The UG-5 duct smoke detector has been developed to detect smoke in ventilation ducts and combines a smoke detector and an adaptor system where both tube and housing are specially designed for optimum airflow through the smoke detector. The detector can be installed on any side of the duct in four different positions: 0°, 90°, 180° and 270°. The smoke detector has 2 alarm relays to control fire-safety dampers, to stop ventilation fans or to activate acoustic and optical alarms etc., 1 service alarm relay to let you know when maintenance is needed, before a false alarm occurs, 1 system error relay indicating fault on the smoke detector circuit, 1 low-flow relay indicating that air flow in the duct is low or that the UG-5 is installed in an inexpedient place on the duct. The smoke detector contains an intelligent controlling circuit. This circuit is adjusting the sensitivity to give an optimal function during the whole life time of the detector. When the controlling circuit has reached the maximum sensitivity compensation for contamination, a service alarm is indicated. The detector has a bayonet fitting to simplify mounting and removal.