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The way I look at it is that you need to start off with a basic sound wave signal (what waveform would you suggest). Then, the schmitt trigger comes into the picture by acting as an oscillator. That is it "comes on" at a certain frequency and "turns off" at a certain tone frequency.
Then, the "two tone" bit would have to work in the on -part of the schmitt trigger frequency. Would it be ok to say that the base frequency the schmitt trigger oscillates around depends upon on its R and C values. At this stage, I am looking to work with a 1 - 5 kHz frequency.
As another option, what would happen if I play around with the amplitude of the input signal? would I still get a two tone signal????
Am I on the right track here with this chain of thinking????
To me, a two-tone siren means the sort of thing you hear from a fire truck. It is a harmonic-rich two-tone oscillator which switches from tone A to tone B at about 1/2 second intervals. It doesn't ever put out both tones at once.
If that is what you want, get two CMOS 555 (7555?), set one 555 up to run more or less 50% duty-cycle at about 2Hz; set the other up to produce about 1000Hz (or whatever sounds good to you); and then wire a resistor from the output of the first 555 to the "frequency modulation" input of the second one. That will warble the tone of the second 555 between two fixed tones. The size of the resistor will determine how much the tone shifts.