This is the circuit connection of home telephone call recorder that uses very few components. In order to understand its working, we must first have the basic knowledge of standard telephone wiring and a stereo plug. In some countries, 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 phone line goes to IN1, while the positive wire goes to IN2. Further, the negative 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 / Audio Input of any recording device, such as computer, audio cassette player, CD player, DVD player, etc. In this case, we will use a computer desktop / notebook to make a call record.
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 / Audio In port of a computer, in most of notebook devices, it is same between Audio Input / Mic Input.
Any audio recording software can be used to record the home telephone call, i.e AVS Audio Editor, Audacity Audio Recorder, or Audio Recorder for Free. 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 “audio input” or “mic input” port of the PC/Notebook. Install the audio recorder software on your PC/Notebook. Run the software and check the setting to ensure that the input is correct. Now you are ready to record any call. As soon as a call comes in, press the record button on the software and then pick up the telephone receiver and answer the call. Press the stop button once the call ends and then save the file in a desired location. Each software may has different setting, you can read the manual of each software about how to record an audio.
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 home telephone call recorder circuit. The recorded audio clip can be edited using different options in the Audio Recorder software.
You can built the circuit on a general-purpose PCB and enclose it in a small box. Use an RJ11 connector and stereo jack for connecting the telephone set and computer desktop/laptop (for call recording). Telephone cords can be used to connect to the phone line and the home telephone call recorder circuit. Use of a shielded cable is recommended to reduce disturbances in the recording so it we will get good audio performance from the recording. These disturbances can also be reduced by increasing the value of R2 to about 15K.
This circuit was tested, come from EFY mag, Dec 2009. Download the PDF version: