They’ve been staples
of business offices
for years, but
now occupanc y
and vacancy sensors
are increasingly
installed in homes.
Used to switch off
room lights after
occupants have left,
their numbers are
growing thanks not
only to the green
movement, but also
because of state mandates for homeenergy
savings.
Watt Stopper/Legrend in Santa
Clara, Calif., was one of the first firms
to produce room sensors for controlling
lights. The firm branched out into
the residential market in 2004. A basic
Watt Stopper wall switch contains a
pyroelectric sensor and lens system,
an ambient light sensor, a microprocessor,
and a switching element.
The switches can distinguish people
in a room from other sources of
motion such as pets or even drapes
dancing in a breeze. The key to this
filtering, says Watt Stopper Marketing
Director John Null, is in microcomputer
programming and in the
Fresnel lens beaming room light to
the infrared sensor. The Fresnel lens is
a special design that has both vertical
and horizontal elements. Room light
passes through it onto a two-element
IR detector. “We’ve refined the detection
technique by making sure the
IR source is moving across the face of
the lens. The processor applies rules
about speed-of-movement and the
amount of energy in the target before
it makes a decision about whether to
open or close the switch,” says Null.
The same basic electronics go into
both occupancy sensors and vacancy
sensors. The difference is in the programming
of the microcomputer. Vacancy sensors turn off lights after
a preset time delay once people have
left the room. Occupancy sensors
turn on lights when they detect people
moving around, then douse lights
once everyone has left. Both include
time delays to allow for people sitting
motionless.
Setups get more complicated
with multiway sensors, as would be
the case with three-way switches at
both ends of a staircase. “When one
of these detects motion, the other has
to know about it because you don’t
know which is connected to the load,”
explains Null.
The only downside to these switches
is that their sensors must draw power
all the time. “We generally leak power
to the sensor through the neutral or
through the lamp, though we can’t leak
current through CFLs or fluorescents,”
says Null. Fluorescent bulbs also need
wall switches having relays rather than
triac switching elements.
There are more developments in
store for automatic light controls. Null
says ultrasonic sensors may be the
next item to come off the drawing
boards as a way to detect motion out
of the switch’s line of sight.
Make Contact
Watt Stopper, wattstopper.com