Smart phones with caller ID "Rus" different versions have the function "answering machine", but its implementation in the majority of the devices required an additional set of so-called "voice" of the Board. In addition, installation fees should be made by the specialist, and it is not always feasible. Slightly altering any transistor tape recorder, you can use the "answering machine" without any interference to the phone. When the recording time depends on the capacity of the cassette (in the "voice" Board time records of tens of seconds).
The proposed device has a high input impedance and virtually no affect the operation of the telephone line.
A tape recorder is installed automation fee collected by the circuit shown in the figure below.
The Zener diode VD1, the resistor R1 and the transistor VT1 form a condition sensor the telephone line. While on-hook line voltage is 60 V. The Zener diode VD1 is opened, the transistor VT1 is opened, VT2 is closed, the relay K1 de-energized, the power supply circuit of the tape recorder is broken.
When an incoming call in the telephone mode is "auto-pick", the line voltage drops to 8...10 V. the Transistor VT1 is closed, VT2 open, relay K1 and its contacts K1.1 connects the power circuit tape, permanently included on the record. The audio signal from the line through the trimmer capacitor C1 and the resistor R5 is input to the recording tape. To import the tape that has no input connectors, the signal may be fed in parallel to the built-in microphone. Diode VD4, VD5 limit the voltage in the call time.
In the tape you need to break the supply circuit and in the gap to turn on the relay contacts. Additional switch SA1 is for listening to records. Led HL1 indicates whether to include the prefix.
Author: G. Voronin, G. N.-Tura, Sverdlovsk region.