If for some reason you really need to know when a cat or other animal has made a waste deposit in a certain place - this is the project for you. Arduino forum member Mastino created a system that monitors methane levels in his cat box, and when they reach a certain level the system tweets the status to its' own account. If you don't have a cat, don't despair - this project provides the framework for monitoring variables and sending results via twitter - so you can use it for all sorts of tasks.
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