At $99, the SmartThings Hub is the brain of the system, and will monitor and control all of your sensors. Even if you don't have any sensors, it'll be able to track when your smartphone comes and goes, then use that as a trigger.
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The Hub is easy to install: just plug it into your router and power it up. The SmartThings app, free for iOS and Android devices, will take care of the rest.
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Motion detection and more
The SmartThings motion detector doubles as a range extender for the Hub.
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The magnetic entry sensor is especially clever. In addition to monitoring when the door or window it's attached to opens or closes, it'll detect vibrations and also track the temperature.
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I sense a presence...
Want the door to lock automatically when you leave, and the lights to turn on automatically when you get home? Clip a SmartThings presence sensor to your keychain, book bag, or purse, and your system will be able to track when you come and go.
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No place like home
You'll be able to teach your system where "home" is from within the app, and also customize how far away you have to be before anything triggers.
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Other sensors, too
SmartThings is compatible with a wide variety of sensors. This moisture sensor will alert you if there's a leak.
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Works with IFTTT
With IFTTT's SmartThings channel, you'll be able to use your sensors to trigger other IFTTT-friendly devices like the Belkin WeMo Switch or Philips Hue bulbs. You also be able to hook them up with popular online services like Twitter, Google Drive, and Foursquare.
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Consider a kit
If you want to try out a variety of sensors, you might go with one of SmartThings kits. Prices start at $199.