DF 503-MD conventional photoelectric smoke/heat detectors are state-of-the-art detectors suitable for connection to conventional fire detection control and indicating equipment, or to addressable fire detection control and indicating equipment that can accept conventional type detectors. Advanced electronics in conjunction with a photoelectric smoke sensing chamber and an electronic thermistor provide early detection of fire and high immunity against unwanted alarms. The heat sensor provides fixed temperature operation when the response threshold value is exceeded. The DF 503-MD range of conventional detectors provides fire detection and alarm system designers with a Standards-compliant, economical fire detector for life safety and property protection applications.