handbook of modern sensors

240 6 Occupancy and Motion Detectors
in energy management (to turn lights on and off), and in the so-called “smart homes,”
in which they can control various appliances, such as air conditioners, cooling fans,
stereo players, and so forth. They also may be used in robots, toys, and novelty
products. The most important advantages of an optoelectronic motion detector are
simplicity and low cost.
6.5.1 Sensor Structures
The general structure of an optoelectronic motion detector is shown in Fig. 6.9A. Regardless
of what kind of sensing element is employed, the following components are
essential: a focusing device (a lens or a focusing mirror), a light-detecting element,
and a threshold comparator. An optoelectronic motion detector resembles a photographic
camera. Its focusing components create an image of its field of view on a
focal plane. Although there is no mechanical shutter like in a camera, a light-sensitive
element is used in place of the film. The element converts the focused light into an
electric signal.
Let us assume that the motion detector is mounted in a room. A focusing lens
creates an image of the room on a focal plane where the light-sensitive element
is positioned. If the room is unoccupied, the image is static and the output signal
from the element is steady stable. When an “intruder” enters the room and keeps
moving, his image on the focal plane also moves. In a certain moment, the intruder’s
body is displaced by an angle α and the image overlaps with the element. This is an
important point to understand: The detection is produced only at the moment when
the object’s image either coincides with the detector’s surface or clears it; that is,
no overlapping—no detection. Assuming that the intruder’s body creates an image
whose electromagnetic flux is different from that of the static surroundings, the lightsensitive
element responds with a deflecting voltage V . In other words, to cause
detection, a moving image must have a certain degree of optical contrast with its
surroundings.
Figure 6.9B shows that the output signal is compared with two thresholds in the
window comparator. The purpose of the comparator is to convert the analog signal
V into two logic levels: ∅=no motion detected and 1 = motion is detected. In most
cases, the signal V from the element first must be amplified and conditioned before it
becomes suitable for the threshold comparison. The window detector contains both
the positive and negative thresholds, whereas the signal V is positioned in between.
Whenever the image of a moving object overlaps with the light-sensitive element, the
voltage V deflects from its baseline position and crosses one of two thresholds. The
comparator generates a positive voltage (1), thus indicating a detection of movement
in the field of view. The operation of this circuit is identical to the threshold circuits
described earlier for other types of occupancy detector.
It may be noted from Fig. 6.9 that the detector has quite a narrow field of view:
If the intruder keeps moving, his image will overlap with the sensor only once; after
that, the window comparator output will a produce steady ∅. This is a result of the
small area of the sensing element. In some instances, when a narrow field of view is