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M300 SERIES
01A-02-D4
002 02/96
The amplifier output is fed to a pulse height discriminator
which compares the signal with a preset threshold level. So-
phisticated synchronous detection techniques are used to
eliminate virtually all the effects of noise and spurious tran-
sients.
If the signal amplitude exceeds the threshold level then the
emitter pulse period is reduced to two seconds. The pulse pe-
riod remains at two seconds if the signal is above threshold.
When the counter has counted three consecutive pulses
above the threshold, the output stage is latched into the
alarm condition. If, however, the amplitude of the second or
third pulse is below threshold, then the pulse period reverts
to 10 seconds and the counter resets. The switching of the
output stage lights the integral alarm indicator [LED] and
provides drive for an external [remote] LED indicator.
2.3.2 INTRINSICALLY-SAFE VERSION
The operation of the MR301Ex detector is the same as that
of the standard MR300 range detector. Circuits are modified,
however, to ensure that safety is maintained under normal
conditions and with up to two zone circuit faults.
3. MECHANICAL CONSTRUCTION
The major components of the detector are:
• Body Assembly
• Printed Circuit/Optical Array Assembly
• Labyrinth and Insect Screen
• Outer Cover
An exploded view of the detector is given in Fig. 4.
3.1 BODY ASSEMBLY
The body assembly consists of a plastic moulding to which
are secured the four detector contacts which align with con-
tacts in the M300 Base. The moulding incorporates securing
and polarising features to retain the detector in the base.
The inside surface of the moulding is metallised to provide
shielding against EMI [Electro-Magnetic Interference]; the
connections between the contacts and the PCB are made us-
ing feed-through capacitors.
3.2 PRINTED CIRCUIT/OPTICAL
ARRAY ASSEMBLY
All electronic components, including the LED alarm indica-
tor, are mounted on a single printed circuit board. The Opti-
cal Array Assembly is also mounted on the PCB. The
combined assembly is fitted inside the body and connected
to the feed-through capacitors. All external connections to
the circuit are thereby provided with EMI filtering. The
Body Assembly is then filled with epoxy to provide mechan-
ical strength and to protect circuit components against cor-
rosive atmospheres.
3.3 FINAL ASSEMBLY
The two labyrinth mouldings are fitted on the encapsulated
body and the smoke entry area is sealed by a wire mesh to
keep out larger insects. The labyrinth is retained by the outer
cover which is a snap fit on the body.
4. TECHNICAL SPECIFICATION
4.1 GENERAL
Several detector variations are offered within the MR300
Range. The technical specifications of these are very similar
and the details given below are applicable to all unless oth-
erwise stated.
4.2 MECHANICAL
Dimensions
The dimensions of the MR300 detector are shown in
Fig. 5.
Materials
Body and cover: “BAYBLEND” ABS
/Polycarbonate alloy
Thermo-plastic,
self-colour white.
Weight
Detector: 0.2kg
Detector
plus base: 0.3kg
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