8
MAN3037-2
XP95 PRODUCT GUIDE
6
Optical Smoke Detector
6.1
Operating Principles:
The XP95 optical detector uses the same outer case as the
ionisation smoke detector and is distinguished by the indicator LED
which is clear in standby and red in alarm. Within the case is a
printed circuit board which on one side has the light proof labyrinth
chamber with integral gauze surrounding the optical measuring
system and on the other the address capture, signal processing
and communications electronics.
An infrared light emitting diode within its collimator is arranged at
an obtuse angle to the photo-diode. The photo-diode has an
integral daylight-blocking filter. The IR LED emits a burst of
collimated light every second. In clear air the photo-diode receives no light directly from the IR LED because
of the angular arrangement and the dual mask. When smoke enters the chamber it scatters photons from the
emitter IR LED onto the photo-diode in an amount related to the smoke characteristics and density. The
photo-diode signal is processed by the optical ASIC and passed to the A/D converter on the communications
ASIC ready for transmission when the device is interrogated.
Figure 7 – Top Section – XP95 Optical Smoke Detector
6.2
Electrical Description:
The detector is designed to be connected to a two wire loop circuit carrying both data and a 17V to 28V dc
supply. The detector is connected to the incoming and outgoing supply via terminals L1 and L2 in the
mounting base. A remote LED indicator requiring not more than 4mA at 5V may be connected between the
+R and -R terminals. An earth connection terminal is also provided.
When the device is energised the ASICs regulate the flow of power and control the data processing. The
optical ASIC is controlled by the communications ASIC and pulses the IR LED. The signal from the photo-
diode is processed by the optical ASIC and transferred to the A/D converter in the communications ASIC
where it is then stored. When smoke enters the chamber the photo-diode signal increases. The information
to the A/D converter is updated once per second or when either the monitor or the preceding address is
interrogated. Whenever the device is interrogated this data is sent to the control equipment. EN54 threshold
alarm levels are calibrated within the processing ASIC. If the device is not addressed within 1 second of its
last polling and the analogue value is greater than the EN54 alarm level the alarm flag is initiated and the
device address is added to the data stream every 32 polling cycles from its last polling for the duration of the
alarm level condition, except when the alarming device is being interrogated. This can provide a location
identified alarm from any device on the loop in approximately 2 seconds.
The detector is calibrated to give an analogue value of 25±7 counts in clean air. This value increases with
smoke density. A count of 55 corresponds to the EN54 alarm sensitivity level. See Fig.10.
