How do I write a patent claim for this?

For example , A device will send a signal to the main control room when there is a lack of item in the room?

Just some advice. If I am an audience, I want to know something like these:

how you came up with this idea (show them that you originate it)

if there is any marketing demand, the prospect of the patent, (maybe convenient storage, show your product deserve an approvation)

how you complished the whole patent (the process),

some technology involved with your patent (try to make it scientific),

your uniqueness, the strength of your patent, (show your distinctiveness and innovation, make them believe nobody has done this before)

what kind of patent you want to claim ( I am not really sure about this point. I remember there are maybe two categories of patents: (1)people can change the shape or other details to make use of your patent to be their own, which is low-protective (2)the law protect your patent right much more than the former one, which is high-protective)

I don't know if this answer could help you. Anyway, wish you success!