Here is my use case: I have a battery powered Raspberry Pi reading a couple of sensors that I want to turn on when I walk near it (within a couple hundred feet) and turn off when I walk away. I want it to do this to preserve battery life.
I'm planning on using Bluetooth Low Energy for this. This might not be the right route; I'm open to other suggestions. I know that some products like Tile have managed to squeeze out almost a year of always-on battery life by using BLE. I want to take advantage of this to detect when my phone comes in range and turn the Pi on. From there, it can shut itself off via software.
I've used this Pololu power switch with great success in the past. What I am looking for is the BLE half of the equation, mainly, some kind of board I can throw into my project that (ideally) sets a pin to HIGH whenever a paired device connects and puts it back to LOW when that connection breaks. I don't necessarily care about being able to communicate with the connected device other than knowing if it's paired or not (I can just have my phone pair with the Pi once it boots up and communicate through there). The board should use the least amount of energy possible and put up numbers close to Tile's (a year+ of battery life, considering the battery I'm using for my Pi will definitely store more energy than whatever they put in the Tile).
The only hard design requirement is that the battery, under any circumstance, must not fall below a certain voltage. This is because the battery I'm using belongs to my car, and, well, I need that to stay alive. I can devise a system to turn off the receiver if this situation occurs, but obviously it would be better if the power drain was so little that the device stays on basically for days at a time.
The part I'm concerned about is the feasibility of such a device existing openly (and also cheaply). I'm not a BLE expert by any means, so I don't know if Tile is able to do what it does by some miracle (read: patented) technology breakthrough, or if that kind of hardware can be found online. I would also be open to using an actual Tile for this, if I can somehow flash my own firmware onto it (I don't care about the warranty and I'm not planning on selling this as a product), but I doubt they have exposed JTAG pins (lol) and as such the difficulty might make it not worth it.
Is such technology available? Could I get a microcontroller with always-on BLE (or polling at the 100-200ms range) that would fit the requirements I want?