FM wireless transmitter assembled by MC2831A

This article recommends to readers a wireless transmitter assembled with the single-chip FM transmitter integrated circuit MC2831A produced by Motorola, USA. It has the advantages of good sound quality, low distortion, no frequency deviation, and stable RF transmission.

The input end of this transmitter can be plugged into an electromagnetic microphone for communication during the convoy's movement. It can also play music programs from MP3, CD and other audio sources.

It can also forward TV audio and receive it with headphones of FM radio to avoid disturbing others' sleep at night. When used in the car, it can be powered by the 12V cigarette lighter in the car, and at home, it can be powered by AC/DC regulated power supply.

MC2831A transmitter-specific integrated circuit has the characteristics of low voltage power supply and low power consumption. It uses DIP-16 plug-in package. Its main technical parameters are as follows:

1. Operating frequency: 14MHz～60MHz;

2. Working voltage: 3V～8V;

3. Working current: 4mA (when VCC=4V);

4. Microphone amplifier: The input of IC's 5-pin is 1mV, and when f=1.0kHz, the closed-loop gain is 30dB. See the figure on the right for its internal block diagram.

To keep the circuit simple, the audio pager and voltage detector inside the MC2831A are not used.

The working principle of the circuit is shown in the figure below.

The microphone BM or audio signal is input from pin 5 and amplified by the operational amplifier inside the integrated circuit, and then output to pin 3 through pin 6 to modulate the high-frequency signal. Inductor 11 and crystal BC are connected in series between pin 1 and pin 16 to make the internal circuit oscillate. 11 also has the function of expanding the frequency deviation. The carrier frequency signal modulated by audio output from pin 14 is amplified by VT1. At the same time, 1C on the collector of VT1 forms a 4-fold frequency resonance and further expands the frequency deviation. The output signal is coupled to VT2 through C10 and then amplified, and then transmitted to the antenna through C13.

In the circuit, R9, C14, C11, C2, C3 and VD1 play the role of voltage reduction, voltage stabilization and filtering, making the operating voltage of MC2831A stable at 6V.

Component selection: VD1 uses a 5V~6V voltage regulator. The quartz crystal is a 22M~26.9M crystal. Since the crystal determines the transmission frequency, the crystal can be replaced to avoid the radio station frequency. This machine uses a 25M crystal. VT1 uses 9018, and β is preferably 70~100. VT2 can choose domestic medium-power tubes such as 3DA18 and 3DG12B, and β greater than 50. 11 is a color code inductor of 5μH~10μH. 12 and 13 can be wound with 0.8mm enameled wire on a φ6mm drill bit for 6 turns.

The antenna base can use the Q9 socket used for walkie-talkies to connect to the vehicle antenna.

See the picture on the right for the printed circuit board. It is recommended that the integrated circuit use a 16-pin integrated circuit socket to avoid damage to the integrated circuit due to poor welding skills, and it is also convenient for maintenance!

After checking, turn on the power supply and use a multimeter to measure the pin voltage of MC2831A. As long as it is close to the value in the attached table, it is fine. Adjust R6 and R8 so that the collector current of VT1 is 15mA and the collector current of VT2 is 40mA.

Plug in an electromagnetic microphone, connect a 75cm long copper wire as an antenna, turn on the FM radio, adjust the frequency to 100MHz, and the radio speaker should make a "pop" sound when you clap the microphone. You can also make a simple field strength meter. Carefully adjust the transmission field strength to the maximum.

When forwarding MP3, you need to make a connecting cable as shown below.

At short distances, the antenna can be replaced by a 50Ω dummy load on the computer terminal. For example, a VU segment vehicle antenna can be connected to the car, and the car radio can receive satisfactorily within 400m!