MP3 repeater made with BA1404

How the Circuit Works

The left and right channel audio signals are input to IC1 (BA1404) from pin 18 and pin 1 respectively. After being amplified by the internal L and R amplifiers, they enter the balanced modulator, and the modulated stereo signal is output from pin 14. R1 and C1 (R2, C2) form an input pre-emphasis network with a time constant of 50μs to match the frequency characteristics of the FM radio. C3 and C4 are input coupling capacitors. RP1 is a balance adjustment potentiometer. The 38kHz oscillator needs to be connected to an external 38kHz crystal. After the oscillation signal is buffered and divided by 1/2, a 19kHz pilot signal is output from pin 13. R3, C7, R4, and C8 form a mixing network to mix the stereo signal output from pin 14 with the pilot signal output from pin 13 into a composite signal, which then enters IC1 from pin 12 to modulate the RF signal. L1, C10, C11, C12 form a radio frequency oscillator with the internal circuit of IC1 through pins 10 and 9. The oscillation frequency depends on L1 and C10, which is about 92MHz. The radio frequency signal modulated by the composite signal is amplified by the internal radio frequency amplifier, output from pin 7, and coupled to antenna W by C16 for transmission.

The power supply voltage of the integrated circuit BA1404 is allowed to range from 1.0V to 2.5V. The 12V power supply of the car must be stepped down before it can be used as the working power supply. In the circuit diagram of Figure 1, the working voltage of BA1404 is designed to be 2V, R5 is a voltage-dropping resistor, VD1 is a voltage regulator, and a light-emitting diode is used as the voltage regulator here. The voltage drop of the light-emitting diode is generally 2V, which has certain voltage-stabilizing characteristics and can also serve as a working indicator light, killing two birds with one stone. C17 and C18 are power filter capacitors.

Production, debugging and use

Except for the electrolytic capacitor, the other capacitors should use high-frequency ceramic capacitors. RP1 uses a linear trimmer potentiometer. VD1 is a light-emitting diode, and you can choose your favorite color. Since VD1 also serves as a voltage regulator, the forward tube voltage drop of the selected light-emitting diode must be between 1.5V and 2V. The inductor needs to be self-made. L1 is a hollow coil. Use 0.5mm diameter enameled wire to wind 4 turns on a round rod with a diameter of about 3.5mm (such as a ballpoint pen core). Remove the coil from the round rod and stretch it to 6mm. L2 uses a 100kΩ resistor as a skeleton, and uses 0.1mm diameter enameled wire to wind 80 turns on it. The two ends of the coil are welded to the two pins of the resistor. The audio signal input uses a 3.5mm stereo socket. Antenna W is replaced by a 20cm long wire. The 12V power input is connected to a car cigarette lighter plug. The whole machine is installed in a small plastic box.

The debugging includes two items: ① Adjust the transmission frequency. Turn on the power of the MP3 repeater, connect the audio signal to the input terminal, and then use an FM radio to receive the FM signal sent by the repeater. The signal should be received near 92MHz. If the signal cannot be received, you can use an insulating object to gently turn L1 to change its turn distance. Increasing the turn distance can increase the transmission frequency, and reducing the turn distance can reduce the transmission frequency until the FM radio receives the signal from the repeater. During the adjustment process, be careful to avoid the frequency of the local FM radio station. ② Adjust the channel balance. Adjust RP1 to make the left and right channel signals balanced and consistent.

To facilitate the connection between the MP3 player and the repeater, you should make an audio cable yourself. The method is to use a two-core shielded wire to connect two 3.5mm diameter stereo plugs, as shown in Figure 3. The two cores of the shielded wire connect the R end of the two plugs to the R end and the L end to the L end, and the shielding layer connects the common ends of the two plugs. Don't make a mistake.

When using, insert one end of the self-made connecting line into the headphone jack of the MP3 player, and the other end into the audio input socket X of this transponder. Insert the power plug of the transponder into the car cigarette lighter, and the power indicator VD1 should be on. Turn on the MP3 player and use the car FM radio to search and receive the stereo signal played by the MP3. If it cannot be received due to the shielding of the car body, move the transponder antenna closer to the car window glass, or install an in-car antenna for the FM radio in the car.