This hobby circuit uses a LTC1799
precision frequency generator from Linear Technology. A 74HC14 hex Schmitt
trigger from Texas Instruments is also used to perform several other functions.
One section is wired as a simple 7Hz square wave oscillator.
The triangle waveform across that
capacitor generates the low frequency sweeping signal for the siren. Two
resistors bias the LTC1799 for a center frequency of about 2KHz. A flyback
DC to DC converter circuit, produces a 40v peak signal, which is turned on and off
according to the output of the LTC1799.
The output is connected to a quality piezoelectric beeper, which has a resonant
frequency of about 2.5KHz. The result is a siren which is quite loud but
draws only 40ma from a 3v supply. A piezoelectric device from Kobitone, part
number 254-PB515-ROX, (Mouser part number 245-PB516) shown above works well.
If you want something much smaller, although
not quite as loud, try the Murata PKM17EPP-2002-BO shown above (Digikey part