motion-alarms-135448943We’ve all seen them in our favorite action thrillers, and we experience them every time we walk through the automatic doors at the grocery store: motion detectors are amazing security sensors, and they can certainly go a long way toward making our homes safer places. So, how do they work, what types are available, and how do you go about choosing the sensors right for your home? There are two main types of motion detectors: active and passive. Here we’ll take a basic look at an overview of each category and how they can be a benefit to your home security system.

Active Motion Detectors

What do we mean by “active”? Active sensors emit signals and then monitor the signal that “bounces back” in order to determine any sudden changes or movements in the environment. These typically work great for covering entire rooms indoors, but would easily tend to set off false alarms if placed outdoors. The three types of active motion detectors we’ll cover today are microwave, ultrasonic, and photo-sensor.


This active motion sensor emits electromagnetic pulses that help it determine motion within its “field of vision.” The returning reflection waves will change in frequency if an object is moving, due to the Doppler Effect.


Ultrasonic sensors act like mechanical bats, using sound waves to “see” the world around them. These ultrasonic waves reflect differently off of different objects and can alert the emitter to their location. If the location of an object changes, this sensor will pick up the difference in the reflection and signal to the control box to set off the alarm.


Infrared motion sensor 466282673These are not technically motion detectors, but they work effectively as such. What sets them apart from the other active motion sensors is that their range rarely covers an entire room. A photo-sensor usually has two parts: a source of light, and a light sensor placed directly opposite of it. Have you ever seen those James Bond movies where he has to get past the room filled with what look like red lasers? Each of those lasers represents an individual photo-sensor, which is why these sensors are great for protecting valuables or for detecting motion in very specific areas of your home. If something interrupts the flow of light to the sensor, the sensor detects it and immediately alerts the home of the potential danger.

Passive Motion Detectors

Passive motion detectors don’t actually emit any kind of energy, but rather simply monitor their surroundings in the particular way that they are programmed to. When a rapid change is detected, the sensor sounds the alarm. We will only go over one main type of passive motion detectors: Passive Infrared, or PIR.

Passive Infrared (PIR)

PIR sensors effectively detect changes in temperature around them. They can “see” around them by picking up the particular heat signatures of the objects moving around them. In a way, they aren’t actually “motion” detectors per say. They instead are most accurate at detecting the sudden presence of an intruder’s body heat, seeing as the presence of body heat is much more recognizable to these PIR sensors than any other kind of temperature change in the environment. Because of this, PIR sensors are highly effective when placed in outdoor settings. They won’t be affected by the movement of critters or foliage, but will immediately detect the presence of a human.

Whichever type you choose, motion detectors are a great advantage to any homeowner concerned with security, which, hopefully every homeowner is! Talk with your professional home security provider about what motion sensor options they can offer you.