Most of the motion sensors available that integrate with smart things do not do well outdoors. This is because they are actually heat sensors that respond to a very small change in heat moving across the sensor. When they are outdoors just a cloud going across the sun or a warm breeze can create enough of a variance that the sensor will report. (In fact, several multisensor manufacturers, including Aeon, recommend disabling the motion sensor reporting if you’re going to use the device outdoors for light and temperature measurement.)
There are a couple of different things you can try for this. In particular Mike Maxwell has a nice smart app which looks at multiple sensors in the zone in order to reduce the number of false alerts.
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A lot depends on the specific use case. To be honest, the easiest, least expensive way just to have lights come on when motion is detected outdoors is to use one of the good quality nonnetworked motion lights. I personally like the Mr. beams brand a lot. Very well engineered, excellent value, easy to buy from Amazon, several different form factors. We use these for path lights at our house.
http://www.amazon.com/Beams-MB393-300-Lumen-Weatherproof-Spotlight/dp/B00BCCNZ7K/
If you want to combine that with some kind of smart things notification, the easiest thing is to use one of the outdoor multisensors which includes a lux sensor. The nonnetworked light’s own motion sensor will cause it to come on, and then that bright light will cause the lux sensor to report to Smart-things.
I know this is the reverse set up of what many people think of it first, but it’s very reliable and quite inexpensive without requiring any additional wiring as all the devices are battery powered. You set up as many nonnetworked motion lights as you need, and just position one lux sensor in each zone.
There are a number of threads in the forums discussing possible outdoor motion sensors that do integrate directly with smartthings, but they almost all come to the same conclusion, which is just this is not very easy to do without setting up a lot of relatively expensive motion sensors so you can do zone reporting.
From a security standpoint, the self-contained nonnetworked motion lights will also work even if your Internet is down, or SmartThings is down, or your power is off. So for something like evacuating during a fire or another natural disaster, something like the Mr. Beams will work even if nothing else does. So that’s another point to consider. 