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
Typical malfunctions of FM modulators (transmitters) can be divided into 3 types. I will talk in detail about only one, since the other two in 50% of cases lead to the complete unusability of the device!
The first type of malfunction is the failure of the stabilizer microcircuit. Figure 1 shows the disassembled modulator itself.
FIG.1 (click to view)
Figure 2 shows a stabilizer board with a 7805 chip (letters may be different) in a TO case.
FIG.2 (click to view)
It is in this microcircuit that the reason for the failure of the device lies.
The microcircuit lowers the battery voltage to 4.5 - 5v and stabilizes it. From its output, this voltage is supplied to the main board to power the modulator circuits. For various reasons, the microcircuit can fail. If a break occurs inside it, then power is not supplied to the main board. But this is half the trouble. Simply replace the chip and everything is in order.
It is much worse when a breakdown occurs - then it is almost a 50% guarantee of the complete unsuitability of the entire device for repair. So, as the main board will receive voltage from the battery directly. And this will most likely kill the device completely!
In Figure 2, this chip is marked with an arrow. There is a noticeable swelling on its case, which indicates a 100% failure of the microcircuit.
If necessary, the microcircuit can be replaced by any one that is suitable in terms of parameters. The main thing is that it has a stabilized output voltage of about 4.5 - 5v! I even had to put 7805 in the usual case instead of 7805 in the TO case! Just need to grind the metal part a little.
If, after replacing the microcircuit, the device still refuses to show signs of life, then there is still a chance to revive it. On the main board, not far from the soldering point of the conductor, through which the supply voltage + 5v is supplied, there is an SMD transistor (not in all models). Through it, voltage is supplied to the board. It's most likely that he's in a 'break'. It is necessary to short-circuit the conclusions of its collector and emitter.
If the device then works normally, then you can either replace the transistor, or simply leave the jumper. Everything will work fine with a jumper, tested on more than a dozen devices!
If there is no such transistor on the board or the device did not turn on after installing the jumper, leave this idea, more serious components are most likely damaged!
There were cases when the device turned on after the indicated replacements, but the number 9 was displayed on the screen, which slowly began to go out. In this situation, further attempts to restore the device's performance were unsuccessful. Unfortunately, it was not possible to determine the exact reason for this behavior.
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.
Many users from different sites, as well as YouTube subscribers after the video, asked for instructions on assembling a bug from an FM modulator. The FM modulator has become quite a popular device in recent times. It is designed to decrypt audio tracks and transmit over the air. Range up to 10 meters.
In order to assemble a bug from a modulator, special skills and knowledge in radio electronics are not needed, a basic knowledge of a beginner radio amateur is enough. We disassemble the FM modulator and throw out the case. The filling is powered by 5 volts, the voltage from the vehicle's on-board network is supplied to a linear voltage stabilizer, at the output of which we get the indicated 5 volts, it is this voltage that feeds the digital part of the modulator.
Any modulator consists of a small processor (in our case it is not needed, but it is impossible to separate it from the main board) and a transmitter. The main advantage of such a bug is that the circuit has quartz stabilization, so the transmitted signal is of very high quality and there is no frequency slope here. 
The circuit can be powered even from a single mobile phone battery with a voltage of 3.7 volts. On the main board, we can see the transmitter chip. 
We turn on the radio transmitter and tune the modulator to any frequency, tune the receiver itself to this frequency. Next, we take a thin and sharp metal object (necessarily bare), for example, a needle and alternately touch all the pins of the microcircuit with the tip of the needle. When touching some of the findings, we observe distortion in the radio receiver. 
We select the output that responds to touch more than the others, that is, the distortion in the receiver is louder than in the case of the others. Directly to this output we connect (solder) a non-polar capacitor (ceramic or film does not matter) a capacitor with a capacitance of 0.1 μF.
We give a sound signal to a free output, when an electret microphone is connected, we will get a radio transmitter.
