Abstract: An ionization type smoke detector includes an element for giving a visible indication of operativeness of the system together with a component responsive thereto for representing impending failure of the power source. A housing for the system includes a battery-containing well that includes a channel permitting finger-access to the batteries. A pushbutton is located in a wall of the housing. An electrically-conductive probe is positioned within an ionization chamber and is associated with the pushbutton for addressing the probe in a manner that creates a simulation of the existence of combustion. In response to a signal from the ionization chamber, digital processing is employed in order to enable alarm indication as appropriate as well as to monitor continued capability of the battery power source. Special circuitry cooperates advantageously with those features as well as in interrelating the different features so as to minimize complexity.