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You can setup an access point that the smart devices can connect. Then you can connect the RasPi wired to your internet router but don't enable routing. This way the smart devices can use data on the RasPi but cannot connect to the internet. Only the RasPi can get into the internet so it can get data from there and store it local. You can find an example at ...


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Using a relay should be your best bet. Also, if the device states that it requires 12V, it doesn't matter how small it is, you would need 12V. It's probably using an extremely low current: W = V*I. You might have better luck asking the EE Stack Exchange.


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