FM modulator or transmitter is a cool thing. Convenient very thing, uploaded a lard of songs tired of rewrote. You can still listen to music from one modulator together. We have one in the garage bay, it is constantly on, so everyone tuned in to it. It looks different, I have such a modulator.
They all have the same functions. Different design of the appearance of the modulator and its memory, some do not have it, and you have to insert an external flash drive.
Okay, more to the point. I did not long rejoice at the flawless operation of the modulator. When it broke, there was an opinion like Chinese garbage. I threw it away on the shelf and somehow managed without it.
Recently got to the modulator. Dismantled: the breakdown was nonsense. There was a fuse in the modulator plug, which is inserted into the cigarette lighter socket. Burnt!
The same was not at hand.
We take a stranded, copper wire. We select two veins from it. I have them with a diameter of about 0.2 mm. Someone will say that this is too much, but did the fuse blow? And it was 250V 0.25A, which is also not a little.
We twist two veins into one cord.
We tie first one contact of the fuse, then the other.
We fix our jumper with electrical tape or paper adhesive tape. It is necessary that the contacts remain bare.
If you need to transmit audio sound 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 sound transmission volume using a 100 kilo-ohm variable resistor, as well as on the receiver itself. An LED with a 330 ohm resistor is optional, 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 a short distance from home, for example, in a garage, and receive a signal on an ordinary FM radio. There is a lay format printed circuit board - download.
Analogue of 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 the sot-23 package: BC847. The picture below shows the location of the base, collector and emitter.
The best, in my opinion, power for the circuit will be two batteries AA 1.5 V connected in series. Together they will give a voltage of three volts. The operating time depends on the current consumption, as well as on the capacity of the batteries. Usually 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 without interruption, roughly speaking, 387 hours. The problem is that it is very difficult to “suck out” the entire charge of the battery. Therefore, in reality, the circuit will work without turning off 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 coil on the paste from the handle.
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, shifting and pushing its turns. I "caught" my transceiver at a frequency of 89.90 MHz.
I assembled this circuit on smd parts, only I took transistors in the TO92 package. The antenna is a piece of copper wire, the more the better. If you just touch the antenna wire, then the frequency does not go away, but if you pick it up, noise starts in the receiver's headphones.
I tried to transmit sound both from a computer and from a phone. A signal that is too loud is transmitted with numerous noises and wheezing, the optimal sound strength is adjusted by a trimmer resistor. In general, the audio quality is quite good. Received on a black and white Nokia phone, and listened to the sound in headphones. There were no major problems with acceptance.
Video of the sound transmitter below. Song: bwb - my boys.
Transmitter operation video
On this I apologize. was with you EGOR .
Discuss the article HOME-MADE FM TRANSMITTER
Hello dear friends!
I want to talk about a compact FM transmitter for connecting to audio devices via a 3.5 mm audio jack. The transmitter allows you to broadcast an audio signal over radio frequencies in the range from 87.5 to 108.0 Mhz. I ordered for a friend, with an eye on the price and attractive design (he asked for an inexpensive one to listen to audiobooks in the car). Before giving it to him, I disassembled it a little and tested it. The review contains detailed photos and impressions of operation.
Delivery within 19 days, packed in a bag with a bubble wrap, the contents are intact.
IN bundled the device itself and a micro USB cable for charging the battery, the cable length is 30 cm. The design of the transmitter is simple and concise, stylized as Apple products. 
During charging, the degree of charge is displayed (4 gradations): 
The transmitter can be connected to any 3.5mm headphone output: phone, laptop, mp3 player, computer, etc. 
Connection and setup are elementary:
1) Turn on the device with the on\off slider
2) Set the desired transmission frequency, for example 87.5 Mhz
3) Apply an audio signal from the source
4) Tune in on the receiver (another phone, radio in the car, etc.) to a frequency of 87.5 Mhz
5) Adjust by experience the sound level at the receiver and source (to minimize radio interference)
Snap-on housing, easy to disassemble with a plastic card: 
A microcircuit without designations, just in the form of a drop. The battery is also unnamed. 
The backlight is quite bright, the numbers are easily distinguishable. After a few seconds, it goes off on its own. 
The battery lasts for 5-6 hours of continuous operation. Estimated battery capacity is 180 mAh.
The audio signal transmission quality is average for this class of devices. In a quiet room at a distance of 10 m from the source, you can hear weak radio interference in the headphones. To check, I transmitted an audio signal from phone to phone (the antenna in this case was the phone's headset).
In principle, these interferences are barely distinguishable, I removed them by adjusting the volume of the audio signal on the transmitter and receiver. In the car, these interferences are not noticeable.
Purchase:
After buying and receiving the device, I found out that there are transmitters with a similar design, but with more solid characteristics.
For example, here is such a more expensive and better performance model
As far as I can tell, at the moment, this is the best option that can be ordered on the Internet, based on a specific design.
Conclusion: The universal FM transmitter has been tested and found to be operational, all functions are normal. During testing, I did not find any anomalous behavior.
Pros:
1. Nice laconic design
2. Versatility and ease of use
3. Autonomy (5-6 hours)
4. Compact size
5. Good audio signal transmission quality
6. Display backlight
7. The transmitter can work while charging
Minuses:
1. There is no instruction
2. Chip on the board without designations
3. No data on battery capacity
Thank you for your attention, all successful online shopping!
P.S.
About stereo:
The number of wires entering the plastic seal of the audio plug and the designation on the board indirectly hints at the fact that there is a stereo. Since the main chip is not signed and it is impossible to look at the specification for it, it is difficult to say anything definite about it.
It would be interesting if the owners of such devices, of which there are many, judging by the comments, posted a photo of the disassembly and it would be possible to compare more objectively 
I will add a useful comment from a Muscovite with the nickname lockeron:
“Good transmitter, I took one myself. You can watch my video review of it:
I plan to buy +38 Add to favorites Liked the review +22 +49
FM modulator or transmitter is a cool thing. Convenient very thing, uploaded a lard of songs tired of rewrote. You can still listen to music from one modulator together. We have one in the garage bay, it is constantly on, so everyone tuned in to it. It looks different, I have such a modulator.
They all have the same functions. Different design of the appearance of the modulator and its memory, some do not have it, and you have to insert an external flash drive.
Okay, more to the point. I did not long rejoice at the flawless operation of the modulator. When it broke, there was an opinion like Chinese garbage. I threw it away on the shelf and somehow managed without it.
Recently got to the modulator. Dismantled: the breakdown was nonsense. There was a fuse in the modulator plug, which is inserted into the cigarette lighter socket. Burnt!
The same was not at hand.
We take a stranded, copper wire. We select two veins from it. I have them with a diameter of about 0.2 mm. Someone will say that this is too much, but did the fuse blow? And it was 250V 0.25A, which is also not a little.
We twist two veins into one cord.
We tie first one contact of the fuse, then the other.
We fix our jumper with electrical tape or paper adhesive tape. It is necessary that the contacts remain bare.
Seal
Modulator repair
basically comes down to repairing the power supply, which is made in most car FM modulators, according to a simple scheme: a stabilizer on a microcircuit 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 I have already indicated is a polarity reversal, then a power surge in the car's 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 the modulator, these malfunctions are most common. 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 commensurate with the purchase of a new car FM modulator and not all customers go for it).
Let's focus on repairing the power supply of the fm modulator.
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 start with a visual inspection. We check: the capacitors are not swollen, for a wire break and for damage to the stabilizer. 
