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.
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, and 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.
On this I apologize. was with you EGOR .
Discuss the article HOME-MADE FM TRANSMITTER
An electric current flowing in any conductor generates an electromagnetic field that propagates in the space surrounding it.
If this current is variable, then the electromagnetic field is capable of inducing (inducing) E.D.S. in another conductor located at some distance - electrical energy is transmitted over a distance.
This method of energy transfer has not yet been widely used - the losses are very high.
But for the transmission of information, it has been used for more than a hundred years, and very successfully.
Radio communication uses electromagnetic oscillations of the so-called radio frequency range directed into space - radio waves. For the most efficient radiation into space, antennas of various configurations are used.
The simplest antenna - a half-wave vibrator, consists of two pieces of wire directed in opposite directions, in the same plane.
Their total length is half the wavelength, and the length of a separate segment is a quarter. If one of the ends of the vibrator is directed vertically, ground can be used instead of the second, or even the common conductor of the transmitter circuit.
For example, if the length of the vertical antenna is - 1 meter, then for a radio wave 4 meters long (VHF band) it will represent the greatest resistance. Accordingly, the efficiency of such an antenna will be maximum - precisely for radio waves of this length, both during reception and transmission.
To tell the truth, in the VHF band, the most reliable reception should be observed when the antenna is horizontal. This is due to the fact that transmission in this range is, in fact, most often performed using horizontally located half-wave vibrators. Therefore, namely, a half-wave vibrator (and not a quarter-wave one) will be a more efficient receiving antenna.
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Reception of a signal from this simplest VHF radio transmitter to date is carried out on a standard (portable, stationary, built-in cell phone) at a frequency of 90-100 megahertz. The circuit is very simple and even for a person who is just starting his amateur radio activity, its assembly will not be difficult.
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It can be used to solve various typical tasks, for example:
1) wireless headphones.
2) Electronic nanny to control the baby.
3) Tracking bug.
In the presented version, it will work as a set-top box, which will turn ordinary headphones into wireless ones. The radio transmitter is plugged into the headphone jack that your TV has, that is, instead of wires, this simple circuit will now work. Such a refinement can save money by making the device with your own hands.
For work we need:
Soldering iron.
Copper wires.
A plug that matches the one used to connect headphones to a 3.5mm TV jack.
Batteries with a voltage of 3 to 9 volts.
Copper wire with lacquered sheath (will be used for the coil).
Glue Moment if needed.
Old boards (if possible).
A piece of textolite or thick cardboard.
All necessary radio components for the transmitter
The coil must be wound with 7-8 turns of copper varnished wire with a diameter of 0.6-1 mm, on a tube with a diameter of 5 mm, for example, you can use a drill for 5). The ends of the wires on the coil must be cleaned of varnish.
For the body of the created transmitter on one transistor, you can use any suitable box. In the example shown - a container for batteries, from which all unnecessary partitions and other parts have been taken out and removed.
Now we make a textolite socket of the right size and make a lot of holes for parts. The more of them you get, the more convenient it will be for further assembly and soldering of parts.
Now we solder the wires to the plug in accordance with the diagram (the part that is the input)
At the next stage, we put the circuit assembled on the board in a box, for reliability, you can fix it with any suitable glue, but this is not necessary. Just make sure that everything is done neatly and during the operation of the radio transmitter
It remains to set up our transmitter. To do this, use the plug to connect it to the TV. On an FM (VHF) receiver, for example, on a cell phone, we find a free frequency (that is, on which there is no transmission of any radio station) and tune our device to this wave. Frequency adjustment is carried out by a trimmer capacitor using a screwdriver. We smoothly rotate it until the sound from the switched on TV appears on the FM receiver.
That's all, you can turn on your mobile phone's headphones and watch TV without worrying about the noise that might annoy others.
To adjust so as not to permanently open the housing, make a hole in the transmitter housing. 
If the audio jack is replaced with a microphone, then you will have a radio transmitter that you can put next to the baby and turn on the radio in another room, know that the child is awake, etc.
Most likely you will be interested.
Radio transmitting devices (Fig. 13.1 - 13.5) can be obtained by simply combining an amplifier (or generator) of a low frequency (VLF, LLF) and a high frequency generator (HHF).
A block diagram of an amplitude modulated (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 generated by the ULF or LLF is allocated to the load resistance Rh, which is included in the power circuit of the MHF circuit. Since the supply voltage of the RF generator changes in proportion to the audio frequency signal, the amplitude of the high frequency signal is modulated. The generator shown in fig. 13.6. Points A, B, C, D on the generator diagram correspond to the points of its connection on the block diagrams (Fig. 13.1 - 13.5).
One of the ways to obtain amplitude modulation of a signal using a low-frequency choke or winding of an output low-frequency transformer is shown in fig. 13.2. The use of inductors, whose resistance to alternating current increases with increasing frequency, allows you to increase the depth of modulation. In addition, the amplitude of the higher frequencies of the audio range increases, which significantly increases the intelligibility of the signal during reception.
With frequency modulation (FM), usually used in the ultrashort wave range, the frequency of a high-frequency signal is changed. To obtain a frequency-modulated signal, the circuits shown in Fig. 1 can be used. 13.3 and 13.4. In the transmitter circuit (Fig. 13.3), the frequency modulation of a high-frequency signal occurs by applying an audio frequency signal through a relatively small capacitor to the base or emitter of the HHF transistor. In this case, the interelectrode capacitances of the active element (transistor) change, and, consequently, the resonant frequency of the oscillatory circuit is modulated, which determines the generation frequency. Strictly speaking, with this type of supply of the modulating voltage, 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 from the value of the applied voltage (Fig. 13.4). In this circuit, the modulation is turned on / off by the SA1 switch. The RA potentiometer is designed to check the frequency limits of the generator tuning.
Amplitude modulation of a high-frequency signal can be obtained by turning on the GHF instead of the VLF (LLF) load resistance (Fig. 13.5). Capacitor C is designed for high-frequency grounding of the MHF power supply circuit.
In addition to amplitude and frequency modulation of the signal for data transmission, radio communication organizations often use single-sideband, less often phase and other types of modulation.
On fig. 13.7 - 13.16 shows practical diagrams of micro-transmitting devices operating in the VHF-FM band (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 the signals of such devices can be detected (Fig. 13.7 - 13.16) usually does not exceed a few meters. Note that the power of local oscillators - high-frequency generators used in any radio or TV set, often exceeds a few mW.
In the structures 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 field-effect transistor preamplifier. Instead of an electret microphone, an electromagnetic telephone capsule can be used, connected between point A and a common wire (Fig. 13.7, 13.9, 13.10 and 13.12) or a power bus (Fig. 13.8). In this case, resistor R1 is optional. When replacing a microphone, the signal amplitude may decrease, therefore, to increase the bass 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 scheme of the 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, like two drops of water, resembles the previous design (Fig. 13.7) [Рl 10/99-40]. The difference lies in the fact that the diagram (Fig. 13.8) is, as it were, “turned” upside down. Such an unusual location next to almost similar schemes makes it possible to accustom the eye to the "recognition" of structures similar to each other. Schemes fig. 13.7 and 13.8 differ in "electrical" terms by the way the modulating voltage is applied: in the first case, it is applied to the base of the generator transistor; in the second - to the emitter. The inductor contains 7 turns of PEV wire 0.7 ... 0.8 mm and has an inner diameter of 5 mm. The current consumed by the device is 15...20 mA.
On fig. 13.9 shows a diagram of a radio microphone in the range of 66 ... 74 MHz, in the base bias circuit of which an electret microphone is included as a controlled resistor [RL 2 / 97-13]. The antenna is a piece of flexible stranded wire 20 ... 40 cm long. The current consumed by the device is about 1 mA.
The cascode switching 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 the transistor VT2 is an RF generator made on the transistor VT1. In turn, the high-frequency current in the emitter circuit of the transistor VT1 is modulated by a signal from the low-frequency signal amplification stage taken from the microphone.
On fig. 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 taken from an ULF electric player, tape recorder and other devices. The amplitude of the low-frequency signal at the input is within 10 ... 500 mV. Coil I without a frame, has an inner diameter of 4 mm and contains 15 turns of PEV 0.5 wire. Coil L2 is wound over resistor R3 (MLT-0.5) and contains 50 ... 100 turns of thin insulated wire.
On fig. 13.12 and 13.14 are practical diagrams of microtransmitters based on an analog of a lambda diode. A forward-biased junction of a semiconductor diode (LED) was used as a controlled element. Frequency modulation is carried out by changing its dynamic resistance. For the high-frequency component, the capacitance of the LED is much lower than its ohmic resistance. Simultaneously with the function of controlling the frequency of generation, 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 home-made condenser microphone was 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 into a photo slide frame; its capacitance is several picofarads.
For comparison, in Fig. 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 [RL 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 band (66 ... 74 MHz), frameless inductors are used, having an inner 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 approaching / moving apart the turns of the coil, selecting the number and diameter of its turns, as well as the capacitance of the oscillatory circuit capacitor. The body of the electret microphone is connected to a common wire. Reception of high-frequency signals is possible on a portable FM receiver.
To create a video transmitter (wireless transmission of a video signal from a VCR to a TV), G. Roman's scheme [RL 3/99-8] can be used. The oscillatory circuit L1C2 (Fig. 13.16) is tuned to the frequency of one of the channels free from television broadcasting.
Literature: Shustov M.A. Practical Circuitry (Book 1), 2003
Many beginners (and not only) radio amateurs, sooner or later become interested in the topic of transmitters. Indeed, the construction of VHF transmitters for the 88-108 MHz band is a fascinating and useful topic. Radio microphones, bugs and other devices can be assembled on the basis of FM radio transmitters. There are many schemes for such devices, but finding a simple, powerful and at the same time stable generator with UHF is a problem. After a long search, the choice fell on the following scheme.
The block was built on the basis of known schemes, but several modifications were added. The system works almost perfectly, the range is large, the sound quality is good. BF240 transistors are used, but others can be installed here, from the list below. Frequency change is carried out using a potentiometer.
There is only one, very simple coil to wind. Many people have problems with this, but winding 5 turns of 1 mm wire on a 5 mm mandrel will be within the power of everyone.
As for shielding - tin does its job. When tests were done without a screen, the frequency swam and reacted to the approach of the hand. After applying the screening, the circuit worked stably and no longer reacts to the approach of the hand.
Capacitors and power chokes can be useful to prevent self-excitation. During the tests, this did not arise - therefore, the denouement was not set.
In addition to the output power level of the radio transmitter, much depends on the antenna. You can even receive a signal from it at a distance of up to 1 km, if you put a long pin a couple of meters.
