Simple audio transmitter. Simple and cheap DIY radio transmitter

The presented radio bug with its own hands can transmit sound over a distance of up to 500 meters. You can also use it to make an FM tuner and transmit a signal from your phone to the radio.

Radio transmitter for KT368

DIY radio transmitter for KT368

In this article I want to talk about a radio transmitter using a single transistor.

It can be used both for wiretapping, and you can also use it to make a repeater, replacing the microphone with an audio signal input.

DIY radio transmitter on MC2833

DIY radio transmitter on MC2833

Using the MC2833 chip you can make a fairly high-quality FM transmitter. This chip contains an oscillator, an RF amplifier, an audio amplifier and a modulator. Available in a miniature plastic housing with end leads for surface mounting and a standard housing.

DIY FM transmitter for 1 km and above

DIY FM transmitter for 1 km

This is a fairly powerful 2 W FM transmitter that will provide up to 10 km of range, naturally with a well-tuned full antenna and in good weather conditions, without interference. The scheme was found in the burzhunet and seemed interesting and original enough to be presented to your consideration))

DIY stereo radio transmitter circuit

DIY stereo radio transmitter

In a car, when it is not possible to turn on music from other sources like the radio, and at the same time you want to listen not only to what the radio presenters provide, but to your own music, as an option you can use the one made DIY FM stereo transmitter .

The radio transmitter is assembled in a standard plastic case from some device. The front panel has an audio jack input and a setup button. There is a power connector on the back surface. The filter output is connected to the +12V terminal, so the power cable is used as an antenna. The PCB is secured with only one screw inside the box.

Audio transmitter

DIY audio transmitter (music transmitter)

In this article I want to introduce music transmitter. I tried to assemble a radio transmitter using a varicap in the modulator. Since it was needed to transmit an audio signal and not a conversation, I installed a plug instead of a microphone. Coil of 9 turns of wire with a diameter of 1 mm, the middle tap is sealed. I pushed a small piece of foam rubber inside the coil and dripped it with paraffin (a candle) so that the coil would not bend when touched, because the frequency depends on this, and it is very easy to knock it down.

DIY stereo transmitter circuit diagram

Radio stereo audio transmitter circuit

For stereo transmitters there is specialized chip BA1404.ABOUTfeature transmitter on BA1404 is high quality sound and improved stereo sound separation. This is achieved by using a 38 kHz crystal oscillator, which provides the pilot tone frequency for the stereo encoder.

A stereo transmitter can be used both at home and in a car to transmit sound from a storage device (phone, player, etc.), since it does not transmit stereo sound.

Such a small stereo transmitter will be a good replacement for an FM tuner.

DIY FM transmitter

FM radio transmitter

Do-it-yourself VHF-FM radio transmitter, operates in the non-traditional range of 175-190 MHz. This radio microphone is easy to assemble. In order to increase the frequency stability of the master oscillator, the base circuit of the power amplifier transistor is powered from a voltage stabilizer (R5, LED1).

Used SMD RED LED. The frequency drift when the power supply drops from 3 to 2.2 volts is no more than 100 KHz. When you touch the antenna with your hand, the frequency also deviates slightly. If you have a receiver with a good AFC, it tracks this change and the frequency shift does not occur at all during the operation of the transmitter.

Do-it-yourself powerful radio transmitter for 500 meters

DIY radio microphone for 500 meters

I want to present the design enough powerful radio bug, Range which amounts to 500 meters with line of sight. The device was assembled almost a year ago for my own needs. Beetle showed amazing results: The frequency hardly fluctuates (every 100 meters by only 0.1-0.3 MHz). The device does not respond to touches of the antenna and other parts (except for the circuit and the frequency-setting circuit) - this is very important point, since almost all schemes from the Internet have such a problem.

In the practice of creating radio bugs, we often encounter the problem of the minimum possible size of a bug. Today we will talk about just such a bug: NEMESIS-2, as it was named. Nemesis was assembled on SMD components, due to which it became possible in a significant way downsize bug several times, the radio bug is so small that it will fit, for example, in one cigarette, lighter or mobile phone. A little about the parameters: frequency range within 88-108 megahertz, microphone sensitivity about 5 meters, you can hear ticking in a quiet room wall clock. So this signal is easy to receive from this bug to the radio receiver, whether it’s on a phone or just a landline one. Let’s move on to the diagram and details.

The circuit of the proposed miniature radio transmitter is easy to manufacture and consists of accessible components, so it will not be difficult for a novice radio amateur to assemble it. Signals can be received using any FM radio you have.

In this case, the range can reach 100 m (depending on the supply voltage). The operating frequency is somewhere between 80 – 100 MHz.

The radio transmitter (bug) circuit can be divided into two blocks: low-frequency and high-frequency. A microphone amplifier built on a single transistor Q1 occupies most of the device and is highlighted in the diagram green.

The RF generator on transistor Q2 is highlighted in red.

To assemble the radio transmitter you will need the following components:

• Transistor 2N3904 (Q1,Q2)
• Resistor 2.2 kOhm (R1)
• Resistor 22 kOhm (R2,R3)
• Resistor 4.7 kOhm (R4)
• Resistor 1 kOhm (R5)
• 100 ohm resistor (R6)
• Capacitor 0.047 µF (C1)
• Electrolytic capacitor 10 µF (C2)
• Capacitor 0.22 µF (C3)
• Electrolytic capacitor 0.47 µF (C4)
• Trimmer capacitor 10-50 pF (C5)
• 5 pF capacitor (C6)
• Capacitor 0.022 µF (C7)
• Contour coil (L1) frameless with a winding diameter of 6 mm, contains 5 turns of enameled wire
• Antenna (Ant 1) - a piece of multi-core insulated wire 15-30 cm long.
• Any electret microphone from a telephone, tape recorder, etc.
• Battery (3-9 v)

As for the contour coil, it is not difficult to make. A piece of enameled wire with a diameter of about 1 mm is wound turn to turn on a 6 mm frame. You can use a drill, screw or any other suitable material.

Before assembly, it is advisable to sort the components according to schematic diagram, this will give some idea of ​​how to arrange the parts on the board.

It is best to start with the amplifier stage (marked in green in the diagram).

We solder the leads of the parts together according to the diagram.

During the assembly process, it is important to properly solder the microphone to the board. To do this, you need to understand where his plus is and where his minus is. This is easily determined visually: the minus is usually electrically connected to the microphone body.

After all amplifier components are assembled breadboard, temporarily solder a small piece of wire to control point TP1 and power the cascade from a source with a voltage of 3 to 9 volts. This will allow us to check this part of the transmitter for functionality. To do this, we connect some headphones to the above wire (TP1) and to the negative of our circuit (GND). By clicking on the microphone we should hear cheeks.

After we have made sure that the audio stage is functioning normally, we assemble the rest of the transmitter.

Well that's it! There's only one thing left to do - we need to configure our bug.

Using a plastic screwdriver, we adjust the transmitter circuit to the frequency of a nearby radio receiver, which manifests itself strong sound acoustic resonance.

Radio transmitting devices (Fig. 13.1 - 13.5) can be obtained by simple union low frequency amplifier (or generator) (ULF, LFO) and generator high frequency(GHF).

The block diagram of an amplitude modulation (AM) transmitter, which is used primarily in the long, medium and short wave bands, is shown in Fig. 13.1. The audio frequency output signal produced by the VLF or LFO is isolated at the load resistance Rh, which is connected to the power circuit of the HHF circuit. Since the RF generator supply voltage varies proportionally to the audio frequency signal, the amplitude of the RF signal is modulated. The generator shown in Fig. 1 can be used as an HHF. 13.6. Points A, B, C, D on the generator diagram correspond to its connection points on the block diagrams (Fig. 13.1 - 13.5).

One way to obtain amplitude modulation of a signal using a low-frequency choke or winding of a low-frequency output transformer is shown in Fig. 13.2. The use of inductors whose resistance alternating current increases with increasing frequency, allowing you to increase the modulation depth. In addition, the amplitude of higher frequencies increases sound range, which significantly increases the intelligibility of the signal upon reception.

Frequency modulation (FM), usually used in the ultrashort wave range, changes the frequency of a high-frequency signal. To obtain a frequency-modulated signal, the circuits presented in Fig. 1 can be used. 13.3 and 13.4. In the transmitter circuit (Fig. 13.3) frequency modulation High-frequency signal generation occurs by applying an audio frequency signal through a relatively small capacitor to the base or emitter of an HHF transistor. In this case, the interelectrode capacitances of the active element (transistor) change and, consequently, the resonant frequency of the oscillatory circuit, which determines the generation frequency, is modulated. Strictly speaking, with this type of modulating voltage supply, shallow amplitude modulation is simultaneously carried out, since the voltage at the base (or emitter) also changes in proportion to the modulating signal.

Frequency modulation “in its pure form” can be obtained using the property of a varicap, or its analogue, to change its capacitance depending on the magnitude of the applied voltage (Fig. 13.4). In this scheme, modulation is turned on/off by switch SA1. The RA potentiometer is designed to check the frequency limits of the oscillator tuning.

Amplitude modulation of a high-frequency signal can be obtained by turning on the HHF instead of the load resistance ULF (LFO) (Fig. 13.5). Capacitor C is designed for high-frequency grounding of the MHF power circuit.

In addition to amplitude and frequency modulation of the signal for data transmission and radio communications, single-sideband, less often phase, and other types of modulation are often used.

In Fig. 13.7 - 13.16 are given practical schemes micro-transmitting devices operating in the VHF-FM range (66...74 or 88...108 MHz). The power of these transmitters is low (from fractions to units of mW), so their radiation does not interfere with radio and television reception. The distance at which signals from such devices can be detected (Fig. 13.7 - 13.16) usually does not exceed several meters. Note that the power of local oscillators—high-frequency generators used in any radio or TV—often exceeds several milliwatts.

In designs according to Fig. 13.7 - 13.10 and 13.12 used electret microphones such as MKE-333 or MKE-332, as well as MKE-3, which contain a built-in preamplifier field effect transistor. Instead of an electret microphone, an electromagnetic telephone capsule can be used, connected between point A and the common wire (Fig. 13.7, 13.9, 13.10 and 13.12) or the power bus (Fig. 13.8). In this case, resistor R1 is not necessary. When replacing a microphone, the signal amplitude may decrease, so to increase the low-frequency gain, it is advisable to use a composite transistor, or use a more sensitive ULF (see Chapters 4 and 5). In most cases (Fig. 13.7 - 13.10 and 13.12), the electret microphone can be replaced with a miniature carbon one (with the selection of resistor R1).

The diagram of a radio microphone designed by D. Volontsevich is shown in Fig. 13.7 [Rl 10/99-40]. With a supply voltage of 3 V, the device consumes a current of 7 mA. The inductors are wound on a mandrel with a diameter of 6 mm with wire /73/7-0.5. L1 has 6 turns, and L2 has 4 turns. A piece of mounting wire 70 cm long was used as an antenna.

A. Ivanov’s VHF radio microphone resembles the previous design like two peas in a pod (Fig. 13.7) [Рл 10/99-40]. The difference is that the diagram (Fig. 13.8) is, as it were, “turned” upside down. This unusual arrangement of almost similar schemes next to each other makes it possible to train the eye to “recognize” structures that are similar to each other. Schemes Fig. 13.7 and 13.8 differ “electrically” in the way the modulating voltage is supplied: in the first case, it is supplied to the base of the generator transistor; in the second - to the emitter. The inductor coil contains 7 turns of PEV wire 0.7...0.8 mm and has an internal diameter of 5 mm. The current consumed by the device is 15...20 mA.

In Fig. Figure 13.9 shows a diagram of a radio microphone in the 66...74 MHz range, in the basic bias circuit of which an electret microphone is included as a controlled resistor [Рл 2/97-13]. The antenna is a piece of flexible multi-core wire 20...40 cm long. The current consumed by the device is about 1 mA.

Cascode connection of transistors is used in the circuit in Fig. 13.10 [Rl 2/97-13]. In this case, for low-frequency signals, the load of transistor VT2 is an RF generator made on transistor VT1. In turn, the high frequency current in the emitter circuit of transistor VT1 is modulated by a signal from the amplification stage low frequency signals, taken from the microphone.

In Fig. Figure 13.11 shows a diagram of a VHF-FM microtransmitter designed by V. Ivanov [R 10/96-19]. The transmitter is capable of broadcasting a signal received from a ULF electric player, tape recorder and other devices. The amplitude of the low-frequency signal at the input is within the range of 10...500 mV. Coil I without a frame, has an internal diameter of 4 mm and contains 15 turns of PEV 0.5 wire. Coil L2 is wound on top of resistor R3 (MLT-0.5) and contains 50... 100 turns of thin insulated wire.

In Fig. 13.12 and 13.14 show practical circuits of microtransmitters based on an analogue of a lambda diode. A forward-biased transition is used as a controlled element semiconductor diode(LED). Frequency modulation is carried out by changing its dynamic resistance. For high frequency component capacitance LED is much lower than its ohmic resistance. Simultaneously with the function of controlling the generation frequency, the LED indicates the on state of the device and stabilizes its operating point.

To implement frequency modulation in the circuit (Fig. 13.14), a homemade condenser microphone is used. It is made in the form of a deployed capacitor with two flat fixed electrodes, parallel to which a membrane (thin foil, metallized dielectric film, etc.) is fixed, electrically isolated from the fixed electrodes. The microphone can be assembled in a photo slide frame; its capacity is several picofarads.

For comparison, in Fig. Figure 13.13 shows a diagram of the simplest microtransmitting device made on a tunnel diode with an operating point stabilizer on a germanium diode VD1 [Рл 9/91-22, 10/97-17]. A microphone design similar to that described above can be used in the circuit in Fig. 13.15. The parameters of inductors (oscillatory circuits) can be transferred from one design to another.

In the circuits (Fig. 13.9, 13.10, 13.13, 13.15) for the VHF range (66...74 MHz) frameless inductors are used, having an internal diameter of 4 mm and containing 5...6 turns of PEV-2 wire with a diameter of 0.56 mm. Winding pitch 1.5 mm. The operating frequency of generation is set by bringing the coil turns closer together/moving apart, selecting the number and diameter of its turns, as well as the capacitance of the oscillating circuit capacitor. The body of the electret microphone is connected to a common wire. Reception of high-frequency signals is possible with a portable FM receiver.

To create a video transmitter ( wireless transmission video signal from a VCR to a TV), G. Roman’s circuit can be used [Рл 3/99-8]. Oscillatory circuit L1C2 (Fig. 13.16) is tuned to the frequency of one of the free television broadcasting channels.

Literature: Shustov M.A. Practical circuit design (Book 1), 2003

When using a compact antenna, this device provides a communication range of about 100 meters, and when using a full-size whip antenna - more than 600 meters. The transmitter circuit is shown in Fig.

The signal from the microphone goes to a low-frequency amplifier (transistors VT1, VT2) with direct connections. Boosted signal through filter R9, C4, R10 it is supplied to varicap VD1 type KV109, connected to the emitter circuit of transistor VT3 type KT904. The varicap bias voltage is set by the collector voltage of transistor VT2. The HF generator is made according to a common base circuit. The collector circuit of transistor VT3 includes circuit C8, C9, L1. The tuning frequency is determined by the inductance of the coil and capacitances C8, C5, VD1. Capacitor C9 sets the depth feedback, and C10 - coordination with the antenna. A choke of any type with an inductance of about 60 μH. Coil L1 is frameless, with an internal diameter of 8 mm, has 7 turns of 0.8 mm PEV wire. The length of the full antenna is 0.75...1 meter. Transmitter power is about 200 mW. If such power is not needed, you can reduce it by using resistor R2 with a resistance of 50..100 kOhm and replacing the choke with a resistor of about 300 Ohm. In this case, the transistor can be replaced with KT368. Low power transmitter frequency stability is higher and battery life is increased.

High power radio transmitter without additional amplifier power

The proposed radio transmitter differs from previous devices in the design of the master oscillator, which makes it possible to obtain increased radiation power without the use of an additional power amplifier. The radio transmitter (Fig. 1) operates at a frequency of 27-28 MHz with amplitude modulation. The carrier frequency is stabilized by quartz, which allows you to increase the communication range when using a receiver with quartz frequency stabilization. The device is powered by a power source with a voltage of 3-4.5 V. The audio amplifier is made using a VT1 transistor, type KT315. To power the microphone and set the DC modes of transistors VT1, VT2, VT3, a parametric voltage stabilizer is used on resistor R2, LED VD1 and capacitor C1. A voltage of 1.2 V is supplied to an electret microphone with an amplifier Ml type MKE-3, "Sosna", etc. The audio frequency voltage from the microphone Ml through capacitor C2 is supplied to the base of transistor VT1. The DC operating mode of this transistor is set by resistor R1. The amplified audio frequency signal, removed from the collector load of transistor VT1 - resistor R3, goes through the capacitor SZ to the master oscillator, thereby realizing amplitude modulation transmitter. The master oscillator of the transmitter is assembled on two transistors VT2 and VT3 of type KT315 and is a push-pull self-oscillator with quartz stabilization in the feedback circuit. The circuit, consisting of coil L1 and capacitor C5, is tuned to the frequency of the quartz resonator ZQ1. The circuit, consisting of coil L2 and capacitor C7, is designed to match the antenna and transmitter. The device uses MLT-0.125 resistors. Capacitors are used for voltages greater than 6.3 V. Transistor VT1 can be replaced with any p-p-p transistor, for example, on KT3102, KT312. Transistors VT2, VT3 can be replaced with KT3102, KT368 with the same current transfer coefficient. Good result can be obtained by using the KR159NT1 microcircuit, which is a pair of identical transistors. The contour coils are wound on a frame with a diameter of 5 mm, which has a tuning core made of carbonyl iron with a diameter of 3.5 mm. The coils are wound in 1 mm increments. Coil L1 has 4+4 as the supporting element of the parametric voltage stabilizer circuit in Fig. 1 turn, coil L2 - 4 turns. Both coils are wound with PEV 0.5 wire. Choke Dr1 has an inductance of 20-50 μH. A wire about 1 m long is used as an antenna. One KBS-4.5 V flat battery or four elements of type A316, A336, A343 can be used as a power source. LED VD1 type AL307 can be replaced with any other or use an analogue of a low-voltage zener diode with a low stabilization current (Fig. 2.). Setting up the transmitter begins with setting the DC modes of transistors VT2 and VT3. To do this, connect a milliammeter to the power circuit break at point A and select the resistance value of resistor R4 such that the current is 40 mA. Configuration of circuits L1, L2, C5, C7 is carried out according to the maximum RF radiation. Moreover, they roughly tune the operating frequency with capacitors, or more precisely, with the coil core. The trimmer of coils L1, L2 should be located at a distance of no more than 3 mm from the center of the coils, since in its extreme positions generation can be disrupted due to a violation of the symmetry of the arms of transistors VT2, VT3.

5 kilometer transmitter:

20 watt power amplifier

Transmitters with analog frequency stabilization. -> 4 Watt FM Transmitter

This is a small but quite powerful FM transmitter that has three RF stages connected to an audio preamplifier for better modulation. His output power 4 watts and it is powered by 12-18 volts DC, making it portable. This is the perfect project for beginners who want to dive into the exciting world of FM radio broadcasting and want a diagram that will form the basis for experimenting with it.
Technical Specifications - Characteristics
Modulation type:......FM
Frequency range: ...... 88-108 MHz
Operating voltage: ..... 12-18 VDC
Maximum current: ....... 450 mA
Output power: ....... 4 W

How it works As already mentioned, the transmitted signal is frequency modulated (FM), which means that the amplitude of the carrier remains constant, and its frequency changes in accordance with the change in the amplitude of the audio signal. When the amplitude of the input signal increases (i.e., during positive half-cycles), the carrier frequency also increases, on the other hand, when the amplitude of the input signal decreases (negative half-cycles or no signal), the carrier frequency decreases accordingly. In Figure 1 you can see a graphical representation of frequency modulation as it appears on the oscilloscope screen, along with the modulating sound signal. The transmitter's outgoing frequency varies from 88 to 108 MHz, i.e. FM band used for radio broadcasting. The circuit, as we have already said, consists of four cascades. Three RF stages and an audio preamp for modulation. The first RF stage is a generator, it is built on the basis of TR1. The generator frequency is controlled by the LC circuit L1-C15. C7 is there to ensure oscillation continues and C8 regulates the capacitive coupling between the oscillator and the next RF stage, which is the amplifier. The amplifier is based on TR2, which operates in class C, the input of which is adjusted by changing the values ​​of C10 L4. From the output of this last stage, which is adjusted by changing the values ​​of L3-C12, the output signal is removed, which comes to the antenna through the configured chain L5-C11. The preamplifier circuit is very simple, it is based on TR4. Input sensitivity is adjustable to allow the transmitter to be used with a variety of input signals and depends on the value of VR1. The transmitter can be modulated directly from a piezoelectric microphone, small cassette recorder, etc. And of course you can use an audio mixer for more professional results.

Design. First let us cover some assembly basics electronic circuits on printed circuit board. The board is made of thin insulating reinforced material with a thin layer of conductive copper, the conductive layer is shaped to create the necessary connections between the various components on the board. It is highly advisable to use a properly designed printed circuit board, as this significantly speeds up assembly and reduces the likelihood of making mistakes. Plus, the PCB kit comes with pre-drilled holes and component outlines with their designations on the component side to make assembly easier. To protect the board from oxidation during storage and ensure that you receive it in great shape, it is tinned during production and coated with a special varnish that protects it from oxidation and makes soldering easier. Soldering the components is the only way to assemble the circuit, and by the way, your success or failure largely depends on it. It's not too difficult and if you stick to some rules you shouldn't have any problems. The soldering iron you use should be lightweight and its power should not exceed 25 watts. The tip should be thin and clean all the time. There are very convenient specially made sponges for this purpose, which are kept damp, and from time to time you can wipe the hot sting on them to remove any residue that tends to accumulate on it. DO NOT use a file or sandpaper to sand a dirty or worn tip. If the tip cannot be cleaned, replace it. There are many in stores various types solder and you should choose the solder good quality containing flux to ensure a perfect connection every time. DO NOT use soldering flux other than what is already in the solder. Too much large number flux can cause many problems and is one of the main reasons malfunction schemes. If you do have to use additional flux, as is the case when you need to tin copper wires, clean it thoroughly after finishing the job. To properly and properly solder components, you should do the following: - Clean the component feet using a small piece of sandpaper. Bend them at the appropriate distance from the component body and insert it into the board in its place. - Sometimes you may encounter components with legs that are larger than usual, they are too thick to fit into the holes on the PCB. In this case, use a mini drill to widen the holes. - Do not make the holes too large, as this will make soldering difficult later. - Take a hot soldering iron and place the tip on the leg of the component while holding the tip of the solder wire at the point where the leg comes out of the board. The tip should touch the leg slightly above the board. - When the solder begins to melt and flow, wait until it evenly covers the entire area around the hole and the flux boils and comes out under the solder. The entire operation should not take more than 5 seconds. Remove the soldering iron and allow the solder to cool on its own without blowing on it or moving the component. If done correctly, the joint surface should have a shiny, metallic tip, and the boundaries should end evenly on the component leg and board trace. If the solder looks awkward, abnormal, or blobby, then you have made a bad connection and should remove the solder (Use a pump or soldering wick) and repeat the steps. - Be careful not to overheat the traces, as they are very easy to separate from the board and tear. - When soldering sensitive components, it is a good practice to hold the pin on the component side with tweezers to dissipate heat that could damage the component. - Make sure you do not use more solder than necessary, as you may short circuit paths located nearby, especially if they are very close to each other. - When finished, cut off any protruding component legs and thoroughly clean the board with an appropriate solvent to remove any flux remaining on the board. This is an RF project, which requires even more care during soldering, as carelessness during assembly can lead to low or no power output, poor stability, and other problems. Make sure you follow the basic rules of electronic circuit assembly described above and double check everything before moving on to the next step. All components are clearly marked on the side of the board elements, and you should have no problems identifying their location and installation. First, solder all the leads, and then the coils, being careful not to deform them, then the chokes, resistors, capacitors, and finally the electrolytes and trimmers. Check whether the electrolytes are installed correctly, in accordance with their polarity, and whether the trimmers are overheated during soldering. At this point you need to stop to check the work done, and if everything is in order, solder the transistors in their places, being careful not to overheat them, since they are the most sensitive of all the components used in this project. The audio signal is supplied to points 1 (ground) and 2 (signal), power is supplied to points 3 (-) and 4 (+), the antenna is connected to points 5 (ground) and 6 (signal). As we said, the signal you will use for modulation can come from a preamp or mixer, or in the case where you want to modulate the carrier with your voice, you can use the piezoelectric microphone that comes with the kit. (The quality of this microphone is not so high, but it is suitable if you are only interested in speech.) An open dipole or Ground Plane can be used as an antenna (for the diagram of this antenna, see the picture, trans.) Before using or changing the operating frequency, you should perform a procedure called setup and described below.

Parts List

R1 = 220K
R2 = 4.7K
R3 = R4 = 10K
R5 = 82 Ohm
R = 150Ohm 1/2W x2 *
VR1 = 22K trimmer

C1 = C2 = 4.7uF 25V electrolyte
C3=C13=4.7nF ceramic
C4=C14=1nF ceramic
C5=C6=470pF ceramic
C7 = 11pF ceramic
C8 = 3-10pF trimmer
C9 = C12 = 7-35pF trimmer
C10 = C11 = 10-60pF trimmer
C15 = 4-20pF trimmer
C16 = 22nF ceramic *

L1 = 4 turns of silver-plated wire on a 5.5mm mandrel
L2 = 6 turns of silver-plated wire on a 5.5mm mandrel
L3 = 3 turns of silver plated wire on a 5.5mm mandrel
L4 = etched on board
L5 = 5 turns of silver plated wire on a 7.5mm mandrel

RFC1=RFC2=RFC3= VK200 RFC tsok

TR1 = TR2 = 2N2219 NPN
TR3 = 2N3553 NPN
TR4 = BC547/BC548 NPN
D1 = 1N4148 diode*
MIC = crystal microphone

Attention: parts marked * are used to configure the transmitter if you do not have a stationary wave bridge.

Settings

If you expect your transmitter to deliver maximum power at all times, you will need to configure all 3 RF stages properly to ensure that power flows between them in the best possible way. There are two ways to do this, and which way to follow depends on whether you have an SWR meter. If you have an SWR meter, then turn on the transmitter, with the SWR meter connected in series with the antenna, and turn C15 to tune the transmitter to the frequency you have selected for broadcasting. Then adjust trimmers C8,9,10,12 and 11 until you achieve maximum power output on the SWR meter. For those who do not have an SWR meter, there is another method that gives good results. You just need to assemble a small diagram, fig. in Fig. 2, which connects to the output of the transmitter, you connect your multitester, which has a suitable marked volt scale, to its input (on C16). You tune the C15 to the desired frequency, and then adjust the other trimmers in the same order as described above, until maximum value on a multitester. The disadvantage of this method is that you cannot adjust the transmitter with an antenna connected at the output, which may be necessary with a slight adjustment of C11 and C12 for the best antenna matching. Remember to adjust your transmitter every time you change antenna or operating frequency. ATTENTION: In each transmitter, in addition to the fundamental frequency, there are various harmonics, usually having a short range. To make sure you're not tuning into one of them, tune as far away from your receiver as possible, or use a spectrum analyzer to look at the output spectrum and make sure you're tuning the transmitter to the correct frequency.

ATTENTION

If the device does not work. - Check the device for poor connections, shorted adjacent tracks, or flux residue, which are usually the cause of the problem. - Check again all external connections going to and from the circuit, there may be an error in them. - Check whether all components are installed and in their places. - Make sure that all polarity components are installed correctly. - Make sure that the supply voltage is the correct value and is supplied to the circuit in the correct location. - Check the circuit for faulty or damaged components.

10 W transmitter

Scheme 1 (27 MHz):

Q1 KT904 on a radiator with an area of ​​600 cm^2
L1 - diameter 15 mm on a ceramic frame. 5 turns of silver wire with a diameter of 1 mm, winding length - 20 mm, tap from the 2nd turn, counting from the grounded wire.
L3 - frameless, on an 8 mm frame, contains 11 turns of PEV-2 with a diameter of 1 mm.
L2 (choke) type DMM-2.4 (20 µH)
C1, C5, C6 - with air dielectric.
L3 - frameless, on an 8 mm frame, contains 8 (6 by 94 MHz) PEV-2 turns with a diameter of 1 mm. Consists of 2 halves.
L4 - on the same frame and with the same wire, located between the 2 halves of L3 and contains 2-3 turns

Circuit 3 (Frequency modulator):

Q1 KT315
D1, D2 - varicaps KV102D or diodes D220.
VM1 - electret microphone MKE-3

Description and setup: Select one of 2 high-frequency circuits (depending on the receiver) and connect it to the modulator at point A. Next, as a load, connect 2 lamps 6.3 V (0.22 A) connected in series to the antenna and the common wire . Connect the 5 V power. Disconnect circuit L1, C1, and instead apply a signal from the VHF generator to the input. Check the frequency of the output signal with a wave meter (if it is not there or it is not the same as from the generator, adjust the capacitors and coils of the output circuit). Next, connect circuit L1, C1 and increase the supply voltage. Self-generation should occur already at 5 V (if it does not occur, move the emitter along the coil by 0.5...2 turns) - current 250 mA. Do not raise the voltage above 20V (current 750 mA, power 8...10 W). Next, adjust all the circuits, checking the frequency with a wave meter. When mounting (mounted, directly on the radiator), the leads of the parts should be as short as possible, capacitors with the appropriate TKE should be used, and the coils should be wound tightly. Only then will you receive good stability frequency, otherwise it will “float” up to 500 Hz. The frequency modulator is adjusted by selecting R1 when the voltage on the collector of Q1 becomes equal to half the supply voltage. It may also be necessary to connect point A to part of the L1 turns.

A simple spy FM transmitter operates in the range of 88-108 megahertz and allows you to transmit an audio signal to any radio receiver within a radius of 100 meters. The device is assembled based on the MAX2606 chip.

Option with higher range

The built-in generator is controlled by sound vibrations. The nominal oscillation frequency is set by inductance L1 at 390 nH, which lies in the range of about 100 MHz. Resistance R1 allows you to select a channel from 88 MHz to 108 MHz.

Almost any inductance can be used as a frequency-setting coil. You can make it yourself by winding 8 - 12 turns of 0.5 mm copper wire on a mandrel with a diameter of 5 mm. Fine tuning with such a coil can be done by squeezing or spreading the coils.

The circuit is powered from one element with a voltage of 1.5 V transmission audio messages from the M1 microphone at a distance of 30-50 m.

Reception is carried out on an FM receiver in the FM range 88...108 MHz. A piece of insulated wire 20...30 cm long with a diameter of 0.5 mm was used as an antenna. L1 without a frame has 7 turns of PEV-0.35, wound on a mandrel with a diameter of 3 mm. Standard inductor L2 with an inductance of 20 μH (can be wound on an MLT-0.25 resistor with a resistance of at least 100 kOhm - 50 turns of PEL-0.2).

This one is enough simple diagram A radio bug with low power consumption can be used to listen to conversations in an apartment or office, but over a short distance of 50-70 meters.

The sensitivity of the specialized microphone MKE-3 is sufficient for detailed recognition of whispers at a distance of 4-5 meters from the microphone. The operating range of the device is about 50 meters (with a transmitter antenna length of 30...50 cm).

The circuit is easy to assemble in a fairly compact design, powering the radio transmitter from small batteries. The current consumption of this design was 3...4 mA. Radio transmission frequency is 64-74 MHz, i.e. you can use a regular radio receiver

Coil L1 contains 6 turns of PEV-2 0.5 mm and on a frame with a diameter of 4 mm with a winding pitch of 1 mm. The frequency of the bug's radio transmission can be changed by moving the coil turns apart.

This radio circuit is powered by one small 1.5 volt battery, because with radio emission at a frequency of 88 MHz of only 0.5 mW, the consumption is 2 mA. And the transmission range reaches 30-50 meters.

Work of the bug circuit. Audio vibrations from the microphone through the isolation capacitor C1 enter the varicap VD1, which is located in the circuit circuit of the generator, made on a field-effect transistor. When the values ​​of the varicap capacitance change depending on the audio signal, frequency modulation of the generator occurs, and radio transmission begins through the inductive coupling coil L1 and the antenna.

As an antenna, I used a piece of wire about twenty-five centimeters long. L1 - 7 turns with a tap from the third, and L2 only one turn. Both coils are frameless, wound on a handle with a diameter of 4-5 mm with PEV-2 0.44 wire.