The XP95 optical smoke detector uses an
internal pulsing LED and a photo-diode
at an obtuse angle. In clear air conditions
the photo-diode in the XP95 detector
receives no light from the LED and
produces a corresponding analogue
signal. The signal increases when
smoke enters the chamber and light is
scattered onto the photo-diode.
The analogue voltage
signal
is converted to a digital signal by the
electronic circuitry and transmitted to
the control equipment on interrogation.
The micro-processor in the control
equipment then compares the signal
with the stored data and initiates a
pre-alarm or fire alarm as smoke density
increases. When the equipment
determines that a fire condition exists,
it instructs the detector to switch on its
indicator LED and the pre-planned
alarm routine is initiated.
The optical smoke detector is externally
identical to the ionisation detector but is
distinguished by having a clear indicator
LED which emits red light when the
detector is in alarm.