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Do-it-yourself radio microphone 150m
Device Diagram:
About work:
The master oscillator is assembled on a KT368 transistor, its DC operation mode is 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.
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 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 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 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 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 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 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 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 WA1 antenna 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 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 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 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 changing 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 performed 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 altogether.
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 out 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
I don’t know how useful this can be and whether it is necessary in the household, but it can be used as a wireless “nanny” or an additional alarm for a car. Its range is sufficient to hear a child waking up in the next room or a car roaring from the opposite side of the house. The entire modernization will consist in adding one amplifying stage to increase the sensitivity of the microphone, and with a slight movement of the hand, you can increase the sensitivity of the receiver and thereby increase the radio range.
I'll tell you how I finalized the ODEON SD-410 wireless karaoke microphone. By the way, a very convenient model, since it is equipped with a full-fledged superheterodyne receiver, which allows you to receive an FM microphone transmitter outside the FM band completely clogged with radio stations (88 -108). The CD 1191ACB chip itself (CXA 1691BM , its analogue) of the receiver allows direct connection of a headphone or a dynamic head with a resistance of 8 ohms, which makes it possible to mobile use a receiving box, a little smaller than a pack of cigarettes, and not to carry a music center.
Microphone design consists of a dynamic head with a winding resistance of 600 ohms, an audio frequency amplifier and a parametric (soft) high (116 MHz) frequency generator. The low-voltage power supply of one element equal to 1.5 V is converted to a voltage of up to 5 V thanks to a simple voltage converter made on a single transistor.
For me, the most difficult part was to disassemble the microphone housing. I hardly guessed that the circuit board rests on the switch, and the switch body is screwed to the cylindrical body of the microphone.
All work consists in installing an additional audio frequency amplifier board in the gap of the wires coming from the microphone. Thus, input and output wires, including ground or negative wires, are known. It remains to connect the power, or find the output of the voltage converter, from which the radio microphone itself works, since the product itself is powered by a 1.5-volt battery. Converter voltage 4.6 V, from this point and feed an additional sound amplifier stage.
Transistor T1 - BC850
Resistors: R f - 1 kOhm, R k - 10 kOhm, R bq - 910 k, R e - 330 Ohm.
Capacitors: Cp -0.1 uF, Sv - 470 pF, Sf - 4.7 uF.
sound amplifier has a gain of K = 20, you will even hear the baby's breathing. The amplifier is made on a single transistor. Due to two negative feedback and low current mode, it has a low noise level. The cascade includes the simplest correction of the amplitude frequency response, these are separating capacitors Ср, providing a blockage in the low-frequency region and a capacitor Cv, providing a blockage of the high frequencies, thus only the spectrum of the speech signal is distinguished.
A correctly assembled amplifier, when connected, will immediately manifest itself as a howl of the speakers of the music center, due to the increased sensitivity of the radio microphone, which will lead to an increase in positive acoustic feedback, which can be eliminated by moving the microphone to another room or using headphones.
You can further increase the sensitivity of the microphone by reducing the resistance of the resistor R e to 50 ohms. The microphone itself has a narrow radiation pattern and this feature can also be taken into account by directing it towards a stroller or other sound source.
Receiver design. This is a superheterodyne receiver with a single frequency conversion, with an intermediate frequency of 10.7 MHz, with a fixed tuning to one receive frequency. Tuning to the microphone frequency is carried out by a variable capacitor and is held by APCG (automatic adjustment of the local oscillator frequency). The supply voltage from one cell is also converted to a voltage of 3 V.
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| Photo 4. Disassembled receiver. |
When I began to practically study the receiver, I was very surprised. The current consumption was 65 mA! Two free places for electrolytic capacitors caught my eye. I filled the places with denominations of 220 microfarads and the consumption dropped to 19 mA. That's such a savings!
When connecting a measuring generator and an oscilloscope, I was confused by the sensitivity, only 50 microvolts. I took a toothpick and with a slight movement of my hand tried to move the input coil, squeezing and unclenching the turns at the level of a weak signal from the generator, achieving maximum sound with a minimum of noise at the output, and thus additional tuning of the circuit improved the sensitivity of the receiver to 5 microvolts.
The 27th output of the microcircuit through an electrolytic capacitor, then through a divider of two resistors goes to the plug (microphone input of the music center). Directly after the condenser, you can install a headphone jack. The best results were obtained by connecting telecoils in series. Even better, if the resistance of the telephone heads is not 15-17 ohms, but 33 ohms. This is due to a low-current stabilizer, which does not hold voltage with increasing consumption.
Tests of the modified device showed a confident radio range of up to 50 meters in line of sight. In the room, radio communication is provided up to 15 meters, taking into account two solid reinforced concrete walls located between the receiver and the radio microphone.
You can stop at this for now, it’s quite enough for a babysitter radio. True, for this kit this is not the limit.
To increase the communication range, the shortened radio microphone antenna can be replaced with a stranded copper wire 65 centimeters long (a fourth of the wavelength). It is advisable to replace the filamentous receiver antenna with a stranded copper wire of a larger cross section and the same length as the radio microphone antenna.
The receiver has a volume control.
It is better to get rid of the converter completely, whistles and beats will disappear, and power the receiver (26 pin of the microcircuit) from two elements, thus increasing the voltage to 3 volts or from one telephone battery with a voltage of 3.7 volts. In this case, you can use a loudspeaker with a resistance of 8 ohms by increasing the capacitance of the coupling capacitor to 470 uF.
Receiver sensitivity can be improved to 1 microvolt by adding a high frequency resonant amplifier. Cm.
The manufacture of a bug radio microphone attracts many, especially beginner radio amateurs. And most often they try to repeat, considering them easier to manufacture. Yes, on the one hand, this is true, but in terms of tuning, it is better to choose a three-stage radio microphone circuit, where each transistor has its own role: a microphone amplifier, an oscillator and an RF amplifier. In this design, each cascade of the bug can be easily and conveniently adjusted individually. Of course, 3 times more details will go to it, but the characteristics (sensitivity, stability, radiation power) will also improve. It is on this principle that the Filin-3 radio microphone circuit works, which I found on the Internet and successfully repeated.Radio microphone details:
VT1 - KT3130B
VT2 - KT368A
VT3 - KT3126B
R1 - 12 kOhm
R2 - 300 kOhm
R3 - 4.7 kOhm
R5 - 20 kOhm
R6 - 200 Ohm
R7 - 200 Ohm
C1 - 100-300 pF
C2 - 0.03-0.1 uF
C3 - 0.03-0.1 uF
C4 - 500-1000 pF
C5 - 22 pF
C6 - 12 pF
C7 - 39 pF
C8 - 0.1-0.5 uF
Transmitter specifications:
Frequency: 88-108MHz
Range from 100 to 1000 m - depending on the antenna
Power supply 9V (Krona)
Output power 50 mW
Current consumption 25 mA
Microphone sensitivity 5 m
Microphone M1 type MKE-332 or any button microphone. The length of the antenna for a good range is 95 cm. The antenna should be placed vertically and away from metal objects. Reducing the length and using a helical antenna will reduce the range accordingly.
Coil L1 contains 6 turns of 0.4 mm wire on a mandrel with a diameter of 3 mm. We wind 2 turns, make a tap to R7 and wind the remaining 4 turns. Choke DR1 - 20 turns of wire 0.1 mm on a small ferrite ring 2x4x7. Any ready-made with an inductance of 100 μH will do. I took from the Chinese receiver.
The frequency of the device is adjusted by compressing and decompressing L1. You can catch on any mobile phone with an FM band.
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.
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:
The scheme of the device is simple to disgrace, so I ask you not to kick it right away:
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
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:
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.
While lighting, periodically move the exposed area slightly (because the light sources give an uneven radiation flux) to ensure that all areas of the board are exposed to an equal level of illumination.
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 the tracks in the photoresist or draw / correct the missing elements with a special marker. If as a result of developing 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.
Well that's all. 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...
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.
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 radio telephones (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.
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".
