I am new here and would like to know if Arduino would work for my application. I have a rental property that is rented by the day through an agency. I am concerned that they may be renting it out and not posting it to my account. I think a way to check this would be to have a sensor on the fridge door and somehow get a time stamp every time is it opened and a way for me to access that information. Any help would be greatly appreciated.
Lary
You would need to hide the recorder in a way that the agency and the renters don't find it too easily. Put your phone number on the box so that when they do find it, they don't freak out like they're being bugged.
I'm thinking that if your fridge has a plastic housing over the light, you could put it in there and use the light as the input to the Arduino, maybe even for power. An Arduino Pro Mini will fit almost anywhere and you don't need a lot of other components to do this job.
Do you want to access the data remotely? Drive by the front of the property?, Text message to your phone? Can you get access to the box to plug in a laptop regularly?
The obvious thing to do would be to hook into the refrigerator light's power, or the switch that turns it on. However, then the arduino would likely be inside the fridge, which would make it hard to send any information by radio or wifi. It might also be removed or disengaged by your tenants.
I'd be more inclined to go with a current sensor on the fridge's power cable. You'll have to distinguish between the current that the compressor draws and that for the light.
Then you need a way to transmit the data telling you that someone's using the fridge. If you have wifi in the property, a wifi shield will do the job, otherwise you'll likely need a GSM shield, if the place has cell coverage.
How about a [u]trail cam[/u]? It takes a picture when it senses movement.
The cheap ones don't work remotely, so you'd have to retrieve the images "manually" to check it. And, you'd need a place to conceal or camouflage it so it doesn't get stolen. (It would probably be illegal to put it inside where legitimate guests would be spied-on. 