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Ring detection circuits in modems

In computer modems the logical signal from ringing is needed instead of ringing tone. The ring circuit must pass the ring signal information to modem electronics and still provide electrical isolation between telephone line and modem electronics. This ring detection is usually done using one optoisolator circuit, which replaces the raditional ring circuit. The optoisolator output can be easily connected digital electronics, but the optoisolator input side needs more electronics: one capacitor for not letting DC to pass through optoisolator, one resistor to limit the cirrent passing through optoisolator LED and one reverse conencted diode in parallel with optoisolator LED to prevent negative voltages from damaging the LED. This is the basic ring detection circuit.
Usually there is also two zener diodes (usually 10-20V models) to make sure that the ring detection circuit does not detect too small AC signals in the line as ring signal. In the picture below you see a very typical ring detector circuit for modems. The circuit just gives the idea how modem ring detector circuit work. The actual component component values selection must be so that the circuit meets the national telephone regulations (this can be usually easily done by using suitable zener diodes and maybe chancing the resistor value a little).