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 Series 65 wide voltage range consists of ionisation, integrating ionisation and optical smoke detectors, 4 grades of heat detector and a range of bases. Optical smoke detectors incorporate a pulsing LED located in a chamber within the housing of the detector. The chamber is designed to exclude light from any external source. At an angle to the LED is a photo-diode which normally does not register the column of light emitted by the LED. In the event of smoke from a fire entering the chamber, the light pulse from the LED will be scattered and hence registered by the photodiode. If the photo-diode “sees” smoke on the two following pulses, the detector changes into the alarm state and the indicator LED lights up. The detector housing is identical to that of the ionisation detector but has an indicator LED which is clear in quiescent state but produces red light in alarm.