Overall module design of smartphone remote control circuit

　　Power supply circuit: Since the power supply circuit of this system needs to supply power to two modules, the single-chip microcomputer and the voice module ISD1820, which have relatively high power requirements, the transformer is used to step down the voltage, and then the integrated voltage regulator 7805 is used to stabilize the voltage to provide +5V voltage to the entire system. The circuit diagram is shown in the figure, where IN-AC is the input end of the 9V AC transformer, D1, D2, D3, and D4 form a bridge rectifier circuit, which is filtered by the large filter capacitor C6 (3300uF) and sent to the input end of the integrated voltage regulator 7805. After being output from the output end, it is filtered by C7 (1000uF) to power the entire system. In this power supply circuit, C4 and C5 are AC high-frequency coupling capacitors to prevent AC high-frequency signals from entering the system through the power supply circuit and interfering with the system. R5 and LED form a power indicator circuit to show whether the circuit is working.

　　Dual audio decoding circuit: It converts various control signals sent from a remote location on the line into 4-bit binary digital signals. When a mobile phone presses a button, each time a number is dialed, a dual audio component composed of two audio signals of different frequencies is sent out, so it is called "dual audio". For example, when dialing 0, the two audio signals sent out are 941Hz and 1336Hz respectively, which means that the remote control instructions are transmitted on the telephone line as dual audio signals. Since the single-chip microcomputer cannot recognize dual audio signals, the test board must first convert the dual audio signals into digital signals after receiving the dual audio signals before they can be recognized and processed by the single-chip microcomputer.

　　MT8870 is a special decoding chip that converts dual audio signals into digital signals. The typical application of this chip is shown in Figure 11. It has 18 pins and the peripheral circuit is very simple. The circuit works in an orderly manner. As shown in Figure 11, the 18th and 19th pins are connected to a 12MHz quartz crystal, and a 30pF capacitor is connected to the ground at both ends of the crystal. The 9th pin is the reset terminal. A 10k resistor R8 is connected to the ground at this pin to ensure that the pin is at a low level of 0 when it is working normally. At the same time, in order to give this pin a high-level reset pulse when power is applied, a 10μF electrolytic capacitor C4 is connected to the power supply VCC. A manual reset button SB1 is also connected to both ends of the capacitor C4. The function of R11 is to prevent the button contacts from being impacted by a large current when the button is pressed, and to prevent the contacts from being oxidized. The other pins can be connected as shown in the figure.

　　Password lock decoding circuit: The password lock circuit is mainly composed of decoder U7, trigger U6 and related components. The input terminals of U7 are A, B, C, D, and the output terminals are Q0 ~ Q9. It can decode the binary code sent by the decoder into decimal code output; U6 contains 4 RS triggers. Each trigger has two input terminals R and S and one output terminal Q. When a high-level pulse is input to the R terminal, its output terminal Q changes to a low-level "0" state and maintains the "0" state; when a high-level pulse is input to the S terminal, its output terminal Q is a high-level "1" state and can maintain the "1" state. Connect the circuit as shown in Figure 12, and the power-on password is 1359. After the circuit is connected, the lock can be unlocked if the password input sequence is correct. When the other party presses the "1" key, after decoding and decoding, the output terminal Q1 of U7 outputs a high level, that is, the high level at point A. This level is added to the S0 terminal of U6A through R23, making its output terminal Q0 a high level and D14 cut off; when the other party presses the "3" key, after decoding and decoding, the output terminal Q3 of U7 outputs a high level, that is, the high level at point b. This voltage is added to the S1 terminal of U6B through R12, making its output terminal Q a high level and D13 cut off; and so on, when the other party presses the "5" key, the output terminal Q2 of U6C outputs a high level, and when the other party presses the "9" key, the output terminal Q3 of U6D outputs a high level, and this high level is used as the power supply of U4, so that U4 starts working.