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.
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.
With this I say goodbye. I was with you EGOR .
Discuss the article HOMEMADE 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 unsuitability of the device!
The first type of malfunction is the failure of the stabilizer chip. Figure 1 shows the modulator itself in disassembled form.
FIGURE 1 (click to view)
Figure 2 shows a stabilizer board with a 7805 chip (the letters may be different) in a maintenance package.
FIG.2 (click to view)
This microcircuit is the reason for the device failure.
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 may fail. If a break occurs inside it, then power does not flow to the main board. But this is half the trouble. You just need to replace the chip and everything is fine.
It is much worse when a breakdown occurs - then this is an almost 50% guarantee of complete repairability of the entire device. So, the main board will receive voltage from the battery directly. And this will most likely kill the device completely!
In Figure 2, this microcircuit is marked with an arrow. There is noticeable swelling on its body, which indicates 100% failure of the microcircuit.
If necessary, the microcircuit can be replaced with any one that has suitable parameters. The main thing is that it has a stabilized output voltage of about 4.5 - 5v! I even had to install 7805 in a regular case instead of 7805 in the TO case! You just need to sharpen 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 +5v supply voltage is supplied, there is an SMD transistor (not in all models). Through it, voltage is supplied to the board. Most likely he is in a “cliff”. It is necessary to short-circuit the terminals of its collector and emitter.
If the device works normally after this, then you can either replace the transistor, or simply leave the jumper. Everything will work fine even with a jumper, it has been tested on more than a dozen devices!
If there is no such transistor on the board or the device does 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 functionality were unsuccessful. Unfortunately, the reason for this behavior could not be determined exactly.
After the video, many users from different sites, as well as YouTube subscribers, asked for instructions on how to assemble a bug from an FM modulator. FM modulator has become quite a popular device recently. It is designed to transcribe audio tracks and transmit over a radio channel. Range up to 10 meters.
In order to assemble a bug from a modulator, you do not need any special skills or knowledge in radio electronics, just the basic knowledge of a novice radio amateur. We disassemble the FM modulator and throw out the housing. 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 powers 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 very high quality and there is no frequency slope. 
The circuit can be powered even from one 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, take a thin and sharp metal object (necessarily bare), for example a needle, and one by one touch all the pins of the microcircuit with the tip of the needle. When we touch some pins, we observe distortion in the radio receiver. 
We choose the output that responds to touches more than the others, that is, the distortion in the receiver is louder than in the case of the others. Directly to this pin we connect (solder) a non-polar capacitor (ceramic or film, it doesn’t matter) a capacitor with a capacity of 0.1 µF.
We send an audio signal to the free output; when we connect an electret microphone, we get a radio transmitter.
FM MODULATOR FOR CAR
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:
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:
FM modulator operating instructions. 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 5 V and consumes 5-10 mA. 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 euros, is of interest in terms of its use in various amateur radio devices.
Suggestions and discussions on the device write to
For beginners, I propose a working circuit for driving an FM modulator - a standard 1-milliwatt automotive modulator. The preamplifier is made on 2 KT368, the remaining 2 stages are taken from a 5-watt radio station. I would like to immediately warn you that it is inadmissible to use a transmitter of such power outside the FM broadcast range - 88-108 MHz! Schematic diagram of the driver and in the figures below.
In front of the preamplifier there is a 5th order P-filter, and there is a 5-volt power supply for connecting the modulator itself!
In the version assembled in the photo, instead of the domestic KT610 microwave transistors, I preferred the imported analog 2n4427, it is easier to solder - I drilled 3 holes and that’s it.
Instead of a permanent capacitor, which goes from the 2n4427 collector to the KT920B base, I always use a trimmer - this makes it much more convenient to tune the circuits to resonance (to the maximum level of the RF signal).
A correctly assembled circuit with working parts works immediately, the only thing is that you need to select the modes of the KT368 transistors with resistors that are located on the collector base. In the output P-filter, the coils are wound on a mandrel with a diameter of 5 mm and contain 6 turns, the capacitance of the capacitors in the filter is 24pf-43pf-24pf. Author of the design: dola
