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Automatic Telephone call recording circuit

Published on Feb 03 2010 // Telephone Communication Circuits


 Telephone automatic recording control circuit

This circuit is used to record the calls automatically. A idle tape recorder at home connected with the telephone to record the phone calls  automatically. For enabling this, the original recorder feature of the tape recorder can even be used.The circuit comprises of IC1 (LM741) and external components which frames the composition of voltage comparator, to monitor the telephone line voltages between L1 and L2.

At a hanged condition, the voltage level between L1 and L2 will be nearly equal to 48V. During the ringing period, the voltages between the lines will 75V AC signal;  When the handset is picked up ,voltage drops down to 12V. This voltage change is an advantage to find out the picking period of a telephone.Whenever the handset is picked up , the control circuit automatically increases the power to the recorder to start recording; when the phone is hung up, the recorder automatically switches the power off to stop recording.

The resistors R3 and R4 gives a positive input voltage bias V/2, to the Op-amp comparator. So, the input to IC1 is 4.5v, which can be called as a Reference voltage. V is the tape recorder working voltage, usually 9V. R1 and R2 is used to reduce the Line voltage from 60V, and the IC1’s negative input voltage is just 6V. When the input voltage of IC1 is higher than the reference voltage , then the output of IC1 is low, and transistor VT goes to cut-off, there by relay contact J will be disconnected, and so the tape recorders power does not work.

Even at the time of ringing, sometimes IC1’s negative input voltage may be very low nearly to 5V, but still higher than the positive input is 4.5V, so IC1 output is still low, tape recorders are still in power-down state. When the handset is picked up, the voltage between L1, L2 drops to 12V. At this point IC1’s negative input voltage reduced to around 1V below the positive input voltage, so jumping into a high output of IC1, transistors VT turn, relay contacts J Pick, 12V voltage to the recorder for by CZ2 connector, began to recording (recorder should be placed in advance, "Recording waiting" state. When the call is completed and the handset is hung up,then, L1, L2 voltage rose to 60V, which disconnects the relay J, and there by recording stops. After switching S2 to be closed, the users can directly hear the recorded information.

Note that the disconnect switch S2 in idle time should be placed in position. S1 is used to control automatic recording, When the telephone recording is not required, S1 can be kept in open.The transmission of audio output signal from the L1, L2 transmission is utilized by C3 which is  coupled with T1 and to the recorder’s MIC input. The power for the recorder is provided from the three-terminal regulator IC2 . General operating voltage for recorder may be mostly 6V, 9V, or 12V. IC2 should be based on the rated working voltage of the recorder. Usually, its good to use 78 × × series type three-terminal regulator. The different operating voltages can be adjusted by R3 and R4. R3, R4 can be selected between 10K-200K, the two should be equal. R6, LED power supply composed of the instructions. VD1’s role is to eliminate the reverse EMF relay coil to protect VT. 
  IC1 can choose any type of op-amp. Relay J should be based on the recorder’s operating voltage and power selection, generally used relay is JZC-21F .The primary of the transformer is connected to L1, and secondary to L2, and no centre tap arrangement should be used. The polarity of L1 and L2 should be clearly checked before installation of the circuit.

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