ring detection output -plzz help!

hello,
can some1 suggest any ring detection circuit which can give out a logic level signal.
i am using the one with the optoisolator. this one can give out a signal that fluctuates according to the rings. i need a circuit with good logic output signal to drive a flip flop.
plzz help..

I have not done a rigerous analysis of this circuit, but I think it works.