# Detect phone call termination

I am designing an automated telephone circuit that will work with the PSTN line. I already had a question to detect the incoming ring signal. Now I want to detect whether the opponent person in the line terminated the call. Because I need to cut off the line connection automatically since there will be no one to hang the handset on my telephone side.

So far, I learned that there is a constant beep sound which is generated by the telecom company when the opponent hangs his/her phone and terminates the call. The beep sound has ~2V peak-to-peak and a 5V offset, you can see the DC measurement and AC measurement below:

DC Measurement:

AC Measurement:

I am trying to detect the existence of this signal by an MCU. So far, I tried using an optocoupler but the voltage is not enough for driving an optocoupler. I tried the optocoupler circuit because the input (LINE- and LINE+) must be isolated from the output (GPIO of the MCU).

Are there any already-tested solutions for this kind of applications? Do they just use circuits for amplifying or filtering this signal to detect its existence? I wanted to use a comparator by using the Line for both supply and input but I couldn't get it work.

• Have you tried just a straight OP-amp, band pass filter and a comparator? Oct 15, 2018 at 7:48
• I'm not really experienced with circuit designs, so I'm using the classic methods and try to understand the basics. How should I supply the op-amp? If I supply it with another isolated source, how can I give the input line signal to the op-amp with a reference to this source? Oct 15, 2018 at 7:54
• Classic method would not involve an optocopler. How are you powering everything today? Do you need isolation from the PSTN? Oct 15, 2018 at 8:45
• I have multiple power sources in this board by I don't know how to connect the line to an op-amp in this example. Because I can not combine the line- with the ground layer of the power source of the op-amp because there is a potential difference between them. Do you have any reference circuits? Even a block scheme is enough for me to understand what you mean. Oct 15, 2018 at 10:57