Abstract: A smoke alarm device has an ASIC (1) with an integrated photo detector (3) and control circuit. The photo detector output is compared every ten seconds with an alarm threshold level in a comparator circuit (10), with a sensitivity-decrease threshold in a comparator circuit (11), and with a sensitivity-increase threshold in a comparator circuit (12). A logic block (2) causes sensitivity to be increased or decreased in which decreases take place at intervals of six hours and increases take place after 40 secs. The logic block (2) provides for least sensitivity at power-up with rapid increases to the appropriate level according to the level of back-scatter caused by dust contamination. The logic block (2) maintains a high signal on an interconnect terminal (9) for four seconds after a test button (7) is pressed so that a maintenance person can hear remote interconnected devices after the local device has stopped sounding.