A pulsed infrared light-emitting diode serves as the light source, and a high-speed photo-diode as the sensing element. This design has superior protection against false alarms caused by dust, insects, RF, and ambient light. These smoke detectors are especially suited for residential occupancies, including hotels, motels, and dormitories, as well as other commercial and industrial fire-system applications. This series is designed for two-wire and four-wire connection to 6-24 VDC fire alarm control panels, UL Listed for commercial or household fire protection. Features Self-diagnostics including automatic sensitivity testing. Each detector in the series continually monitors its own sensitivity and operational status (see Maintenance). A wide range of optional features are offered as shown in the Product Selection Guide. To provide for almost any application, these options include a built-in sounder, an auxiliary relay, an integral heat detector, and an isolated heat detector.