I want to connect phone line to my PIC's ADC to sample signal changes without interfering the line. Human voice and DTMF signals are all under 4kHz level. So, I am hoping that any 18F PIC can do sampling at 8kHz or over very easily.
On my research, I have learned that the line is DC normally at about 9V, and only while ringing, it is about 90V 20Hz. About 10 minutes ago, I used my voltmeter to measure the line, and it shows about 42.7V in DC which makes my research wrong.
Anyway, since I cannot connect this high voltages to PIC's ADC directly, is using a high voltage Op-Amp for downscaling would solve the problem? Or is there any other convenient way to accomplish this?
P.S. This question is for UK phone lines.
Best Answer
You should look at COTS telephone line interface modules (TLI). They take all of the headache out of connecting an MCU to a phone line.
Typical services include on/off hook signalling, ring detection, AC-coupled line in/out, sometimes a DC output derived from the DC present on the phone line (the 42.7Vdc you measured), and decent isolation (1-2000V) to minimise the risks of damage to the network, your electronics, and you.
Cost is up to perhaps $20, but you can certainly find cheaper ones.