I need to interface door phone system to a pc soundcard to listen and talk to the external unit.

The door phone to interface is an as yc/200 of BPT and its a classic analog 4+n wires Door phone system and I have access only to the internal unit.

In the internal unit I have a cable with 4 wires: a Ground wire, a call wire, an audio in, an audio out; going through the power supply to the external unit.

This is a picture of the phone.

photo 1
(source: intercomsrus.com)

Unfortunately the datasheet doesn't say nothing regarding the audio lines and their signals. What I know is that they are probably standard for every 4+n wires door phone.

Now what I would like to understand is how to connect the audio signals to the sound card without damaging it.

Are those signals usually high voltage or high impedance?

The power supply output 11 volts DC and 15 volts AC. Also the pushbutton that open the door is connected to the audio out towards the external unit. What kind of signal is sent to open the door?


2 Answers 2



I just used a voltage divider between soundcard output and outgoing audio line and a simple resistor for the incoming audio to the pc microphone input.

Thank you :)


To undertake a project like this you need to seriously get an oscilloscope and probe out the signals to see what is going on when the currently installed device is in use. A little bit of investigation will answer virtually all of your questions regarding the signalling of the existing door phone.

  • \$\begingroup\$ Don't forget to check what exactly happens when the doorbell rings, there may be large voltages that you didn't expect. \$\endgroup\$
    – jippie
    Feb 28, 2014 at 13:15

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