In a multistory facility, there may be a relay panel on each floor controlling
all the lights on the floor. Each room on the floor has a local switch and there
is also a master switch for the entire floor. The master switch for the floor may
be programmed to turn lights on at 7AMand off at 6PM; between 6PMand
7 AM, the system may repeatedly perform an “off” sweep to turn lights out
where the programming has been overridden.
Occupancy Sensors
Occupancy or motion sensors are devices that sense the presence or absence of
people within their monitoring range. Unlike scheduling controls, occupancy
sensors do not operate on a time schedule; they merely detect whether a space
is occupied. They may be used in restrooms, utility rooms, conference rooms,
coffee rooms, locker rooms and many other spaces. Typically, the sensor and a
control unit can be enclosed in one unit, such as a wallbox, but for larger facil-
ities, the sensor is tied to a relay panel.
The control unit or the relay is programmed to turn lights “on” when the
presence of people is sensed by the motion detector, and may be programmed
to turn the lights “off” if the space is unoccupied for a predetermined time
period. The sensitivity of the sensor may also be adjustable (Fig. 4.3).
There are several types of motion sensors available, including passive infra-
red (PIR), active ultrasound, and hybrid technologies, such as combinations of
PIR and active ultrasound, or PIR and audible sound. These sensors are typi-
cally used in locations such as hallways, lobbies, private offices, conference
rooms, restrooms, and storage areas.
Ultrasonic sensors emit high-frequency sound waves and sense the fre-
quency of the reflected waves as they return to the device. Movement in the
area where the waves are emitted changes the frequency of the reflected waves
causing the sensor to turn the lights on. Ultrasonic motion detectors provide
continuous area coverage and are best suited for use in open areas such as
offices, classrooms, and large conference rooms. Mechanical devices that pro-
duce vibrations or changes in airflow, such as HVAC systems, can however
trigger ultrasonic occupancy sensors and cause lights to turn on.
PIR sensors detect radiation, that is, the heat energy that is released by bod-
ies. They are labeled “passive” because they only accept infrared radiation and
do not emit anything. PIRs operate in a line of sight and have to “see” an area,
so they cannot be obstructed by open-area partitions or tall furniture. PIRs
use a lens to focus heat energy so that it may be detected. However, the lens
views the covered area through multiple beams or cones and may create cover-
age gaps. Any objects that prevent the sensors from “seeing” portions of its
Lighting Control Systems 51