Application of AT89C51 microcontroller in telephone remote controller

With the development of the communication industry, telephones have entered thousands of households, and the technology of using telephones for remote control is increasingly used in life. This article designs a telephone remote control system, which is mainly used for telephone remote control. For household appliances, users can remotely control various appliances (such as rice cookers, microwave ovens, etc.) through any dual-tone multi-frequency phone (including mobile phones and telephone extensions) outdoors according to voice prompts.

　　1 System structure

This system mainly includes a telephone ringing detection circuit, automatic telephone pickup and on-hook telephone, DTMF signal decoding circuit, voice prompt circuit, and audio amplification circuit. The system structure block diagram is shown in Figure 1.

 

When a call comes in, the phone ringing detection circuit detects ringing and waits for the system's default number of rings before the controller automatically picks up the phone. The user sequentially enters the user's operation password and operation instructions under the voice prompts. The DTMF decoding circuit sends the received DTMF decoded digital signal to the AT89C51. If the password is incorrect, the system will automatically hang up. When the password is correct, just follow the voice prompts to complete a series of operations in sequence. When the operation is completed, the microcontroller will return according to the The confirmation information enables control of the corresponding household appliances.

　　2 System hardware design

2.1 Ringing detection circuit and automatic pickup circuit

When the phone receives the ringing signal, there is an AC 48V voltage signal on the phone line. Capacitor C1 blocks the DC and only allows the ringing signal (about 30Hz) to pass through BR1 to rectify the command signal. D1 stabilizes the signal, and C2 Filter the rectified signal to suppress interference. The DC voltage smoothed by C2 is used to drive the light-emitting diode of the photocoupler. When the ringing signal appears, the collector potential of the photocoupler becomes low. The falling edge of this signal applies for an interrupt to the CPU. The circuit is shown in Figure 2. .

 

When the processor detects the preset number of rings, P3.7 of the CPU changes to high potential, causing Q1 to saturate and conduct, relay K1 closes, and the secondary of transformer T1 and R17 (680Ω) resistor form the 300mA of the telephone line The load on the left and right makes the switch think that the phone has been picked up. Transformer T1 is used to output the voice signal to the telephone line. The circuit is shown in Figure 3.

 

2.2 Audio amplifier circuit

Use the LM386 low-voltage audio power amplifier. LM386 is a power amplifier designed for low-voltage users. The internal gain is 20 times. When pins 1 and 8 are connected to resistors and capacitors, the gain can be increased to 200 times. It is widely used, easy to use, and can be connected externally. The number of components is small. The audio amplifier circuit of this system is shown in Figure 4.

 

2.3 DTMF signal decoding circuit

This system uses the conventional MT8888 decoding chip. This chip has the characteristics of strong function, low power consumption, simple adjustment, strong ability to suppress dial tone and stable and reliable operation. Compared with MT8880, it can be matched with more types of microcontrollers and has an external The circuit is simple. When MT8888 detects a DTMF signal input, demodulates it and latches it into the output register, the IRQ/CP terminal outputs a high level. After the CPU detects the signal, it can control the use of the three-state output latch of MT8888. The terminals RD and CS are valid, and the 4-digit BCD codes Q1-Q4 representing the dial value are sent to the CPU for processing. The circuit is shown in Figure 5.

 

2.4 Voice prompt circuit

When a call comes in and the telephone remote controller picks up the phone, the operator will enter the password at the prompt of the voice prompt circuit, select the channel, set various values, and perform operations such as starting up and hanging up. The voice circuit is adopted by the American TI Company. Voice recording and playback chip ISD4004. Contains oscillator, anti-aliasing filter, smoothing filter, audio amplifier, automatic squelch and high-density multi-level flash memory array. The chip design is based on the fact that all operations must be controlled by a microcontroller. Operation instructions can be sent through the serial communication interface (SPI or Microwire). The chip uses multi-level direct analog storage technology, and each sampled value is directly stored on the chip. In flash memory, it can reproduce speech, music, tones and sound effects very realistically and naturally, avoiding the quantization noise and "metallic sound" caused by quantization and compression of general solid-state recording circuits. The sampling frequency can be 4.0, 5.3, 6.4, 8.0KHz. The lower the frequency, the longer the recording and playback time, but the sound quality will be reduced. It can be stored for 100 years (typical value) under power outage and repeated recording 100,000 times. After the voice information that needs to be prompted is entered into the chip segment by segment, the entered information is sequentially output from the audio output terminal under the control of the CPU, and then amplified by the audio power amplifier and transmitted to the telephone line. The circuit is shown in Figure 6.

 

2.5 Relay control circuit

The relay cannot be driven directly by the integrated circuit chip. The relay is controlled through the drive circuit. The circuit is shown in Figure 7.

 

3 Software design

The response software of this system mainly completes the detection of various states and the sending and receiving of data. The software process is shown in Figure 8.

 

This system can use the telephone to conveniently realize remote operation of remote electrical equipment, for example, it can be used as a remote controller for household appliances. Users can use the telephone to control various household appliances in their residence anywhere.

4 Conclusion

This system has been proven by actual operation to have good pick-up effects. It does not affect the normal use of the phone and greatly simplifies the on-hook circuit. The password setting is simple and practical. The electrical control circuit is stable and reliable. The voice prompt function is powerful and easy to set. Flexible and with clear voice, this controller has strong adaptability and stable performance.