Did you know that it is possible to use the computer as an electronic bug? To use PC to record phone conversations is much more practical – and cheaper – than to do so using a conventional tape recorder, besides being able to record hundreds of hours of conversation, for the audio is recorded in a file in the hard disk of the computer. And you may also convert the audio files into MP3 so they take even less space in the hard disk, or you may even save the files in an audio CD so the recorded phone conversations may be played in any CD player.

Until not long ago voice modems were very popular on the market. These modems had an answering machine, speaker phone, and phone conversation recording system as extra features. For not knowing those features existed, most users did not fully use the potential of their modems, so the manufacturers ended up removing them from the newest models to reduce their price.

Some of the most modern modems have an output channel called TAD (Telephony Answering Device), which can be used to connect the modem to the sound card, through a cable similar to the one used in the connection of CD-ROM drive to the sound card. You may record phone conversations through that connection using your sound card. In order to do that, your sound card also needs to have a TAD input and you will have to install the cable connecting the modem to the sound card.

With the popularization of the broadband Internet connection, less and less users have modems in their computers. The solution to transform the PC into a phone conversation tape recorder is the construction of the small circuit we describe next. This circuit is very simple to be assembled, and it is very cheap too. Even a user without a deep knowledge in electronics can assemble it without great difficulty. The parts you will have to buy are: a 4K7 x 1 W resistor, a 220 nF x 200 V capacitor, two 5 V x 1 W Zener diodes, a 10 K logarithm potentiometer (that will work as the volume control) and a mono P2 male plug. There is also a phone transformer. To reduce the price of the equipment, we suggest you simply remove the transformer of an old modem (from a 2400 bps one, for instance). See in the picture how the modem transformer is. We’ve posted a tutorial called “How to Dessolder Components” that can be of great aid for those who doesn’t know how to remove the transformer from the modem.

Phone Bug Schematics

Figure 1: Phone bug schematics.

Modem Transformer

Figure 2: Modem transformer used in the circuit.

After assembling the circuit, all you have to do is connect it to the phone line on one side and to the Mic In channel of the sound card on the other. To record conversations, you will need a recording program, like Sound Forge. In case you don’t have any, use the Sound Recorder for your Windows. Since the phone line quality is not high, you may record at 11.025 Hz mono so the recorded file is the smallest possible.

Attention: Please remember that recording a phone conversation without the other party consentment can be a crime. Please consult your state/country laws in order to not break the law.

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.