Abstract: An alarm driver is described providing an audio frequency signal for operating the alarm of a smoke detector. The driver comprises a four transistor bridge in which the transistor input bias is momentarily interrupted at an audio rate to permit the bridge current supplied to the alarm to stop and change direction. Cross coupling is provided to achieve bistability in operation of the bridge, while commutation is achieved by utilizing energy stored in the inductance of the alarm to turn one pair of transistors on and the other pair off at each switching transition. The driver is designed for maximum reliability and minimum current drain and it is suited for battery operation. It is preferably fabricated by an integrated circuit process, typically using bipolar transistors and sharing the substrate with the other circuitry of the smoke detector.