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Is it possible to create a script that reads values from an open api that triggers a function on homey that can light a lamp? Example when a value exceeds a sum of 200 a lamp lights up, lamp goes out when value is below 200
Yes, propably, maybe, doubt it and no. 
For more accurate answer you need to give some more info. Connecting to open api can be made with app or webhooks or rest api or mqtt etc.
What kind of api are you trying to connect?
If it is possible to do a get request and you get a json response you can make it work. With homeyscript or http request you can do it. Just du your api request and get the json file. Extract the file in homeyscript or with http request. And make your flows what has to happen
