drift. You can simplify the circuit by eliminating the RF amplifier. In this case, the current consumption is reduced to 5 mA, and the range is reduced to 50 m. Below is a photo of the finished radio microphone, made on planar parts.
Capacitor C3 serves to prevent self-excitation of the radio microphone by HF and its capacitance is selected in the range of 100–1000 pF.
- Scheme and recommendations for assembly
The capacitance of the isolation capacitor C7 is chosen so small in order to reduce the influence of the antenna and the output stage on the frequency of the master oscillator. It is possible to increase the radiation power of the radio microphone, and as a result, the range by increasing the value of this capacitor to 10 pF, however, the influence of the antenna on frequency stability will also increase.
The master oscillator retains its performance even when the supply voltage drops to 0.8V! Therefore, if it is necessary to power the circuit from a low-voltage source with a voltage of 3–5 V, the output stage on the VT3 transistor should be switched to mode A. To do this, we put a 100 kOhm trimming resistor between the base and the plus of the power supply. Having set with its help the quiescent current of the output stage within 5–10 mA and measuring the resulting resistance with an ohmmeter, we replace it with a constant one.
During assembly, many users noted that it is better to choose a better quality Kron battery (from 50 rubles on the price scale), since cheap ones quickly fail.
In practice, it has also been shown that the current consumption varies between 18-25 mA, depending on how it is configured. At a current of 15 mA, the generation in the generator starts to break down. Above 25 mA on these parts (in particular transistors) the UHF may overheat due to a high signal level, which leads to excessive current consumption, inefficient use and, as a result, failure of the third transistor.
At a current of 20 mA, as a rule, the RF indicator goes off scale at the antenna. If the transistor heats up at a current of 20 mA, then something is not configured or done incorrectly, probably a mismatch between the generator and UHF stages. For some reason, some users put a capacitor there over 30 pF and consider this the norm. The place there for the capacitor is 3–10 pF and no more. There is no need to overload and deactivate UHF, it is better to tune the generator than load it with harmonics and poor narrow deviation.
In the ULF, instead of more than 400 kOhm, it is better to set the resistor to 100 kOhm. A capacitor that sends a signal to the base of 0.01 microfarads will more lead to level locking. With such ULF parameters, the sound is clear and a good new microphone catches even how you turn the pages in a book at a distance of 6–7 meters!
The microphone itself produces a powerful signal. In single-transistor beetles without an amplifier, it can give out 3-4 meters of good audibility, so there is no need to drive ULF into extreme modes either, so that later you don’t have to worry about how to remove distortion.
In UHF, transistors behave well, except for c9018, and in the generator this is the best option.
ULF can be set with s9014, as an option, something Soviet, since there are many such colors (KT315, for example)
More about the capacitor. As a rule, in the circuit the best option is 12 pF. We solder it closer to the circuit and subsequently fill it with silicone along with the coil and the generator transistor. In terms of power, the choke is imported small-sized at 100 microhenries. If you put a 47 microfarad capacitor, it will smooth out everything superfluous.
Haven't you found a scheme that combines the quality of work, cost, lightness and the most minimal current consumption parameters providing reliable communication at a distance? Then this article is for you!
After assembling a miracle radio microphone made in China, which I bought on Aliexpress for $1.63, I released this video:
And I'm not the only one who got the same results after the build:
the board is simple, when soldering the contacts from the textolite sometimes fall off, which is a big minus, and the delivery was fast, the transmitter works, but not far, I would add a sound amplifier there, since the sound from the microphone is very quiet and you can only hear it when you speak directly into microphone
- real customer review from the seller's product page
That is why I propose to read this article, which I wrote back in 2007, the figure below shows a schematic diagram of a transmitter designed to operate in the VHF band:
Rice. 1 Schematic diagram of the transmitter
The signal from the microphone is taken through the resistor R2 and the capacitor C2, the sensitivity of the microphone is set to the resistance R1, but you need to make sure that the voltage on the microphone is not exceeded, its maximum value.
Next, the signal passes through a filter consisting of R3 and C3, and is fed to the base of the transistor VT1, moreover, with two intersecting frequencies from the microphone output and filter oscillations. Further, an already amplified signal is removed from the output of the transistor, on the collector, and using a filter built on a capacitor and an inductor (C4, L1), we select our operating frequency of the radio transmitter, the capacitor C5 serves as a load for high frequency, thereby creating capacitive resistance.
The circuit uses low power resistors mlt-0.125 W, if necessary, if it is necessary to develop a large transmitter power, it is advisable to use the resistance R4 brand mlt-0.5W. Capacitors used series k10-17, although any ceramic will do.
The transmitter consumption voltage is from 1.5 V to 3.5 V. To operate the transmitter over a voltage of 3.5 V, it is necessary to replace the resistors R1, R3, R4.
Replacing parts when powered by 3 Volts, some components did not change, so I left them unchanged so as not to mislead you:
- R1 - 10 kOhm
- R2 - 18 kOhm
- R3 - 36 kOhm
- R4 - 75 Ohm
- C1 - 0.47 uF
- C2 - 0.1 uF
- C3 - 1000 pF
- C4 - 33 pF
- C5 - 10 pF
- C6 - 47 pF
- L1 - 5 turns (on paste d = 3 mm)
- Antenna 20-40 cm
The low-frequency part of the transmitter, assembled on an electret microphone, has a certain spread of parameters when the voltage across it changes, this is especially reflected in its sensitivity. Electret microphones have good electro-acoustic and technical characteristics:
- wide frequency range;
- small uneven frequency response;
- low non-linear and transient distortions;
- high sensitivity;
- low self-noise level.
Electret microphones, according to the principle of operation, are the same as condenser ones, but the constant voltage in them is provided by an electret charge, a thin layer deposited on the membrane and retaining this charge for a long time (over 30 years).
The L1 coil of the radio microphone is wound on a 3 mm frame, the basis of which is an ordinary ballpoint pen paste, with a PEV 0.8 wire of 4-5 turns (in my case 5) wound turns to turn, this coil is from me, and the standard one is drawn on the board, with tracks in spiral shape: 
The current consumption from 1.5 Volts is only 2 mA and the range at the same time reaches 27 meters, with an antenna length of only 15 cm.
I continue my description, but now the goal is not a simple radio microphone, but a real one bug.
The task was to achieve a stable connection at a distance of 50 meters, with the minimum size of the device and a duration of at least 1 hour. At the same time, the sensitivity of the microphone should be sufficient to listen to conversations in small rooms (offices, classrooms). In my case, a small meeting of people in the principal's waiting room.
Printed circuit board:
The power supply voltage of the radio microphone was 3 volts, from two AG13 batteries connected in series, the duration of operation was about 2.5 hours, the current consumption was 7 mA.
As for the sensitivity of the microphone, I selected a resistance of 1.1KΩ, put a variable resistance of 15KΩ in its place, and in working condition achieved the desired signal level. Just before switching on, you need to make sure that this resistance is not too small, because. it is possible to burn the circuit inside the microphone, for safety reasons, I usually solder this resistance in series, which results in 1.1KΩ-constant, 15KΩ-variable, then in this case if the variable is at resistance = 0, the total is 1.1k.
I know about the typo (the photo was taken in my youth, I post it as it is)!
Another plate is put on top of the case, which is screwed onto small screws and presses a small metal plate that tightly fixes the batteries to the tracks and connects them together.
Concluding the article, I will say that this radio microphone has been working since 2007, it is just as stable and resistant to pickups, and for me it has no analogues among similar ones!
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 of copper wire with a diameter of 1-1.2 mm, wound on a mandrel 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 the frequency setting circuits. Capacitors C1 and C7 should 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 in the range of +0.8 ... +1.2 V. Two silicon diodes are connected in parallel with the tuning resistor R8, which stabilize Ucm and minimize the frequency drift of the generator 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 for ensuring the stable operation of the RM is the mechanical strength (immobility) of the components of the 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 PM;
* 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 L1 coil 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 tuning 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 a flexible stranded wire in insulation and this wire is pulled from the antenna PM 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 stabilizer (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 its high gain, effective AGC system, and 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 the high-frequency components of the audio 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. With a very high capacitance (100 uF or more), the 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 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 responds 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 master oscillator of the radio microphone is assembled on a high-frequency (cutoff frequency - at least 900 MHz) transistor VT5 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 equal to approximately 3 V, and on the capacitor C14 - 2 V. After turning off the microphone VM1, a trimming resistor R3 is set at pin 9 of the DA2 microcircuit to a voltage close to 0.25 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 should be heard in the receiver without distortions noticeable to the ear. 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
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 the 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 the 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.
Download: Simple Radio Microphone
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The idea of creating this radio microphone was born on the day when I was engaged in the manufacture of the PM on the PIC12LF1840T48, designed by the famous master of his craft, Blaze.
There was little space left on a piece of textolite, and I was too lazy to cut, so I decided to make a couple more boards, simply replacing the node on the PIC controller with a MAX1472 chip.
Radio microphone circuit
In fact, the radio microphone itself is not something fundamentally new, but is a compilation of well-known blocks that have proven themselves in practice, namely:
- Microphone amplifier, by Christian Tavernier, assembled on a dual, low-noise op-amp TL082 with gain control;
- Master oscillator and modulator - built on the basis of the MAX1472 transmitter chip, which has proven itself well in the "R series" radio microphones;
- UHF on a BFG540 transistor, used in a radio microphone on a PIC controller.
The scheme of the device is simple to disgrace, so I ask you not to kick it right away:
Printed circuit board
The printed circuit board is not the "top" of miniaturization and has dimensions of 33x22 mm. The foil on the back is not removed. There are 3 0.5mm holes drilled in the board. to supply (+) power. They are indicated on the wiring diagram. This connection can also be made from the mounting side of the elements. As you like ... PCB file in Visio2003 format you can
PCB manufacturing (small lyrical digression)
The main difficulty for many novice radio amateurs in the manufacture of such products is the manufacture of a printed circuit board for a modern element base.
Of course, you can order PP in production, but its price will be “golden” in the conditions of a poorly developed technological base of our enterprises and the desire of merchants to get 1000% profit from any order.
Therefore, radio amateurs have to master a variety of ways to produce printed circuit boards at home.
For a couple of years now, I have switched from the LUT method to manufacturing boards using photoresistive technology. With this manufacturing method, the quality of the boards practically depends only on the quality of the drawing,
that your printer can reproduce. This method is more reliable and efficient than LUT, although it does require some initial investment to purchase the necessary materials. Beginners are frightened by the apparent complexity of the technology and the unpredictability of the result.
I believe that this is an international conspiracy of capitalists who do not want young talents to develop in our country and global innovations to be born 🙂 !!!
In fact, everything is simple, no magic and magic, and you don’t need to go to Hogwarts. The process of manufacturing boards by the photoresistive method consists of 6 stages and, on average, takes me from 40 to 60 minutes.
For this process you need:
- Transparency film for laser printers, sold at the stationery store;
- Toner to increase the optical density of printing (Density-toner)
- Small or large can of photoresist Positive 20;
- A piece of transparent plexiglass 1-2 mm thick. (preferably new and not scratched);
- UV lamp (black) or other source of UV radiation (for example, an LED matrix), in extreme cases, a conventional high-power energy-saving lamp of 150-200 W is suitable;
- Caustic soda (NaOH).
All this junk looks like this:
STEP 1. Create a stencil.
We take any drawing program, vector (I use Visio) or pixel editor or specialized software for designing PCBs, of which there are quite a lot.
Figure PP in the "positive" - lanes must be black- printed on a film for a laser printer. If you have a printer with a new cartridge, then your stencil will turn out to be optically dense.
But it is better to sprinkle it with a special toner (I use Density Toner from Kruse, made in Italy), which increases the optical density of the dye by dissolving it. Dry for a couple of minutes and our stencil is ready.
STEP 2. Application of photoresist
This is the most critical stage of the whole process and should be carried out in a darkened room. The textolite blank is well washed with a fine powder for washing dishes (commet or similar). If the foil textolite is completely old or oxidized, it is better to go over it with sandpaper No. 1000-2500. Then degrease with acetone and no longer touch. Shake the can of photoresist for a minute and cover the fat-free workpiece with a thin layer of photoresist. Here you need to adapt a little, you can cover in 1 layer, you can in two (for example, along and across). It has a bluish tint and the thicker the layer, the darker it is. A thicker layer - requires a longer illumination. Do not be embarrassed when you see a lot of air bubbles in the newly applied layer of photoresist - they will disappear when dried. We leave the board in a dark room for initial drying - 3-5 minutes. It is advisable to do this in a room where there is less dust. I do it in the bathroom.
STEP 3. Drying of the photoresist
Preheat the oven to 50-60 degrees. We transfer the board, protected from direct light, to the oven. Maintain the specified temperature for 15 minutes. periodically turning the oven on and off. We do not allow the board to overheat above 70 degrees otherwise the photoresist will lose its properties. Turn off the oven and let the board cool down to room temperature. After cooling, the board is ready for illumination.
STEP 4. Flare
A stencil is applied to the foil textolite coated with photoresist, a piece of transparent plexiglass is placed on top and this whole structure is clamped to prevent the stencil from shifting relative to the textolite. For illumination I use 40W. UV lamp by simply placing it above the stencil at a distance of 5-10 cm. Typically, for small boards, the exposure time is 15-20 minutes. With a more powerful source of UV radiation, it will take less time.
During the lighting process, periodically move the exposed area slightly (because the light sources give an uneven radiation flux) to ensure an equal level of illumination of all areas of the board.
STEP 5. Development
We place the illuminated board in a NaOH solution - a small teaspoon of 0.5 liters. water at room temperature. In this solution, areas of the photoresistive layer exposed to ultraviolet light are washed off (for positive technology). The process usually takes 1-2 minutes. After that, the board is washed and ready for etching. At this stage, need to do quality control your board and correct the flaws that have arisen: using a thin scalpel, cut tracks in the photoresist or draw / correct the missing elements with a special marker. If as a result of development Not all of the drawings are highlighted or due to the high concentration of alkali all the photoresist washed off- you need to return to stage number 2 and start all over again.
STEP 6. Pickling
We poison the board in any, the usual way. I don’t know about acids, but ammonium persulfate, ferric chloride, vitriol with salt - Positiv 20 photoresist withstands easily. We wash the board in running water and wash off the photoresist with acetone. The board is ready to use.
OK it's all over Now. Particularly impressionable people, looking at the board and wiping tears of joy from their cheeks, will ask themselves the question: Why didn’t I do this before? At least I asked myself...
Mounting elements
The radio microphone uses resistors and capacitors of size 0805. The mounting diagram of the elements and photographs will help you figure out what and where to solder.
Radio microphone setup
Properly assembled and well-washed from the flux, the radio microphone practically does not need to be adjusted. I made two instances of the device at different frequencies and both earned without any questions. With a 13 MHz quartz resonator, the frequency of the device was 416.045 MHz.
The trimming resistor sets the required sensitivity for the microphone input. This amplifier is quite "clamped" and does not tend to self-excite due to the rather low overall gain. If necessary, you can still play with the resistor values to get more sensitivity.
But at the same time, it must be remembered that increasing the gain leads to an increase in output noise. I also want to note that a very important element of any radio microphone is the microphone itself (pun, damn it ...). Choosing a microphone for maximum sensitivity and minimum noise is also an important tuning step.
The best result was shown by ordinary electret microphones, torn from old Panasonic cordless phones (not cellular).
Trimmer capacitor C1, - set the device to the maximum current consumption. With the ratings indicated on the diagram, the current consumption should be within 50-55 mA. In this case, the radiated power will be 70-85 mW.
Conclusion
In conclusion, I would like to add that this is one of the best radio microphones(which I managed to collect in my practice) by combining such characteristics as sound quality, frequency stability, output power, practicality and manufacturability. In most cases, if all components are working, it does not need to be configured. You can experiment with microphones, quartz resonators and ogres. resistors to achieve the best sound quality and transmission power.
Radio amateurs who want to assemble this transmitter and conduct experiments with it, manufactured under the brand "MIKROSH".