If you want to use the 555 circuit that Nigel referred to, you'll still need a "D" flip-flop. I made a slight modification to the original circuit, and added the "D" ff. The output is high when the input is above the preset frequency (nominally 30Hz), and low when the input frequency is below the preset. This circuit does require the input to have a 50% duty cycle (which you said you have) with the values shown. The input voltage must swing from 0 to 9 volts.