Andreas1984:
a) getting the signal voltage down to a 5V or 3,3 V level.
Depending on the signal type a diode may do the job (pointing away from the Arduino allowing the external signal to pull the pin low, use the internal pull-up for the high signal).
b) measuring the frequency with an arduino.
Count number of pulses for a certain time, or measure how long it takes for a certain number of pulses to arrive. Just poll the pin and use state change detection; no need to use interrupts for such rather low frequencies.