Telephone call recorder
Today telephone has become an integral part of our lives.
It is the most widely used communication device in the world.
Owing to its immense popularity and widespread use, there arises a need for
call recording devices, which find application in call centres, stock broking
firms, police, offices, homes, etc.
Here we are describing a call recorder that uses very few components.
But in order to understand its working, one must first have the basic knowledge
In India, landline telephones primarily use RJ11 wiring, which has
two wires—tip and ring. While tip is the positive wire, ring is the negative
one. And together they complete the telephone circuit. In a telephone line,
voltage between tip and 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 phoneline goes to IN1,
while the positivewire goes to IN2. Further, thenegative wire from
OUT1 and the positive wire from OUT2
are connected to the phone. All the resistors
used are 0.25W carbon film resistors and all the capacitors
used are rated for 250V or more.
The negative terminal of ‘To AUX IN’ is connected to pin 1 of the stereo jack
while the positive terminal is connected to pins 2 and 3 of the stereo
jack. This stereo jack, in turn, is connected to the AUX IN of any recording
device, such as computer, audio cassette player, CD player, DVD player,
etc. Here we 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
into DC by the diodes and fed to resistor R1, which reduces its magnitude
and feeds it to LED1. The current is further reduced in magnitude by the
resistor R2 and fed to the right and left channels of the stereo jack, which
are connected to the AUX IN port of a computer.
Any audio recording software, such as AVS audio recorder, Audacity audio
recorder can be used to record the call. When a call comes in, one needs
to launch the audio recording software and start recording.
For phone recording, simply connect the stereo jack to the AUX IN
port of the PC. Install the audacity audio recorder (different versions are
available for free for different operating systems at http://audacity.
sourceforge.net/) on your PC. Run the executable audacity file. In the
main window, you will find a drop down box in the top right corner.
From this box, select the AUX option. Now you are ready to record any call.
As soon as a call comes in, press the record button found in the audacity
main window and then pick up the telephone receiver and answer the
call. Press the stop button once the call ends. Now go to the file menu and
select the ‘Export as WAV’ option and save the file in a desired location.
You may change the value of resistor R2 if you want 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 software.
You can assemble the circuit on a general-purpose PCB and enclose it in
a small cabinet. Use an RJ11 connector and stereo jack for connecting the
telephone set and computer (for call recording). Telephone cords can be
used to connect to the phone line and the circuit. Use of a shielded cable is
recommended to reduce disturbances in the recording. These can also be
reduced by increasing the value of R2 to about 15 kilo-ohms.
EFY note. Audacity recording software is included in this month’s
EFY-CD under ‘utilities’ section.
by - sky Group
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