【導(dǎo)讀】本文設(shè)計(jì)的是一種基于AT89C2051單片機(jī)的遠(yuǎn)程電話控制系統(tǒng)。和PSTN公用電話交換網(wǎng)的電話遠(yuǎn)程控制系統(tǒng)。DTMF雙音多頻信號(hào),實(shí)現(xiàn)對(duì)近端電器設(shè)備的實(shí)時(shí)遠(yuǎn)程控制。該電話遠(yuǎn)程控制系統(tǒng)。提示音及時(shí)了解受控對(duì)象的有關(guān)信息。用戶還可以查詢其狀態(tài),提供密碼處理功。能,只有輸入正確的密碼才能控制家電,從而提高了安全性。聲（光）智能控制對(duì)信號(hào)的靈敏度要求較高，控制過(guò)程中易受到外部環(huán)境的干擾。采用有線電話控制器使用方便，又體現(xiàn)了科技改變生活的理念，逐漸在家電。產(chǎn)品設(shè)備中被廣泛采用，因此有線電話控制器有較大的市場(chǎng)價(jià)值。電話遙控作為一較新的課題與常規(guī)的遙控方式相比，顯示出一定的優(yōu)越性，不需要進(jìn)行專門的布線，不占用無(wú)線頻率資源，避免了電磁污染。本課題利用單片機(jī)技術(shù)，通過(guò)電話線實(shí)現(xiàn)用外部的普通電話對(duì)家中電器。遙控技術(shù)是通過(guò)一定的手段對(duì)被控物體實(shí)施一定距離的控制，

　　

【正文】 d an electronic circuit in the keypad to generate two output tones that represented the number. These tones are called Dual Tone Multifrequency DTMF signals. d Connecting the Phones: The exchange office had various switches and relays that automatically connected the caller and the called phones. If the called phone handset was offhook when the connection was attempted, a busy tone was generated by the exchange office and was returned to the caller phone. Otherwise, a ringing signal was sent to the called phone to alert the called party. At the same time, a ringback tone was returned to the caller to indicate that the failed phone was ringing. e Answering the Call: When the called party removed the handset in response to a ring, the loop to that phone was closed and a loop current flowed through the called telephone. The exchange office then removed the ringing signal and the ringback tone from the circuit. f Ending the Call: In today39。s most switching systems, the call was ended, the connection was released, only when the calling arty went onhook. SYSTEM DESIGN The system was designed to receive and respond to the DTMF signals. Hence, the caller party should use a telephone set equipped with a DTMF keypad for sending a number. Telephone sets, using the DTMF method for sending a telephone number, were equipped with a pushbutton key pad with 12 keys which represented the numbers 0 through 9 and the symbol * and . See figure 3. Pressing one of the keys caused the DTMF generator to generate two tones in the voice band 300 3,000 Hz . There was a low frequency tone for each row and a high frequency tone for each column. Pressing key 5, for example, generated a 770 Hz tone and a 1,336 Hz tone. By using the dual tone method, 12 unique binations were produced from only seven tones when the 1 2 position keypad was used. Figure : DTMF Keypad and Frequencies Keypad High Frequency Hz 1209 1336 1477 1633 Low Frequency Hz 697 1 2 3 A 770 4 5 6 B 852 7 8 9 C 941 * 0 D The frequencies and the keypad layout have been internationally standardized, but the tolerances on individual frequencies varied in different countries. The North American standards were*% for the generator and f % for the receiver. The tones have been selected carefully so that the DTMF receiver in the exchange office would not confuse them with the other tones that might occur on th6 line. These DTMF tones from the exchange office were transmitted the same as speech over the telephone line. This permitted us to receive the DTMF tones by our system39。s DTMF Receiver if any number was pressed on the keypad. a DTMF Receiver Circuit: The task of the DTMF Receiver was to detect the presence of a valid tone pair on a telephone line or other transmission medium. The presence of a valid tone pair indicated a single dialled digit. In order to generate a valid digit sequence, each tone pair should be separated by a valid pause. The following table showed the established Bell system standards for a valid tone pair and a valid pause： LowGroup Tone： 697 or 770 or 852 or 941 Hz HighGroup Tone： 1209 or 1336 or 1477 or 1633 Hz Frequency Tolerance： for %+2Hz Amplitude Range: +6dBm @ 600 Ohm Tone Duration:40 ms or longer Pause Duration:40 ms or longer Figure 2 showed the plete circuit for the DTMF receiver. The circuit employed the RadioShack 2761303 DTMF Receiver IC. It was a plete Dual Tone Multifrequency DTMF Receiver circuit detecting a selectable group of 12 or 16 standard digits. Figure DTMF Receiver amp。 Telephone Source controller The block diagram of the DTMF receiver internal structure. The analog input signal was preprocessed by a 60Hz reject filter, a preamplifier and a low pass filter to emphasize the signal part. The output of the preprocessor stage was then split into two frequency bands, each of which contained only one DTMF tone group. The output of each bandsplit filter was amplified and the following zerocrossing detector converted the sinusoidal signals to square waves. These square waves, passed through tone frequency band pass filters to generate the individual frequencies in each tone band pass filter output was detected by an amplitude detector and fed to a timing circuit to determine the detection validity within 40ms. The detector output was also fed to an output decoder to decode the tones to their equivalent binary numbers. The timing circuitry produced the control pulses, for an output register. For example, the data strobe from the timing circuit enabled the decoder outputs to be latched at the register outputs D1, D2, D4 ,D8. The register outputs became valid after a tone pair has detected and they were cleared when a valid pause has arrived. The output format was programmed by the HEX/B28 pin. When HEX/B28 1, the output was hexadecimal and when HEX/B28 0, the output was a binary code. The following tables showed the two output formats. A, B, C and D were used for extended keypads. The Data Valid DV l pin indicated a valid tone pair was sensed and decoded at the output pins of D1, D2, D4 and D8. DV remained high until a valid pause occurred. As shown in figure 4, the input pins 9 and 10 of DTMF receiver IC were connected to an Isolation transformer which was used to isolate the telephone line ground from the digital ground. Pin 10 was tied to the digital ground, which was the input ground. The Ring terminal of the telephone line was connected to one terminal of the Isolation transformer and the other transformer terminal was connected to one end of a relay. The other end of the relay was connected to one terminal of a capacitor which alloyed AC signals to pass through. The second terminal of the capacitor was connected to the Tip terminal of the telephone relay provided the control of co