Telephone automatic recording controller circuit

Circuit working principle: The circuit consists of a rectifier circuit, a control circuit, a receiving circuit and
a power supply circuit. Telephone lines A and B pass through the diode VD. A VD. The rectifier, resistor
Rz and potentiometer RP divide the voltage to pin 2 of the IC (TWH8751). The
output terminal 4 of IC is connected to a relay , and the normally open contact of the relay is used to turn on or off the power of the recorder .
The power supply of the recorder and the normally open contact of the relay are connected in series as shown in Figure 4-llb. When
working, when there is no incoming call from the outside line, the voltage of the telephone line is generally around 38 V. At this time,
it is connected through bridge rectification and voltage dividing. The voltage at IC2 terminal is high potential, then the output terminal 4
is low potential, so relay KS does not pull in. When a call comes from the outside, the voltage of the telephone line drops from the original 38 V to about 6.5 V. At this time, the voltage added to the terminal of
IC after rectification and voltage division is low potential, and terminal 4 outputs high potential, and the relay KS is closed. Turn on the recorder to record. When the outside call is interrupted, the 4-terminal output of IC changes to low level again , the relay is powered off, and the recorder is controlled to stop recording. When using the automatic recording controller , the voltage dividing ratio of the potentiometer RP should be adjusted so that the potential required by IC2 pin can be reached. Telephone voice input is via capacitor C. Couple it with the audio transformer and connect it to the microphone input port of the recorder .