The simple circuit transforms the steady beep of an audible-signal device, such as a Mallory sonalert, into a distinctive warble or chirp. The value of R, C2 determines just what tone color you`ll get. With 1k the 1-I`F value shown, the circuit produces a warble similar to the ring tone of an inexpensive phone. A R, 10-I`F value produces a chirp similar to a truck`s 2ok back-up alarm.
One elaboration of this circuit would be to use the second section of a 555 timer to drive a piezoelectric transducer instead of a sonalert; that modification would vary the tone"s pitch, as well as the chirp rate.