Siemens Smoke Detector: The Photoelectric Smoke Detector  uses state-of-the-art microcontroller circuitry and surface-mount technology for maximum reliability. Model OP921 incorporates an optical sensor using a light-scattering detection principle. The device utilizes advanced software algorithms to analyze the signals providing highly stable and accurate smoke detection. Model OP921 is UL 268 7th edition listed incorporating advanced built-in ISOtechnology - True Class-X SLC operation (use is optional) greatly improving system reliability and circuit integrity while providing advanced addressable fault finding.