Series 65 incorporates well-proven sensing technologies, together with advances in materials and electronics technology, including an IC based on that used in XP95 analogue addressable detectors. Having a wide operating voltage of 9-33V, the Series 65 detectors can be integrated into security systems, when used with a relay base. The A1R, BR and CR (rate-of-rise) heat detectors operate by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions the two thermistors register similar temperatures, but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance, causing the detector to change into the alarm state. Rate-of-rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detector earlier. The CS (static response) heat detector has only one thermistor and changes to the alarm state at a pre-set temperature. Externally, the heat detectors are distinguishable from the smoke detectors by having wide openings to the surrounding atmosphere to allow good movement of air around the external thermistor.