Abstract: Typical apparatus according to the present invention for detecting a predetermined type of condition (such as the presence of smoke) at a given location and providing an indication (typically audible and/or visible) of the presence of such condition, comprises an electrical detector responsive to the type of condition, for providing a potential difference of at least a predetermined amount between two predetermined points; first circuitry for connecting at least a substantial fraction of the potential difference to an electrical relay or relay analogue in the apparatus so as to provide a predetermined first indication; and second circuitry responsive to a condition in the detector or in the first circuitry that is outside of a predetermined range, and thus of possible detriment to the optimum performance of the apparatus, to provide a predetermined indication.