Hello All,
I’m trying to figure out what temp sensor is best to work with my ecobee4 and smart things app
Welcome! 
You should have quite a few choices. We do need to ask a couple of questions first.
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what country are you in? The device selection does vary.
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do you have a smartthings/Aeotec hub, or are you working in a “hub optional“ configuration?
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just to be sure, will the sensor be indoors?
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how often does the sensor need to report? It’s pretty typical for sensors that will control a thermostat that they only report temperature once every 15 minutes. This has the advantage of extending the battery life. but some people do prefer a sensor which will report every five minutes or so.
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also, most people are looking for battery operated sensors, or they don’t care whether it’s battery operated or plug-in. If you prefer a plug-in, let us know because there aren’t as many of those available.
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do you want something that works straight out of the box, or are you OK if it requires custom code?
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do you have a particular budget in mind, or is that not as much of an issue?
Once we know the answers to those, we should be able to give you some options.