Telephone automatic recording control circuit

Telephone automatic recording control device Telephones with recording functions sold on the market are generally divided into tape recording and semiconductor recording according to the recording carrier. The movement quality of the former is generally not very good, and the time of the latter is too short and inconvenient for long-term storage. Information, and the price is much higher than that of ordinary telephones. Maintenance and repair are inconvenient due to compact structure. The device introduced in this article uses an idle tape recorder at home to connect to a telephone. It can automatically record the content of calls during phone calls, and the original functions of the tape recorder can still be used in normal times. working principle    　　

The circuit principle is shown in Figure 1. Integrated circuit IC1 (LM741) and peripheral components form a voltage comparator to monitor the voltage status between the telephone outside lines L1 and L2. When an ordinary dial-up phone hangs up, the voltage between L1 and L2 is about 60V; when there is a ring current, an AC signal of about 100V is superimposed; when the handset is picked up, the voltage between L1 and L2 drops to about 10V. Using this voltage change, the working status of the telephone can be determined. Whenever the handset is picked up, the control circuit automatically powers up the recorder and starts recording; when the phone is hung up, the recorder automatically cuts off power and stops recording. The positive input of op amp comparator IC1 is biased to V/2 by resistors R3 and R4. V is the operating voltage of the recorder, generally 9V. Then the voltage at the positive input terminal of IC1 is 4.5V. In static electricity, the voltage between L1 and L2 is 60V. After being divided by R1 and R2, the voltage at the negative input terminal of IC1 is 6V. Since the wind input terminal voltage of IC1 is higher than the positive input terminal voltage, IC1 outputs a low level, the transistor VT is cut off, the relay J contact is disconnected, and the recorder does not work when the power is turned off. When ringing, although the voltage at the negative input terminal of IC1 sometimes drops to 5V, it is still higher than the 4.5V at the positive input terminal, so IC1 still outputs low level and the recorder is still in a power-off state. When the handset is picked up after ringing, the voltage between L1 and L2 drops to 10V. At this time, the voltage of the negative input terminal of IC1 drops to about 1V, which is lower than the voltage of the positive input terminal, so the output of IC1 jumps to high level, the transistor VT is turned on, the relay J contact is closed, and the 9V voltage supplies power to the recorder through CZ2. Start recording (the recorder should be placed in the "recording standby" state in advance. After the call is completed and the receiver is hung up, the voltage between L1 and L2 rises to 60V again. As mentioned above, the relay J is disconnected again and the recording stops. The user can Switch S2 is closed to directly power up the recorder to playback and organize the recording data. Note that switch S2 should be in the off position at ordinary times. S1 is used to control automatic recording. When phone recording is not needed, S1 can be turned on. The output signal is transmitted by L1 and L2 and coupled to the MIC input port of the recorder through C3 and T1. The power supply of the recorder is provided by the three-terminal voltage regulator IC2. Generally, the operating voltage of the recorder is 6V, 9V, and 12V. Select a 78×× series three-terminal voltage regulator according to the rated working voltage of the recorder. When selecting different working voltages, R3 and R4 should be adjusted to meet the principle requirements. In this example, R3 and R4 can be selected from 10K-200K. , the two should be equal. R6 and LED constitute the power indicator. The function of VD1 is to eliminate the reverse electromotive force of the relay coil and protect the VT. See the table below for component selection and production. Number, model, and quantity. R1, 560K. R3, R4 resistor 62K 3 R5 resistor 10K 1 R6 resistor 1K 1 C1 polyester capacitor 0.1u 1 C2 electrolytic capacitor 470u/16V 1 C3 polyester capacitor 0.047u 1 VD1 switching diode IN4148 1 LED light-emitting diode 1 U full bridge 1A/50V 1 VT Transistor 9013 1 IC1 Operational amplifier LM741 1 IC2 Three-terminal voltage regulator 7809 (select according to the recorder) 1 S1, S2, S3 toggle switch 3 T1 Audio transformer Radio output transformer 1 T2 Power transformer 5W 10V 1 J Electromagnetic relay voltage and current reference Recorder 1 FU fuse 0.5A 220V 1 IC1 can choose any type of operational amplifier. Relay J should be selected according to the operating voltage and power of the recorder. Generally, T1 uses a transistor radio output transformer, and the primary is connected to L1 and L2. , the secondary is connected to CZ1, and the center tap is not used. When installing, first use a multimeter to measure the polarity of the external lines L1 and L2, and then connect the device to the ground.