DIY audio transmitter. Low power radio transmitter circuit

Hello friends. Using 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 one specialized BA1404 chip. This chip already includes a stereo audio 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 - schematic diagram of a stereo transmitter on BA1404

The device is assembled on one-sided fiberglass laminate measuring 35x50 mm. The printed circuit board is shown in Figure 2.

Figure 2 - printed circuit board of a stereo amplifier based on the BA1404 chip

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 interfering with radio stations is punishable. I advise you to read the Federal Law on Communications No. 126-FZ of July 7, 2003. Circuit C13, C14 and L1 are responsible for the operation of the transmitter at a certain frequency. By adjusting capacitor C14 and increasing or decreasing the distance between the turns of coil L1, we can achieve operation of the transmitter 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 adjusted by ear, by increasing or decreasing the distance between the turns of the L2 coil. It is advisable to use an antenna with a length equal to a quarter of the wavelength. You can also use smaller antennas, but the communication range will decrease.

References

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The topic of FM transmitters, including bugs, is covered far and wide. The Internet is full of radio microphone circuits based on transistors and microcircuits... And we decided to make a miniature tube FM sound transmitter using ECC91. The output power of this baby is a whopping 3 watts! Operating frequency 88-108 MHz. The circuitry is standard - the usual switching on of the lamp in generator mode. Adjusting the anode-modulator feedback using a small trimming capacitor. Frequency regulation using a variable capacitor.

Schematic diagram

There is no basic diagram, since everything was done from the head. Here are a couple of options suitable for domestic 6N3P radio tubes, if you want to repeat the design. , which can be used to select a foreign analogue.

The second version of the circuit with an RF power amplifier using a 6P15P (6P14P) lamp.

A couple more options:

Transmitter setup

Tuning the antenna also by adjusting the circuit capacitance. Silver plated coils and connector. The box made of foil fiberglass provides easy processing and good shielding. The dimensions of the transmitter turned out to be only 9x4x6 cm. Power with mains power is 2 W at a voltage of 240 V. In principle, it can be increased to 3 W through the correction of grid resistors and the resistor powering the generator. There were no problems with the launch - it started right away. Power consumption is 20 mA at 250 V, that is, 5 W.

For this FM transmitter, it is also planned to make a very small power supply with a converter from lithium batteries and voltage stabilization.

We warn you: according to the law, broadcasting on FM frequencies without the appropriate permission is prohibited!

Finished design and testing

Measurements of the quality of audio modulation were not made - but the music transmitted to the receiver plays really well by ear, you can listen to it calmly. The antenna is a regular telescopic one - at its base there is an adjustable stretch circuit.

Many beginner (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 range is a fascinating and useful topic. Radio microphones, bugs and other devices can be assembled based on FM radio transmitters. There are many schemes for such devices, but finding a simple, powerful and at the same time stable UHF generator is a problem. After a long search, the choice fell on the following scheme.

The block was built on the basis of well-known circuits, but several modifications were added. The system works almost perfectly, the range is large, and the sound quality is good. BF240 transistors are used, but others can be installed here from the list below. The frequency is changed using a potentiometer.

List of semiconductor elements for assembly

• BB105G
• BB104G
• BF240 (BF199, BF195, BF183,184,185)
• 2n2369
• 1n4007

There is only one, very easy to wind, reel. Many people have problems with this, but anyone can wind 5 turns of 1 mm wire on a 5 mm mandrel.

As for shielding, the tin does its job. When tests were done without a screen, the frequency floated and responded to the approach of a hand. After applying the shielding, the circuit worked stably and no longer reacts to the approach of a hand.

Capacitors and power supply chokes can be useful to prevent self-excitation. This did not occur during testing, so the decoupling was not installed.

In addition to the output power level of the radio transmitter, a lot depends on the antenna. You can even receive a signal from it at a distance of up to 1 km if you place a long pin a couple of meters away.

I am posting a small collection of schematic diagrams of radio transmitters collected from various foreign sites. Starting from low-power, a few milliwatts, and up to powerful multi-watt UHF amplifiers. I haven’t tested the functionality, but the circuitry inspires confidence. All transmitter circuits are designed for the standard broadcast VHF range 88-108 MHz.

FM Transmitter in a pen

The FM bug in a writing pen project is very popular among beginning radio amateurs. In an effort to reduce the size of this design, surface mount components are used. The circuit has low power consumption, but sufficient output power to cover a radius of 50 - 200 m. You can put watch batteries or a lithium-ion battery from the Bluetooth system here.

FM transmitter for 5 kilometers

The proposed broadcast transmitter is indeed very stable, has a complex, but high-quality and well-thought-out circuit design, and uses standard FM frequencies 88 - 108 MHz. Its range of action is a real 5 km. The circuit includes a stable generator powered through an LM7809 stabilizer - this is a 9 V stabilized power source, on transistor T1 and a frequency adjustment element 10K potentiometer. The RF output power of this transmitter is about 1 W. A pair of MV2019 varicaps function as variable capacitors.

The final stage of the FM transmitter is a powerful microwave transistor of at least one watt of power. You need to use transistors 2N3866, 2N3553, KT920A, 2N3375, 2SC1970 or 2SC1971. Do not forget to install an effective heatsink for the T5 transistor, because it becomes slightly warm during operation. The circuit will require a 12V/1A power supply.

Coil winding data:

• L1 = 5 turns on 4 mm frame
• L2 = 6 turns on 6 mm frame
• L3 = 3 turns on 7 mm frame
• L4 = 6 turns on 6 mm frame
• L5 = 4 turns on 7 mm frame

Everything is wound with a wire about a millimeter in diameter. Transistors T1 = T2 = T3 = T4 = BF199, T5 = 2N3866 or 2SC1971, BLY81, 2N3553.

15 W UHF for 88-108MHz range

The RF power amplifier amplifies all frequencies 88-108MHz from 1W input power received from the FM transmitter to 15W. The circuit includes a multi-level low-pass filter and is highly efficient. With a good antenna, the expected transmission radius is at least 20 km. It uses 2SC1972 high power RF transistor (175MHz, 4A, 25W), which should be installed on the heatsink to dissipate excess heat.

Inductors L1-L6 with 0.8 mm wire with a frame diameter of about 5 mm. If you put transistor C2538 here, the power will be even greater.

When debugging, the circuit must be connected with a load equivalent, for example a 50 Ohm 10 Watt resistor. The power source power is at least 2.5 amperes, the antenna resistance is strictly 50 Ohms. Enter the setting only with the supply voltage reduced to 9 Volts; when measuring high-frequency voltage at the antenna output, you do not need to use a conventional multimeter - there will be false readings due to interference on the device microcircuits.

300 mW VHF transmitter

The last scheme is also of interest as it is quite thoughtful and not hackneyed. Although, in principle, everything is as usual here - a generator with a power stabilizer and a high-frequency power amplifier with adjustable harmonic suppression circuits. Due to the 12-volt power supply and the 2SC2538 transistor, it was possible to obtain a range of up to a kilometer on a small helical antenna.

I present to your attention a diagram of a simple and tested radio transmitter - a small one. The circuit is simple and contains a minimum of radio components - just what a novice radio amateur needs. Below are the characteristics and a list of radio components for its self-assembly.

Radio transmitter characteristics

• Radius of action - 50m;
• Supply voltage - 3.7V (I used a Li-Ion battery from a phone);
• Frequency/Band -95 MHz/PM

Parts list for the circuit:

T1 - transistor KT3102 (analogue of BC547)
R1 - resistance 270 Ohm
R2 - resistance 4.7K
C1 - capacitor 1000 pF (code 102)
C2 - capacitor 8.2 pF
C3 - 10 pF capacitor
L1 - coil without frame. Contains 12 turns of wire with a diameter of 0.4 - 0.6 mm, mandrel diameter 4 mm. I wound the coil on a drill. You can also fill the coil with paraffin after tuning so that the frequency does not fluctuate too much.
Bat1 - power supply.
Ant1 - 400mm flexible wire.
Mk1 - microphone from a mobile phone.

Microphone for radio transmitter

Often, after making a radio microphone, a novice amateur complains about its poor acoustic sensitivity. What is the reason? In the microphone itself, in an installation error, in the device circuit? It’s no secret that microphones in “tablet” housings have a wide range of parameters. In addition, performance deteriorates due to overheating during soldering, shock when dropped, etc. Often you can buy a dead microphone. Therefore, the microphone should be checked before use. The simplest test can be carried out using a computer sound card. To do this, take a stereo plug with a diameter of 3.5 mm and solder a piece of two-core wire to its contacts. We solder the microphone to the other end of the wire.

Polarity must be observed: the central contact of the plug is positive, the body is negative. A microphone's minus is always connected to its body. So, we assembled this circuit and inserted the plug into the microphone input of the sound card. Next, check the status of the microphone input. Double-click on the speaker image on the taskbar: the audio settings window will appear. Find "microphone" and uncheck "Off". Also make sure that the microphone volume is not turned down to zero. Now, if everything is working properly and connected correctly, sounds will be heard in the speakers. We have checked the microphone and can continue assembling.

PCB

Surface mount printed circuit board. Also, the printed circuit board can be made in different sizes depending on your convenience. I made a printed circuit board for ordinary parts, but if you make it on SMD, it will be much smaller.

Setting up the radio transmitter

The frequency is selected by stretching and compressing coil L1. I tuned in to the phone radio. There is an automatic search for radio stations - this is what you need. We turn on the beetle and turn on auto search and it finds the desired frequency. That's it, set up. It worked for me right away.