Prefix for recording telephone conversations 2. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Attachment for recording telephone conversations. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Sometimes it becomes necessary to record a telephone conversation on magnetic tape, without being distracted to turn on the tape recorder. The problem will be solved if you connect the tape recorder to the phone through the proposed automatic prefix.

The described device automatically turns on the tape recorder to record the conversation and turns it off when the handset is hung up. When operating the prefix-machine, the tape recorder must be constantly turned on for recording. Switching on and off occurs by switching the power circuit. The scheme of the device is shown in Fig.1.

(click to enlarge)

The telephone line voltage is applied to the divider across resistors R1 and R2. When the tube is on the lever, the voltage in the line is about 60 V, the output of the element DD1.1 is low, the capacitor C1 is discharged, the output of DD1.3 is also low, the transistor VT1 is closed, the relay K1 is de-energized, the power of the tape recorder is turned off.

When the tube is removed, the voltage in the line drops to 5 ... 12 V, the output DD1.1 is high, the capacitor C1 starts charging through the resistor R3. When the threshold level is reached at the terminals of the capacitor, the state of the elements DD1.2 and DD1.3 changes to the opposite, as a result of which a high level appears at the output of DD1.3. Transistor VT1 opens, relay K1 is activated. Through the closing contacts of the relay, power is supplied to the tape recorder. The audio frequency voltage from the line is supplied through the circuit C3, C4, VD4, VD5, C5 to the line input of the tape recorder.

At the end of the conversation, as soon as the handset is placed on the lever, the voltage in the line will increase to 60 V. A low level appears at the output of element DD1.1. Capacitor C1 starts to discharge through resistor R3 and element DD1.1. As soon as the voltage across the capacitor reaches the threshold level, the elements DD1.2 and DD1.3 will change their state. Transistor VT1 will close, relay contacts K1 will open and turn off the power of the tape recorder.

Since the time constant of the R3C1 circuit is much greater than the repetition period of a series of "dialing" pulses, when the handset is picked up and the number is dialed, the tape recorder remains de-energized. However, if you pick up the phone and do not dial the number for a while, the capacitor C1 will have time to charge and the tape recorder will turn on.

Sending a call signal (80 ... 120 V, 25 Hz) also does not change the state of the elements DD1.2 and DD1.3. Diode VD2 limits the voltage at the input of the element DD1.1. The resistance of capacitors C3 and C4 at 25 Hz is high, so they do not bypass the ringing signal. Diodes VD4 and VD5 limit the voltage at the input of the tape recorder at the level of 0,6 ... 0,7 V.

The K561LA7 microcircuit is interchangeable with the K561LE5, as well as with similar K176 and 564 series. The KTS407A (VD1) diode assembly can be replaced with KTS402B, KTS405B or four diodes with a permissible reverse voltage of more than 200 V. The VT1 transistor is an npn structure with a permissible collector dissipation power of at least 150 mW. Relay K1 - for a response voltage of 5 ... 7 V, for example, RES10 (passport RS4.524.302 or 031-04-02), RES15 (passport RS4.591.003).

The details of the device are mounted on a printed circuit board made of foil fiberglass. Its drawing is shown in Fig. 2.

Establishing the device is reduced to the selection of resistors R1 and R2 to ensure a clear operation of the element DD1.1 when raising and lowering the tube. But the resistance of the resistor R1 should not be less than 330 kOhm. The delay time of the relay operation can, if necessary, be changed by selecting the resistor R3.

Author: A.Kurguzov, Moscow; Publication: N. Bolshakov, rf.atnn.ru

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