Using PIR Sensors Outdoors

        PIR sensors are easy to use indoors. Temperatures are relatively stable. There is usually no direct sunshine. So false alarms are infrequent. But outdoor usage has serious problems with false alarms. In applications where PIR sensors merely turn on lights, false alarms are of little consequence. However if the application is for security systems where alarms may be activated, false alarms are a nuisance at best. The good news is that false alarms can be reduced to near zero.
        Our research is based on using PIR sensors to detect motion with low cost circuits that will run for years on common batteries. In our devices a PIR sensor triggers a camera and transmits a snapshot of an intruder to a computer. Also transmitted is an ID code to identify which sensor was activated. We have more than a dozen of these protecting our property. We use the AM312 sensor which is a small PCB with the PIR sensor and a voltage regulator. The quiescent current is only 15uA and works over a voltage range of 2.7-12V. A 9V alkaline battery will operate this device for years. But using it outdoors as-is will certainly result in many false alarms. Following are tips you can use to reduce and nearly eliminate false alarms...

        Tip #1 - Do not try to detect motion over a wide area. PIR sensors will respond to moving shadows which create temperature gradients. Even wind can produce enough of a temperature gradient to trigger a PIR sensor. In most applications a narrow angle of coverage is sufficient. You point the sensor at where you expect an intruder to enter. Think of it as a 'breaking the beam' device except an actual beam is not used. We use a lens which has a 27 degree fields of view. If necessary you can reduce it further by covering the lens and allowing it to see through a small hole about an inch away. We mount our devices to point at doorways and walkways. We even attach sensors to trees to cover entry points of our property.

        Tip #2 - A PIR sensor is extremely sensitive to small temperature gradients across it's detecting element. If the enclosure is in direct sunlight it should be painted a light color to reflect sunlight and heat. As air inside the enclosure changes from external sources, temperatures are not equally spread. It helps to insulate the sensor with a foam sleeve to reduce gradients across the detecting element. Open areas within the enclosure should also be filled with foam to reduce convection currents.

        Tip #3 - We saved the best for last! While the above tips help significantly, a major leap in reducing false alarms was to use dual PIR sensors an inch apart and process their outputs. Here is our suspected cause for reduced false alarms. Others may have a better explanation. Each sensor sees an image with concentric circle lines due to the steps in the fresnel lens. Those lines are a different temperature than the rest of the image. As a focused image of a temperature gradient passes across the sensor those lines are detected. Two sensors will see the lines in different places. One sensor may output a false alarm from wind or other causes but two sensors seeing different views will output a detection event that is significantly less likely to be a false alarm. Using dual sensors in our products nearly eliminated false alarms outdoors.
       
        We've been using the dual sensor versions since early 2019 with only a few possible false alarms. We haven't yet commercialized our sensors but may do so in the future. Ideally we would license the product's manufacture and distribution.