Telephone callback device made by 89C2051 single chip microcomputer

The telephone callback device is controlled by a single-chip microcomputer and has the advantages of simple circuit, reliable contact and easy use.

1. Working Principle

The circuit is shown in the attached figure. The system consists of four parts: off-hook and on-hook circuits, microcontroller dialing control circuits, DTMF number generation and amplification circuits, and power supply circuits. They are introduced below.

1. Off-hook/on-hook circuit

In the figure, J1 is the telephone line socket, J2 is the telephone socket connected in parallel to the device, and R1 is a fuse resistor; DI-D4 is a polarity protection circuit, which is used to convert the voltage with uncertain polarity on the line into a voltage with fixed polarity, ensuring that no matter how the positive and negative poles of the telephone line are connected, the end entering the emitter of the switch tube VI is always a positive voltage; V1 is an electronic switch, and the on and off of the switch is controlled by the P3.7 port of the single-chip microcomputer. When P3.7 is at a high level, V2 and VI are turned on successively, and the 48V voltage on the telephone line is added to both ends of R5 through VI. There is current flowing through the telephone line, and it is in the off-hook state; when P3.7 is at a low level, V2 and VI are cut off. There is no current flowing through the telephone line, and it is in the on-hook state. In the off-hook state, most of the 48V power supply voltage sent by the telephone room is attenuated by the line resistance of the telephone line. The voltage drop generated at both ends of R5 is about 5V~15V.

2. Single chip microcomputer dial control circuit

89C2051 completes the control of the dial chip, R8 and c5 are reset elements to complete the power-on reset, and JTI is a 6MHz crystal oscillator.

Use the 8 wires of the Pl port to control the dial-up chip.

3.DTMF number generation and amplification circuit

The MK5087 DTMF dialing chip is used. This chip was originally a matrix key input chip, that is, 4 row lines and 4 column lines form 16 keys. Practice has proved that this chip not only has matrix keyboard input, but also has good interface performance with 51 series microcontrollers. The functions of each pin of this chip are introduced below.

Pin (1): Positive power supply, operating voltage range is 3.5V-10V. Pin (2): Status output terminal, when the chip sends out DTMF signal, this pin is at low level, otherwise it is at high level. Pins (3), (4), (5), (9): Keyboard column line input terminal, internally connected with pull-down resistor, high level is valid, when this pin is connected to any row line, or a row line and a column line are valid at the same time, the chip sends out DTMF signal.

(6) Pin: Negative pole of power supply. (7) and (8) Pins: Oscillator input and output terminals, connected to a 3.58MHz crystal. (10) Pin: This pin is exactly the opposite of pin (2). When the chip sends a DTMF signal, this pin is at a high level, otherwise it is at a low level. (11), (12), (13), and (14) Pins: Keyboard row line input terminals, with a pull-up resistor inside, low level is valid. When this pin is connected to any column line, or a row line and a column line are valid at the same time, the chip sends a DTMF signal. (15) Pin: Single tone validity control terminal, with a pull-up resistor inside. When this pin is grounded, the chip can only send

Output dual audio but not single audio: When this pin is connected to a high level or left floating, the chip allows both single and dual audio. Only when the row and column lines are not valid at the same time, the chip can send a single audio signal. (16) Pin: DTMF signal output terminal: The DTMF signal sent by pin (16) is amplified by V3 and sent to the line from the collector of V3.

The row and column keyboard of this chip corresponds to the DTMF number. When a key is pressed, it is equivalent to the row line R of the key being at a low level and the column line C being at a high level. Similarly, when a row line R is at a low level and the column line C is at a high level, it is equivalent to the key where the row line and the column line intersect is pressed.

4. Power circuit

In the power supply circuit, the 220V AC voltage is transformed into 9V by transformer B. Then it is rectified by D5~D8, filtered by CI and C2, and stabilized by ICl (7805). After filtering by C3 and C4, a stable Sv voltage is output for use by the microcontroller and number generation circuit.

采用外加交流电源供电的优点是：当电话线路过长(比如大干20km甚至更多).如果电话线上的电压因为线路过长衰耗太大，以致于到了终端，电压降不能维持终端设备工作甚至为零的情况下，该系统仍然能够正常工作。

2. Production and Installation

All components are installed on a 50mmx80mm board. There are 8 screw holes on the board, two for fixing the transformer on the circuit board, one for fixing 7805, and the remaining 5 for fixing the circuit board and the case. The telephone inlet and outlet sockets are installed on the case and connected to the JI and J2 rods on the board through wires. The indicator LED is designed with an installation position on the circuit board, and the indicator light can also be led out according to the design needs of the appearance. J3 is the 220V voltage input terminal. J4 is used to connect to the switch K on the case. In order to facilitate rewriting the program, the microcontroller IC2 is not directly soldered on the board, but inserted through the IC socket, so that when rewriting the program, the microcontroller can be directly removed from the board. It should be pointed out that due to the large power consumption generated by R5, a cement resistor with a power of 1W or 2W must be used. Rl should not be a 51Ω insurance resistor. A 2W transformer can be used, and there are no special requirements for other components.

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3. Program flow (see the figure below)

4. Source Program List

The following is the source code for dialing the mobile phone number 18797130900:

MOVPl, A; dial MOV30H.#2; delay time constant

If the above program is changed to other numbers, you only need to change the data table in one row on the TABF page.

5. Use

Normally, the power switch of the device should be in the off state, and the telephone connected in parallel with the device can be used normally. When the user needs to use the callback service, just turn on the switch K. The indicator light will light up immediately after the system is powered on, and then it will automatically pick up the phone and dial according to the program. When dialing, as the digits are issued one by one, the indicator light flashes at the same time. Every time a number is issued, the indicator light goes out once. After the number is dialed, the indicator light is still on. After a delay of about 2s, the indicator light goes out, and the system hangs up and waits for callback. After the device is used, the power supply should be turned off, because although the line is on the hook, the power supply is still working, which will waste energy.