You’ve described exactly the issue: most of the under $100 motion sensors that work with smartthings are PIR: passive infrared, which means they measure very small changes in heat across the lens. And it’s just very hard to get those to work successfully outdoors because of both sunlight and breezes. And insects.
There are three typical options:
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rely on a sensor built into an outdoor rated camera or video doorbell. These use a different technology and are usually much more reliable.
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use three or four sensors to create a zone and then only count it as a motion event if they all trigger at the same time. This can be done, but obviously significantly increases the total cost.
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use a plastic tube over the lens to narrow the detection field and help protect it from wind. Some community members have been very successful with this if they are only trying to target a very specific area, like someone coming onto the front porch.
you can find all of these options discussed in detail in the outdoor motion sensor FAQ, but sometimes it’s just a matter of trial and error to see which one works for you and fits your budget. There isn’t any simple easy answer because of the technology being used.
( this is a clickable link)