By Miro Oljaca, Senior Application Engineer, Texas Instruments
Signature characteristic makes motion detection more adaptive and reliable.
Pyroelectric or passive infrared (PIR) sensors are very efficient for detecting various infrared emissions from
a heat source, which allow them to be used for motion
Today’s buildings are adding building intelligence for
energy and system efficiency through wireless sensor
nodes. These sensor nodes must maintain a long battery
life (up to 10 years) while constantly monitoring key
parameters like temperature, humidity, and occupancy.
In building automation, PIR motion detectors are
incorporated in overall echo system, including comfort
control to airflow control in heating, ventilation, and air
conditioning (HVAC) systems as well as lighting, safety,
and security. PIR motion detectors require just a PIR
sensor and a few components around the sensor to make
Each pyroelectric element of a PIR sensor (there
are at least two) receives a variation of infrared
radiation when a heat source moves in the sensor’s
area of detection. As the heat source moves, two
elements will receive different amounts of radiation.
The change in received amount of the radiation will
produce the sensor’s output analog signal. Fresnel
lenses shape the detection area and focus the beam’s
on the sensor. The PIR sensor produces an AC signal
with very low amplitude (1 to 3 mV typically). This
signal needs one- or two-stage amplification. Finally,
passing this signal to a window comparator facilitates
binary detection of the motion.
In the age of connected things, new challenges arise.
Having a motion detector connected wirelessly and
ensuring long battery life is the ultimate challenge for
designers. PIR motion detectors that have a sub-1 GHz
connection and a single coin-cell battery with a life
span of more than 10 years will need to operate with an
average current consumption of less than 2.1 μA. This
consumption can be divided as 0.6 μA for communication
and 1.5 μA for the analog signal chain.
Motion detectors containing one PIR sensor, a Fresnel
lens with a cone-type beam, and a binary output
are very efficient for detecting any type of motion,
including human or pet motion. Imagine a customer
application based on this type of motion detector
that sends a notification and switches on a light when
any motion is detected. The amount and type of
notifications received could be annoying, like when
your dog is moving around in your house while you
How to Avoid False Triggering and Detect Only Human Motion
One low-cost solution uses a Fresnel lens with a narrow
vertical-detection area that detects tall moving objects
(humans). Unfortunately, this solution will have a blind
area that can be exploited.
Another more customizable solution consists of two
PIR sensors with different Fresnel lenses (as shown
in Figure 1). One lens with a middle beam detects
tall moving objects like humans, while another with
a lower beam detects short moving objects like pets.
With this configuration, it is possible to differentiate