Abstract: An add-on alarm system for use in conjunction with an audio detector. The system includes an indoor unit that is audibly coupled to the detector and an outdoor unit containing an independent power supply that communicates with the indoor unit to provide an alert signal for directing rescue operations. The outdoor unit includes a relay which is energized by a signal from the indoor unit to close a contact in an alarm circuit and thus generate an alert signal. The relay is held energized by the outdoor power supply so that the alert signal persists in the event the detector or the indoor unit becomes disabled.