Recording Device of Phone Conversation

The phone conversation recorder is a piece of hardware that records audio from phone line into an MP3 format while being powered from the USB.

This device would start recording automatically when picking up the phone is detected and will stop recording when the phone is hanged. The duration of conversations and other status like WAV conversion to MP3 can be seen in the PC software as well as receiving the phone line state via USB. Using the serial to USB connection, the microcontroller is responsible for sending the phone line status to the PC as it checks the line status every 0.2seconds.

The phone line voltage, around 50V when on hook, is measured by the phone line status monitor circuit. The voltage will drop below 15V when the phone line is picked up. This is also a way of detecting if the phone line is used or not. The audio pickup uses a transformer from an old PC. The output voltage of the transformer is limited to below 0.6V by using two 1N4148 diodes. It also includes a resistor and a capacitor that ensures the line is not occupied by the circuit.

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