Can motion sensors be triggered by light?

May 07, 2018

Motion sensors are a common part of our world today. Wave your hand under the faucet and the water runs. Pull open the closet door and the light turns on. Walk near the doors of the grocery store and they automatically slide open.

How do motion sensors work? It depends on what kind of motion sensors they are.
 
Active motion sensors
 
Active motion sensors use microwaves to detect motion. Microwaves are a type of electromagnetic radiation — waves of electrical and magnetic energy moving together through space. This is how radar and pregnancy ultrasounds work. 
 
An active motion sensor emits microwaves and then analyzes the waves that are reflected back at its sensor. When the pattern of waves reflected at the sensor are disturbed, such as when a person walks by, it triggers the sensor to send a signal. This signal may open a door, turn on the water or activate an alarm.
 
These types of sensors are great in harsh, high heat environments, but they also can cause false alarms if a small animal scurries by or the wind blows too hard. Active motion sensors need a continuous power source or strong batteries because they are actively working to send out microwaves and detect motion.
 
Passive motion sensors
 
Passive motion sensors generate energy when exposed to heat or infrared light. Infrared light is a form of light we can’t see with our eyes but can feel on our skin as heat. When a passive motion sensor detects heat, it generates energy that triggers the detection, thus causing the lights to turn on or the alarm to sound.
 
It’s passive because the sensor doesn’t generate or radiate energy for detection purposes, like how the active sensor emits microwaves to test the environment around it. Rather, it works entirely by detecting any energy emitted from other objects.
 
If you live in a hot climate where temperatures can climb above 95 degrees F, this type of sensor won’t work. A high quality passive motion sensor can last multiple years on one battery.
 
What can accidentally trigger your motion sensor?
 
So back to our original question: can motion sensors be triggered by light? Yep. But not likely just your normal lights turning on and off in the living room. A passive motion sensor would more likely be triggered by sun shining on it or an extremely hot bulb close to the sensor. You’ll also want to keep passive motion sensors away from the heating vents and radiators.
 
Active motion sensors aren’t foolproof either. As we already said, small animals, insects or the wind can potentially trigger an active motion sensor.
 
To prevent false alarms, today’s motion sensors are usually a combination of passive and active sensors so both microwaves and heat must be detected to trigger the sensor. Today’s motion sensors also can be set to ignore animals up to a certain weight, helpful if you have pets.

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