Car audio system modification solutions

The car's audio system only has two functions: radio and tape player. Can the existing audio system be used to play MP3 music? The author studied the car audio circuit and found that its function adjustment is controlled by bus. The signal selection IC model is TDA7313, as shown in the middle figure. The left channel signal is input to the integrated circuit through pins (13), (14), and (15), and the right channel is input through pins (9), (10), and (11) for switching. The radio signal is input through pins (11) and (15), and the tape player is input through pins (10) and (14). Although there are two input terminals left unused. However, the circuit is controlled by bus, and it is impossible to change the software. The only way is to find a way to change the hardware. After analysis, it is relatively easy to change the radio part. First, solder off Cl and C6 in the middle figure and solder them to the positions of C6 and C7 in the upper figure. The (1) and (5) pins of the signal terminal CN are connected to the negative poles of the original Cl and C2 positions of the middle picture with shielded wires, the (3) and (4) pins are connected to the positive poles of the original C1 and C2 positions of the middle picture respectively, and the (2) and (6) pins are connected to a signal input socket as the MP3 audio source input. The signal input socket and the signal switch key AN are installed in the appropriate positions of the audio control panel respectively. The +KV power supply in the above figure is connected to the (2) pin of the TDA7313 with a lead, and the +12V is connected to the input power supply of the audio system. The modified circuit is shown in the figure below. In order to ensure that the audio signal is not distorted, a small relay is used here to switch the signal. After the author's design and continuous improvement, the circuit shown in the figure is finally formed.

The switch circuit is composed of two integrated circuits, ICl (NE555) and IC2 (CD4013). ICl and peripheral components form a monostable trigger, and IC2 and peripheral components form a bistable circuit. The control principle is as follows: When the sound system power is turned on, a 5V power supply is input to the +KV terminal, and the voltage across C1 cannot change suddenly. ICl outputs a high level.
IC2 flips, VT turns on, the relay is energized, and the signal is switched to the signal input socket, which is not allowed. So R2 and IC3 are connected to the (4) pin of ICl to form a power-on reset circuit. Make the reset time Tl greater than the charging time TO of Cl. In addition, a capacitor is connected to the negative pole of the (11) pin of IC2 to ensure the accuracy of the control. When the power is turned on, the previous state is still changed. Use MP3 as the sound source input. After inserting the MP3, press the touch key. AN, the voltage of ICl's (2) pin is 0, which is lower than 1/3VCC. (3) pin outputs high level, IC2 flips to output high level, VT is turned on, relay K is energized, and the contact is switched to the signal input socket. Then you can enjoy MP3 music. After releasing the AN button, the power supply charges Cl through R1. When the voltage is higher than 2/3Vcc, it outputs low level and waits for the next button press. When we want to listen to the radio, we just need to press AN again. ICl outputs high level again, and IC2 flips to output low level again. Relay K loses power, the normally closed contact is closed, and the radio signal channel is connected.