Sampling phone line

Question

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.

Answer 1

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.