All individual motion systems have a lot of false alarms outdoors because wind blowing across the sensor will trigger often trigger it. Rain sometimes. And on a bright day, cloud movement may trigger enough of a change in temperature to trigger it.
Professional security firms solve the issue with multiple device systems. There are three common approaches.
- trigger beam. A two part device with a beam between them. When the beam is broken, the notification is sent. Deer can still trigger it, but wind and clouds won’t. In very strong wind conditions, a branch or lawn chair blown through the beam will trigger it, but that’s OK with some people, it just depends on where you want to use it. @johnosstyn has a detailed discussion of the system he uses under the laser trip beam topic:
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zone detection. Multiple sensors are used and the event counts only when all the sensors go off at about the same time. Tricky to get just right, and wind is still a problem.
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pressure plates and mats. Completely different technology, switch closes when pressure is applied. Cheap ones ($60 or less) usually trigger at about 25 pounds, more expensive ones have adjustable minimum limits up to truck size. Many are weatherproofed (sealed in mats). Wireless are more expensive than hard wired. Commercial buildings like these for car detection in driveways. Cities use them to schedule some traffic lights.
Hospitals use light range versions under mattresses to alert when a patient gets out of bed. Many marinas use them as deck sensors, sometimes to act as “doorbells” for docked ships, sometimes for alarm systems. I have a friend with a son in a wheelchair in Arizona who uses a deck sensor pressure mat on the entrance ramp in the back yard so that the door opens automatically when the son is coming in. It’s been much more reliable than a previous motion detector system.
So again a lot depends on what you want to use it for. If you’re trying to detect intruders outdoors, trip beam seems to be the most common if there’s an appropriate “bottleneck” location for the beam. Or use multiple beams to cover a wider area. If you essentially want an outdoor switch for a welcome and cooperative visitor, pressure mats might be a less expensive option, although you’ll likely have to do some wiring and you may have to secure the mat itself.