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Telephone Call Recorder


Today phone has become an integral part of our lives. It is the most widely used communication tool in the world. Owing to its immense popularity & widespread use, there arises a necessity for call recording devices, which find application in call cent-res, stock broking firms, police, offices, homes, etc. Here they are describing a call recorder that makes use of only a few parts. But in order to understand its working, must first have the basic knowledge of standard phone wiring as well as a stereo plug.


In India, land-line rings primarily use RJ11 wiring, which has wireless-tip & ring. While tip is the positive wire, ring is the negative. & together they complete the phone circuit. In a phone line, voltage between tip & ring is around 48V DC when handset is on the cradle(idle line). In order to ring the phone for an incoming call, a 20Hz AC current of around 90V is superimposed over the DC voltage already present in the idle line. The negative wire from the phone line goes to IN1, while the positive wire goes to IN2. Further, the negative wire from OUT1 & the positive wire from OUT2 are connected to the phone. All the resistors used are 0.25W carbon film resistors & all the capacitors used are rated for 250V .


The negative terminal of To AUX IN is connected to pin one of the stereo jack while the positive terminal is connected to pins two and three of the stereo jack. This stereo jack, in turn, is connected to the AUX IN of any recording tool, such as computer, audio cas-setts player, CD player, DVD player, etc. Here they shall be connecting it to a computer. When a call comes in, around 90V AC current at 20Hz is superimposed over the DC voltage already present in the idle line.

This current is converted in to DC by the diodes & fed to resistor R1, which reduces its magnitude & feeds it to LED1. The current is further reduced in magnitude by the resistor R2 & fed to the right & left channels of the stereo jack, which are connected to the AUX IN port of a computer. Audio recording application,can be used to record the call. When a call comes in, needs to launch the audio recording program and start recording.

For phone recording, basically connect the stereo jack to the AUX IN port of the PC. Install the Audacity audio recorder . Run the executable Audacity file. By & sizable window, you will discover a drop-down box in the top right corner. From this box, select the AUX option. Now you are prepared to record any call. As soon as a call comes in, press the record button present in the Audacity main window & then pick up the phone receiver & answer the call. Press the cease button one time the call ends. Now go to the file menu & select the Export as WAVE option & save the file in a desired location.

You may change the worth of resistor R2 in case you require to change the output volume. You can use a variable resistor in series with R2 to vary the volume of the output. The recorded audio clip can be edited using different options in the Audacity application. You can assemble the circuit on a general-purpose PCB & enclose it in a small cabinet. Use an RJ11 connector & stereo jack for connecting the phone set & computer (for call recording). Phone cords can be used to connect to the phone line & the circuit. Use of a shielded cable is recommended to reduce disturbances in the recording. These may even be reduced by increasing the worth of R2 to about 15 kilo-ohms.
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