Figure 8-28.Motion detector rotating up to 180° for
The basis of this type of system is a gas-detection
tube using the Geiger-Mueller principle to detect
radiation wavelengths extending from 2000 to 2450
angstroms (Å) (1 Å = 10-8 cm). Figure 8-34 displays
the radiation sensitive area of the tube and compares
this area to other forms of radiation. It should be noted
that visible radiation does not extend into the sensitive
area of the detector. Similarly, radiation from artificial
lighting sources does not extend into the sensitive area
of the detector.
Figure 8-29.Audio detector.
Welding arcs and lightning strikes, however, will
generate radiation to which the detectors are sensitive
and precautions must be taken to minimize these
The ultraviolet-radiation detectors focus of
sensitive points is a 60-degree spherical cone whose
apex lies at the detector tube. Figure 8-35 indicates the
relationship between viewing angle and relative
sensitivity. The sensitivity of the detector tube is a
characteristic of its cathode material and is fixed, but
its voltage-pulse output rate varies both with flame size
and flame viewing distance. The pulse output rate is
directly proportional to flame size; that is, it increases
when larger flame fronts are presented to the detector.
The pulse output rate is also inversely proportional to
the distance of the flame front from the detector
tube-the pulse output rate decreases as the distance
from the detector tube to the flame front increases.
To illustrate, a l-foot (0.09 m2) hydrocarbon fire
will cause a pulse output rate of 3 pulses per second at a
viewing distance of 30 feet (8 m). This same fire will
cause a tube pulse output rate of 20 pulses per second at
a viewing distance of 20 feet (6 m). In a like manner, 1-
foot (0.09 m2) flame front must be located at a distance
of 5 feet (1.5 m) to create a pulse output rate of 30
pulses per second; a 16-foot (1.4 m2) fire will create the
same pulse output rate at a distance of 25 feet (7.6 m),
and so forth.
A schematic wiring diagram of a typical telephone
dialer is shown in figure 8-36. The two cooperating