Abstract: A circuit for switching between two signals of different frequency, such as "wail" and "yelp" of an electronic siren, by switching between different capacitive values in a multivibrator circuit. A monostable switch couples a reference level voltage to an integrated flip-flop circuit. The integrated circuit receives a reference level pulse when the monostable switch is closed and supplies a switching voltage to a transistor circuit. The transistor circuit places capacitances in parallel to change the time constant of the multivibrator circuit and also to change the characteristics of wave-shaping circuitry coupled to the multivibrator circuit. A reference level circuit accepts either of two voltage levels and provides a reference level voltage.