How can I boost/limit a signal to TTL levels?
You may wish to try an open collector for the TTL input. Get some cheap high gain npn transistor:


connect its emitter to ground; 
connect its collector to the TTL input; 
add 1k resistor between the collector and +5V rail; 
add (if needed, check the transistor's cpecs) a diode between base and emitter in the direction reverse to "base-emitter" junction to save the junction on negative half-periods of your AC;
connect the AC output to the base of the transistor through, say, 100k resistor;


So, at 5V AC the peak of base current will be approx 0.05ma - more than enough to handle 1k resistor on the collector and TTL input (for high gain transistor). At 50V AC the peak of base current will be approx 1ma, quite within the specs for most transistors.