Recording circuit of phone conversations using PC

Posted on Jun 25, 2017

This circuit is used to record telephone conversations on your landline telephone. All you will need is this small circuit and subscription of your PC. There are few times when we forget what we said or told us. So we need recording. Sometime, not many years ago, recording of this kind of conversation was done with a small tape recorder. The line signal came to the microphone input, while a simple circuit activated the tape deck when the handset came up and stopped it when the conversation ended. The cost of this method was not great and the recording worked well. But it had a big drawback. The tape ended quickly and had to be replaced by another. Those who neglected to change a tape simply lost recordings, so the system was useless. On the other hand, those who were consistent with the changes ended up filling their cupboards with wasteful cassettes. So the idea of ​​recording was lost after it failed practically.

Recording circuit of phone conversations using PC
Click here to download the full size of the above Circuit.

But times changed and technology evolved significantly. Today we all have at least one computer in our office. Hard drives are of huge capacity so using our computer as a sound recorder is something extraordinarily simple. With this in mind, the idea of ​​recording is coming back to the forefront.

The circuit is extremely simple. It is a telephone transformer that interfaces between the telephone line and the telephone. In the secondary winding of the transformer, the speech signal is displayed. We signal this signal to the microphone input of the computer's sound card. No power, no other connections. The circuit simply supplies the phone signal to the computer. The computer in turn takes over the rest.

The software that we used to test the circuit is the "RecordPad Sound Recording Software" which is a simple sound recording program for Windows and Mac OS X. RecordPad is ideal for recording voice and other audio to add to digital presentations, creating an audio book , Or for simply recording a message. And because it has a voice activated recording feature makes it ideal for our project. Offcourse you can use any software you want without any problems.


recordpad software


Audio Recording Features of RecordPad

  1. Record sound, voice, notes, music or any other audio
  2. Save recordings to your hard drive in wav, mp3 or aiff format
  3. Record only when you are talking with voice activated recording
  4. Recordings can be burned directly to CD using Express Burn
  5. System-wide hotkeys allow keyboard control over recording while working in other programs
  6. Find and Play recordings by format, date, duration and size. Includes keyword search for audio recording when a speech engine is installed
  7. Optionally, you can send a file by email or upload to an FTP server
  8. MPEG Layer-3 encoding offers content of variable bit rates from 8 to 320kbps with optional crc error checking and joint stereo modes
  9. Wave coding at sample rates between 6000 and 196000Hz in PCM or many other codecs and bits
  10. It can be automated and integrated to run from other programs with powerful command line options
  11. Integrate with WavePad Professional Audio Editing Software to edit recordings and add effects
  12. Designed to be simple and robust for professional and corporate recording applications

The construction of the logger will not make it difficult for you, it is realized with only 5 simple parts. Even if you are not familiar with electronic constructions, if you observe the polarity of the diodes, the circuit will work perfectly.

The circuit


phone recording circuit pcb and parts


In the figure we see the theoretical circuit of the logger. It is essentially a circuit that makes the signal fit. It connects to the 'MIC' input of the sound card and between the telephone line and the phone. The work of the circuit is to transfer the line signal to the sound card. The primary winding of the T1 transformer is connected in series with the line and the telephone device. Thus, the line signal is transferred to the secondary. The D3 D3 diodes limit the 3V pick to pick level to protect the sound card input from the ringer over-voltages. The Kl output is connected to the microphone input of the sound card. The circuit is passive so no power is needed.



D1,D2,D3,D4 = 1N4148, T1 = Phone transformer 600/600 Ω


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