Not so long ago, a Chinese manufacturer developed this device for the happy owners of cassette recorders who do not have enough money to buy a normal MP3 player.
Cheap and cheerful - it is relatively cheap (I took it for 200 rubles), while it has a number of advantages, including a remote control remote control. It's simple: plugged it into the cigarette lighter, plugged in a USB flash drive with your favorite music, tuned the radio to the frequency of the transmitter and that's it! Click from the remote without touching your hands from a distance.
I don't have a car, but I decided to use this thing in my own way. Like a stereo transmitter. Why do I need it? And in order to broadcast sound from a laptop to a music center. The thing is, I like to watch movies on big screen, on the projector. I connect the video from the laptop directly, and to connect the sound you need to pull a long wire to the center. So that this was not, I decided to get rid of the wires in my own way.
Bought, disassembled. Having made a peculiar bunch of details.
Divide the device into two parts: a small board is a stabilizer. It reduces the voltage to 5 volts and stabilizes it accordingly. There are 3 wires going to it: two power wires and a third antenna (white in color). The big board with the display is the MP3 player itself.
We solder all three wires from a large board. We solder a longer wire in place of the antenna wire to increase the transmission radius. We take a USB adapter and solder power from it to the board as shown in the figure.
Next, we connect the sound to the transmitter. We find the transmitter chip. It receives sound from the processor through two chip-capacitors. We remove these capacitors, I just gently knocked them down with a screwdriver. The work is painstaking. We solder two capacitors with a nominal value of 0.01 ... 0.1 microfarads to the output of the microcircuit and apply sound to them. We take the common wire from the minus of the board. That's actually all. It would be nice to add a divider from resistors to each input, say 1: 2, otherwise the laptop output is more high-voltage than necessary. But then I realized it.
Close, check. Works!
Hello, friends. With this transmitter, you can easily transmit a stereo signal from a smartphone to a car radio with an FM receiver. This stereo transmitter is very easy to manufacture, it is built on a single specialized BA1404 chip. This microcircuit already includes a stereo audio frequency amplifier, a multiplexer, a subcarrier frequency generator, a carrier frequency generator, and a radio frequency amplifier. The supply voltage of this microcircuit is 1-2V, current consumption is up to 5 mA. Coils L1 and L2 are wound with PEV-2 wire with a diameter of 0.5 mm. on a mandrel with a diameter of 3 mm. and contain 4 turns. The device diagram is shown in Figure 1.
Figure 1- BA1404 Stereo Transmitter Schematic Diagram
The device is assembled on a single-sided fiberglass 35x50 mm in size. Printed circuit board shown on Figure 2.
Figure 2 - printed circuit board stereo amplifier chip BA1404
Radio elements and analogues
Transistor VT1 KT368 can be used with any letter index, transistor KT399 is also suitable
Trimmer capacitor C14 - CTC-05-10RA, ceramic capacitors K10-17 or similar imported ones, for example CL0805.
Resistors are ordinary MLT or similar imported ones.
Setting up and configuring the device
First of all, the transmitter should be tuned to a frequency free from radio stations. Remember that interference with radio stations is punishable. I advise you to read the Federal Law on Communications No. 126-FZ of 07.07.2003. The circuit C13, C14 and L1 is responsible for the operation of the transmitter at a certain frequency. By adjusting the capacitor C14 and increasing or decreasing the distance between the turns of the coil L1, the transmitter can be operated at the frequency we need. Circuit C20, C21 and L2 are responsible for matching the device with the antenna. To adjust the matching, you can use the field strength indicator, if it is not there, then the receiver should be moved away and tuned by ear, by increasing or decreasing the distance between the turns of the L2 coil. It is desirable to use an antenna with a length equal to a quarter wavelength. Antennas can also be used smaller, but the communication range will decrease.
Bibliography
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The use of superheterodyne receivers, the bandwidth of which can be narrowed down to the value active width spectrum of the received signal, can significantly increase the noise immunity of receivers and their sensitivity. As a result, the range of the equipment increases without increasing the transmitter power.
However, superheterodyne receivers, the bandwidth of which does not exceed 10-12 kHz, require such a high stability of the transmitters that the radiated frequency deviations will not exceed 5-10% of the bandwidth. In absolute terms, this is 0.5-1.2 kHz. Consequently, the relative instability of transmitters in the range of 27-28 MHz should not exceed 1.8-10 "5. Only quartz-stabilized oscillators can provide such high requirements.
It has already been noted above that it is not advisable to carry out modulation in the master oscillator itself, so the transmitters are obtained at least two-stage. On fig. 3.24 shows a diagram of such a transmitter, in which quartz ZQ1 operates on the third mechanical harmonic. The resonator is connected between the collector and the base of the transistor, which, as practice shows, allows you to combine the simplicity of the circuit with the high reliability of its operation, even with low-activity quartz. The output power of the transmitter does not exceed 10 mW.
It must be borne in mind that quartz operating at the first harmonic is produced mainly up to 20 MHz. Since the resonant frequency depends on the geometric dimensions of the quartz plate, at higher frequencies the dimensions are so small that it is technologically difficult to manufacture a plate with the
acquired characteristics. For this reason, if a frequency of more than 20 MHz is applied to the quartz body, then this quartz is most likely harmonic.
Since opposite potentials must be induced (due to the piezoelectric effect) on opposite sides of the plate, excitation is possible only at odd harmonics, usually not higher than the seventh. To prevent such quartz from being excited at the fundamental frequency, the circuit must necessarily have a circuit tuned to the required harmonic. In the circuit under consideration, this circuit consists of an inductor L1 and a capacitor C2.
On the transistor VT2, a power amplifier is implemented that operates in class B mode due to the absence of a constant bias based on the transistor. The emitter circuit of this transistor is switched by an electronic key VT2, controlled by modulating pulses from the output of the encoder. The antenna is connected to the output circuit through an extension coil L3. Partial switching is applied through a capacitive divider С6С7, which provides the matching mode.
Details and construction
The printed circuit board is shown in Figure 3.25. The ZQ1 quartz resonator is used at a frequency of 27.12 MHz. You can apply and quite common at 27.14 MHz. Coil L1 is 24 turns of wire with a diameter of 0.12-0.15, wound on an MLT-0.5 resistor with a resistance of at least 100 kOhm. L2 is wound on a 6 mm diameter carcass with a carbonyl iron tuning core and has 9 turns of 0.5 mm wire.
The L3 extension coil is a standard DM (PDM) 5 uH choke. All transistors can have any letter index, it is also possible to replace it with KT3102.
Ceramic capacitors, type KM-5, KM-6 or similar. Whip antenna, 40-60 cm long.
Setting
The setting is reduced to setting the L2 core to a position that ensures the maximum amplitude of the output oscillations. The pre-modulation input is connected to the plus of the power supply. The amplitude is controlled with an oscilloscope as described in the previous paragraphs. Between the VT1 base and the case on the diagram, a dotted line shows a capacitor (a place is provided for it on the board). If the generator does not self-excite (due to the low activity of quartz), a capacitor must be soldered to this place, choosing it in the range of 120-180 pF according to the maximum output oscillations. In the absence of these quartzes, you can try to set the resonators to a frequency three times lower than the required one (9.04-9.046). The transmitter works confidently when the supply voltage drops to 5 V.
The presented do-it-yourself radio bug can transmit sound at a distance of up to 500 meters. You can also use it to make an FM tuner and transmit a signal from your phone to the radio.
Radio transmitter on kt368
Do-it-yourself radio transmitter on kt368
In this article I want to talk about a radio transmitter on a single transistor.
It can be used both for wiretapping, and also make a repeater with it, replacing the microphone with an audio signal input.
DIY radio transmitter on MC2833
DIY radio transmitter on MC2833
Using the MC2833 chip, you can make a fairly high-quality FM transmitter. This microcircuit contains an oscillator, an RF amplifier, an audio amplifier and a modulator. Available in miniature plastic package with surface mount end leads and standard package.
Do-it-yourself fm transmitter for 1 km and above
DIY FM transmitter for 1 km
This is a fairly powerful 2W FM transmitter that will provide up to 10 km of range, of course, with a well-tuned full-fledged antenna and in good weather conditions, without interference. The scheme was found in the bourgeoisie and seemed interesting and original enough to be presented to your court))
Stereo radio transmitter do-it-yourself circuit
DIY stereo radio transmitter
In a car, when it is not possible to turn on music from other sources like radio, and at the same time you want to listen not to what the radio hosts provide, but to your own music, as an option, you can use the made diy FM stereo transmitter .
The radio transmitter is assembled in a standard plastic case from some kind of device. The front panel has an audio jack input and a setup button. There is a power connector on the back surface. The filter output is connected to the +12V terminal, so the power cable is used as an antenna. The printed circuit board is fixed with only one screw inside the box.
Audio transmitter
DIY audio transmitter (music transmitter)
In this article I would like to present music transmitter. I tried to assemble a radio transmitter using a varicap in the modulator. Since he was needed for the transfer sound signal, and not a conversation, put a plug instead of a microphone. Coil 9 turns of wire with a diameter of 1 mm, the middle tap is soldered. I shoved a small piece of foam rubber inside the coil and dripped it with paraffin (candle) so that the coil would not bend when touched, because the frequency depends on it, and it is very easy to knock it down.
Do-it-yourself stereo transmitter circuit
Radio stereo sound transmitter circuit
For stereo transmitters, there is ASIC, BA1404.ABOUTfeature transmitter on BA1404 is high quality sound and improved stereo sound separation. This is achieved by using a 38 kHz crystal that provides the pilot tone frequency for the stereo encoder.
A stereo transmitter can be used both at home and in a car to transmit sound from a carrier (phone, player, etc.), since it does not transmit stereo sound.
Such a small stereo transmitter will be a good replacement for an FM tuner.
DIY FM transmitter
FM radio transmitter
Do-it-yourself VHF-FM radio transmitter operates in the non-traditional range of 175-190 MHz. These radio microphones are easy to assemble. In order to increase the frequency stability of the master oscillator, the base circuit of the power amplifier transistor is powered by a voltage regulator (R5, LED1).
Used SMD RED Light-emitting diode. The frequency deviation during the “drawdown” of the power supply from 3 to 2.2 volts is no more than 100 kHz. When you touch the antenna with your hand, the frequency deviates slightly too. If you have a receiver with a good AFC, it tracks this change and the frequency drift does not occur at all during the operation of the transmitter.
Powerful radio transmitter for 500 meters with your own hands
Do-it-yourself radio microphone for 500 meters
I want to present the design enough powerful radio bug, Range which is up to 500 meters with line of sight. The device was assembled almost a year ago for our own needs. beetle revealed amazing results: The frequency almost does not float (only 0.1-0.3 MHz every 100 meters). The device does not respond to touches from the antenna and other parts (except for the circuit and the frequency-setting circuit) - this is a very important point, since almost all circuits from the Internet have such a problem.
In the practice of creating radio bugs, we often encounter the problem of the smallest possible size of a bug. Today we will talk about just such a bug: NEMESIS-2, as it was called. Nemesis was assembled on smd components, due to which it became possible in a significant way downsize bug several times, the radio bug is so small that it fits, for example, in one cigarette, lighter or mobile phone. A little about the parameters: the frequency range within 88-108 megahertz, microphone sensitivity about 5 meters, in a quiet room you can hear the ticking of a wall clock. So this signal is easy to receive from this bug to the radio, whether it is in the phone, or just stationary. Let's move on to the diagram and details.
Answer
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A radio transmitter converts sound into an electrical signal, amplifies it, converts it, and emits it as radio waves. It is a small compact device capable of hidden bookmarking in the listening room. To increase battery life and detection, it is usually manufactured with low power. One of the most successful FM radio transmitter circuits is shown in the figure.
Radio transmitter circuit:
Coil L1 - 5 + 5 turns of wire 0.8mm. Inductor Dr1 - any design (factory, home-made on a ferrite ring, on a low-resistance resistor), with an inductance of 10-100 μH. Microwave transistors are interchangeable with C9018, BFR93A, BFR92, BFS17A, BFR91, BFR96, BFR90, BFG67, BFG591. The pinout of the most popular transistors is shown in the figure.
An FM radio transmitter usually consists of five main stages:
ULF - low frequency amplifier; ZG - master oscillator; UM - power amplifier; SC - matching stage: BP - power supply (battery, stabilizer).
The principle of operation of the transmitter.
The electrical sound signal from the microphone is fed to the ULF (low frequency amplifier), where it is primary amplified, which achieves high sensitivity. This allows even a whisper to be heard in the room. Some professional devices have an Automatic Gain Control (AGC) system, so that a loud sound signal is not distorted. AGC principle - weak signal is enhanced by 100%, and the strong is weakened. After the amplifier, the signal is fed to the ZG (master oscillator). The CG generator produces undamped high-frequency oscillations of a certain frequency, into which it inserts low frequency(frequency modulation occurs). ZG is essentially the "heart" of a radio bug, to which stringent requirements are imposed. It must maintain the specified frequency and prevent the interruption of generation.
To increase the range use PA (radio frequency power amplifier). And in order to match the radio transmitter with the antenna, a matching stage (SC) is used. It allows you to squeeze the maximum out of the circuit and prevents the frequency drift when changing the length and direction of the antenna. But to simplify the design, and due to the low power, the SC is not used in this circuit. To receive the signal, an FM radio receiver is used, which is tuned to the frequency of the radio transmitter.
