This remote alarm for smoke detector circuit is an application note from the maxim integrated webpage. Here this circuit uses MAX921 Comparator IC, this will detect supply current increase triggered by activation of sensing device.
Construction and Working
Main part of this circuit is IC MAX921 which will sense supply current increase due to the activation of detector. Many detectors consumes low current during monitor mode and consumes high current during active, this difference is detected by max921 comparator and gives remote output.
Here 9volt battery source gives power supply to the smoke detector and comparator circuit.
You can get datasheet of IC MAX921 here.