Is there an electronic device that will notify you if the item it is on is more than 4-5 feet away from you?

Question

For example, something that could be attached to your keys so if you laid them down and walked 4-5 feet away from them it would beep to remind you that they were laid down. This device could have so many applications, especially in law enforcement.

For example, it could be attached to or embedded in my pda so if i laid it down and started walking away from it, it would beep to notify me that it is out of range. This would keep me from leaving it behind.

Answer 1

Yes, there are called proximity sensors.

However, since they need to be constantly on they drain batteries quite fast. I think I have seen ads to use them for pets