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:
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.