If you need to transmit audio over a relatively short distance, then you can assemble the circuit presented on this page. The circuit is based on two NPN transistors BC547. The range at best will be 70 meters. You can adjust the volume of sound transmission using a 100 kiloOhm variable resistor, as well as on the receiver itself. It is not necessary to install an LED with a 330 Ohm resistor; it serves as an indicator.
Schematic diagram of a simple transmitter
I used this device for broadcasting sound so that I could listen to the music I needed while being at a short distance from the house, for example in the garage, and receive the signal on an ordinary FM radio. Lay format printed circuit board available - download.
An analogue of an imported silicon bipolar n-p-n transistor bc547 is domestic kt3102. The higher the gain of the transistors, the more powerful the audio transmitter will be. If you want to make the device miniature, use transistors in a sot-23 package: BC847. The picture below shows the location of the base, collector and emitter.
The best, in my opinion, power supply for the circuit would be two batteries A.A. 1.5 V connected in series. Together they will produce a voltage of three volts. The operating time depends on the current consumption, as well as on the capacity of the batteries. Typically, the higher their cost, the better they are. For example, if you use fairly expensive batteries GP Ultra Alkaline, with a capacity of 3.1 A declared by the manufacturer at a current in the circuit of 8 mA, this device will be able to work, roughly speaking, 387 hours without interruption. The problem is that it is very difficult to “suck” all the battery power. Therefore, in reality the circuit will operate without shutdown and with stable signal transmission for approximately 150 hours, or almost 7 days.
The coil has six turns of insulated copper wire with a cross section of 0.3-0.5 mm. We wind this reel using pen paste.
When testing the device, the current in the circuit was almost 10 mA.
It is very easy to catch the frequency of the transmitter by twisting the subscript capacitor and “playing” with the coil, moving and spreading its turns. I “caught” my transceiver at a frequency of 89.90 MHz.
I assembled this circuit using SMD parts, only using transistors in a TO92 package. The antenna is a piece of copper wire, the larger the better. If you simply touch the antenna wire, the frequency does not go away, but if you pick it up, noise begins in the receiver's headphones.
I tried to transmit sound both from the computer and from the phone. A signal that is too loud is transmitted with a lot of noise and wheezing; the optimal sound strength is adjusted using a substring resistor. In general, the quality of audio transmission is quite good. I received it on a black and white Nokia phone and listened to the sound on headphones. There were no major reception problems.
Video of the sound transmitter in operation below. Song: bwb - my boys.
Video of the transmitter working
With this I say goodbye. I was with you EGOR .
Discuss the article HOMEMADE FM TRANSMITTER
Seal
Modulator repair Basically it comes down to repairing the power supply, which is done in most car FM modulators, according to a simple scheme: a stabilizer on a chip and capacitors. Even the diode shown in the diagram (see below) and capable of protecting against polarity reversal is not present in all modulators. 
Several reasons can lead to the failure and repair of the FM modulator, one of which I have already indicated is polarity reversal, then a voltage surge in the car network (malfunction of the charging relay, disconnection of the battery while the car is running) and a break in the wires from the power supply to the circuit. I already wrote about these malfunctions in the article “FM modulator malfunctions”. When repairing a modulator, these faults occur most often. Often there is a failure in the firmware (not in all models and I do not repair such a malfunction, for the simple reason that the repair will be comparable to the purchase of a new car FM modulator and not all clients agree to this).
Let's focus on repairing the fm modulator power supply.
As already mentioned, the power supply is quite simple - it is a stabilizer installed on the board on an IC type SP7805V2-L/TR (5 volt stabilizer) and capacitors. At modulator repair You need to start with a visual inspection. We check: the capacitors are not swollen, the wires are broken, and there is no damage to the stabilizer. 
An FM modulator is a small device that allows you to listen to music from any audio source or flash drive to any FM radio receiver. The modulator is connected to the car's cigarette lighter socket, from which 12V power is supplied. USB flash drives serve as storage media for audio files. Just record MP3 files onto a flash drive, insert it into the modulator and listen to it in the car. After all, it’s easier to record the desired list of songs on a flash drive than on an MP3 CD.We leave the modulator in the car, and take home only a flash drive to download new music or audiobooks from the computer onto it. If you have an audio output on your mobile phone, you can connect it to the modulator via the headphone jack and listen to music from your car speakers or a conversation as if over a wireless channel. FM modulators with memory can be used as a regular USB flash memory.
There are also such interesting varieties on the market as Car Handsfree Bluetooth Adapter and Car FM Transmitter. CharacteristicsFM modulatorfor car:
LCD screen 1.4".
Connection via: Bluetooth, USB, SD card, linear input for PDAs, MP3 players, laptops.
Display on the screen: FM wave number, track number, artist name, song name, equalizer, caller number.
FM frequency range: 87.5-108FM.
Support USB, SD cards up to 16GB.
Display backlight.
This device can be used as a headset in the car. On your mobile phone, in the search mode for Bluetooth devices, find this modulator and connect it as a headset. When a call comes in on your mobile phone, press the off-hook button on the FM modulator control panel and talk without picking up the phone. When the conversation is over, press the end call button on the FM modulator remote control.When you turn on the ignition in the car, the device automatically connects to the mobile phone if the Bluetooth function is turned on on the mobile phone.
Technical characteristics of a standard PM modulator:
The FM modulator is powered from a car cigarette lighter 9 - 24 V. Rated current: up to 100 mA. There is a USB connector for reading files from a flash drive.
Signal-to-noise ratio: 90 dB.
Output frequency range: 20 Hz - 20 kHz.
Equalizer modes: Normal, Rock, Pop, Classic, Soft, Bass.
Audio input - reading files from the audio output of any player. Supports MP3 and WMA files.
Display to indicate volume, number, song title.
About two hundred fixed FM frequencies are used to communicate the modulator with the receiver.
The optimal operating distance is up to 10 m.
Operating instructions for the FM modulator:
Using a USB cable, connect the MP3 FM modulator to your computer. Copy the necessary music files to the root folder of the modulator.Place the MP3 FM modulator in the cigarette lighter socket, first making sure that the voltage is 9-24 volts.
Press the "Play/Stop" button (bottom center button on the device or green top left button on the remote control), the melody will start playing.
By pressing the “CH” button on the modulator or the “CH-” and “CH+” buttons on the remote control, you can change the FM frequency.Turn on your radio and switch to the FM band, tune in to the frequency you selected on your auto MP3 modulator. Make sure the frequencies match.
Initially, music files play in order. To switch melodies, use the “Back” and “Forward” buttons (the extreme left and right buttons of the modulator). To stop playing a melody, press the Play/Stop button. If you want to listen to melodies in a chaotic order, press the number buttons on the remote control.By pressing the EQ button on the remote, you can control the built-in equalizer.Before using the remote control, remove the protective film from the battery compartment.To improve the sound of the music being played, set the volume on the modulator to less than 50 units. The volume level is adjusted using the Vol + and Vol - buttons (on the remote control) or “Back” and “Forward” (on the modulator panel (press and hold the button, you will find the optimal ratio of sound volume and sound quality)).Advanced models of FM modulators have an equalizer function, balance function, quick song search, display brightness adjustment, voice recorder, and language menu settings.
A typical PM modulator circuit is shown in the figure below:
The basis of the device is the BH1417 chip. It is powered by a voltage of 5V and consumes 5-10mA. Despite the small output power, it seems interesting to connect to it a simple RF power amplifier using two transistors KT368 and KT610, in order to increase the range to a kilometer or more. The sound quality of this BH1417 microcircuit is much better than that of the well-known BA1404 stereo modulator. According to some information received from radio amateurs, with an antenna installation height of 10 meters - then when using a 2-watt amplifier - the radius was not about two kilometers. If you raise the antenna higher, a radius of up to 7 km is covered with reliable reception. In general, this device, at a relatively low price of 15-25, is of interest in terms of its use in various amateur radio devices.
There are quite a few FM transmitter circuits on the network, but most of them are micro-power (100-300 mW).
Using a modern element base, it is easy to assemble a power amplifier of minimal dimensions and quite decent power...
The proposed transmitter, the diagram of which you see below, is powered by 12 V, is small in size, and operates in stereo. Only 9-12 mW is supplied to the input of the power amplifier and this is enough to drive it up to 1.3...1.7 W. In this case, I did not have to assemble a modulator and an FM generator; I was quite satisfied with a car FM transmitter powered by a cigarette lighter. Its power output is just right. Since the weak point in these transmitters is the voltage stabilizer, made on a 5-volt bank, we had to lower the voltage to 9 V, adding another stabilizer to the circuit.
Now let's take a closer look at the power amplifier itself. It is assembled on two transistors of the BFG-591 type (VT1 can be replaced with BFG-135), as mentioned above, it is powered by a voltage of 12 V. A correctly assembled transmitter begins to work immediately, all that remains is to adjust the tuning capacitors (their capacitance with the given coil data within 22-30 pF, capacitor C12, if necessary, can be excluded from the circuit).
The gain bandwidth of the PA is quite wide, so it is quite suitable not only for amplifying signals in the FM range, but also on almost all television channels of meter waves, however, in this case its output power will drop to 0.7...0.9 W at the same input level (90-120 mW).
List of parts required to assemble the amplifier:
Capacitors:
C1, C4, C5, C7, C9, C10 - 3300 pF
C2, C6, C11, C12 - 6-30 pF
C3, C8 - 100-200 µF
Resistors:
R1, R5 - 270 Ohm
R2, R6 - 1.5 kOhm
R3, R7 - 2.2 kOhm
R4 - 100 Ohm (0.5 W)
R8 - 51 Ohm (0.5 W)
Transistors: VT1-VT2 - BFG-591
(VT1 can be replaced with BFG-135)
Coils L1, L3, L5 - 5 turns of PEL-2 wire with a diameter of 1.0 mm, on a mandrel with a diameter of 5.5 mm
L2, L4 - contain 7 turns of the same wire, on a mandrel of the same diameter. (all coils are wound turn to turn)
The amplifier is assembled using SMD parts on one-sided foil fiberglass. All coils and tuning capacitors are installed on one side of the board, and the rest of the installation of parts is on the “printing” side.
Although this amplifier, assembled in SMD design, is not prone to self-excitation, just in case I separated the cascades with shielding partitions. Coils L2-L3 and L4-L5 are located perpendicular to each other.
At the transmitter output it would not hurt to install a P-circuit tuned in the desired frequency range, although you can use an amplifier without it.
PS. This amplifier was tested with modulators for cable television and showed good results when transmitting a TV signal on the air.
http://radio-device.ru/radio.php?p=prostoy_peredatchik_iz_car_fm_transmittera
I am posting a small collection of schematic diagrams of radio transmitters collected from various foreign sites. Starting from low-power, a few milliwatts, and up to powerful multi-watt UHF amplifiers. I haven’t tested the functionality, but the circuitry inspires confidence. All transmitter circuits are designed for the standard broadcast VHF range 88-108 MHz.
FM Transmitter in a pen
The FM bug in a writing pen project is very popular among beginning radio amateurs. In an effort to reduce the size of this design, surface mount components are used. The circuit has low power consumption, but sufficient output power to cover a radius of 50 - 200 m. You can put watch batteries or a lithium-ion battery from the Bluetooth system here.
FM transmitter for 5 kilometers
The proposed broadcast transmitter is indeed very stable, has a complex, but high-quality and well-thought-out circuit design, and uses standard FM frequencies 88 - 108 MHz. Its range of action is a real 5 km. The circuit includes a stable generator powered through an LM7809 stabilizer - this is a 9 V stabilized power source, on transistor T1 and a frequency adjustment element 10K potentiometer. The RF output power of this transmitter is about 1 W. A pair of MV2019 varicaps function as variable capacitors.
The final stage of the FM transmitter is a powerful microwave transistor of at least one watt of power. You need to use transistors 2N3866, 2N3553, KT920A, 2N3375, 2SC1970 or 2SC1971. Do not forget to install an effective heatsink for the T5 transistor, because it becomes slightly warm during operation. The circuit will require a 12V/1A power supply.
Coil winding data:
- L1 = 5 turns on 4 mm frame
- L2 = 6 turns on 6 mm frame
- L3 = 3 turns on 7 mm frame
- L4 = 6 turns on 6 mm frame
- L5 = 4 turns on 7 mm frame
Everything is wound with a wire about a millimeter in diameter. Transistors T1 = T2 = T3 = T4 = BF199, T5 = 2N3866 or 2SC1971, BLY81, 2N3553.
15 W UHF for 88-108MHz range
The RF power amplifier amplifies all frequencies 88-108MHz from 1W input power received from the FM transmitter to 15W. The circuit includes a multi-level low-pass filter and is highly efficient. With a good antenna, the expected transmission radius is at least 20 km. It uses 2SC1972 high power RF transistor (175MHz, 4A, 25W), which must be installed on the heatsink to dissipate excess heat.
Inductors L1-L6 with 0.8 mm wire with a frame diameter of about 5 mm. If you put transistor C2538 here, the power will be even greater.
When debugging, the circuit must be connected with a load equivalent, for example a 50 Ohm 10 Watt resistor. The power source power is at least 2.5 amperes, the antenna resistance is strictly 50 Ohms. Enter the setting only with the supply voltage reduced to 9 Volts; when measuring high-frequency voltage at the antenna output, you do not need to use a conventional multimeter - there will be false readings due to interference on the device microcircuits.
300 mW VHF transmitter
The last scheme is also of interest as it is quite thoughtful and not hackneyed. Although, in principle, everything is as usual here - a generator with a power stabilizer and a high-frequency power amplifier with adjustable harmonic suppression circuits. Due to the 12-volt power supply and the 2SC2538 transistor, it was possible to obtain a range of up to a kilometer on a small helical antenna.
