Check the listing for details. SIEMENS FDOT421 Multi-Sensor Smoke Detector Model free ship.. Condition: New. Listed at 53.50 USD. The SIEMENS FDOT421 Multi-Sensor Smoke Detector Model is a reliable and advanced fire protection equipment designed for business and industrial facilities. With its advanced multi-sensor technology, this smoke detector can quickly detect smoke and fire, providing a safe environment for employees and customers. The Siemens brand guarantees quality and performance, making this model a trusted choice for smoke detection needs.