Telephone answering machine. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Telephone answering machine. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Caller ID telephones have a mode in which the telephone operates as an answering machine. A standard phrase recorded in ROM is pronounced into the line. To record your telephone message, you need an additional sound card, which is not particularly common due to its relatively high cost.

The proposed scheme operates independently of the telephone set and outputs a telephone message pre-recorded on a cassette recorder to the line.

(click to enlarge)

The circuit consists of an input amplifier on transistor VT1, inverters DD1.1, DD1.2, a timing circuit on resistor R4 and capacitor C2, an output power amplifier on transistor VT3, the load of which is the winding of electromagnetic relay K1, and a control transistor VT3.

When a call signal is received, relay K1 is activated and, with its contacts, supplies power to the tape recorder, from the line output of which the signal is fed to the transistor VT3 for transmission to the line. At the same time, transistor VT3, opening, shunts the line with resistor R8, which is equivalent to picking up the handset on a telephone set. Potentiometer R6 sets such a transistor operation mode, in which the loudest and most undistorted signal enters the line. By adjusting this resistor, the device is consistent with both paired lines and conventional ones. The auto-raising time is determined by the value of the resistor R4 and the capacitance of the capacitor C2, it is chosen equal in time to the length of the phrase recorded on the tape recorder.

The disadvantage of the scheme is that repeated playback of the same phrase requires rewinding the tape recorder. To eliminate this shortcoming, it is necessary to record the same phrase on a tape recorder several times with an interval equal to the auto-up time. If it is necessary not only to transmit, but also to record a telephone message, then the same pair of relay contacts can be used to supply voltage to a second tape recorder in recording mode. In the circuit, transistors with a static current transfer ratio of at least 100 can be used. Diode VD1 - on direct current not less than 50 mA Relay K1 - RES-9, passport RS4 524 202

Authors: N.Martynyuk, Kobrin, Brest region; Publication: N. Bolshakov, rf.atnn.ru

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