Using a simple but effective technique, the EV-P Optical Detector incorporates a chamber housing an Infra-Red (IR) LED light and a photodiode light detector set at an obtuse angle preventing it from seeing the light. When smoke enters the chamber the light bounces off the smoke particles and strikes the photodiode, causing it to generate an alarm condition when a set threshold value is met. The EV-P Optical Detector is therefore more sensitive to larger smoke particles which tend to be caused by smouldering fires rather than fast flaming fires.