I am a product developer but not an electronics engineer. I am doing a cost analysis of a product idea for my company. The product will need something like this.... There will be two small portable devices (A and B) 1 meter apart (not in line of sight since A can be in pocket). Product A will keep pinging product B at regular intervals. When pinged, B will send response to A for that ping. A knows that B is still available. Now, if we move B away from A (e.g. > 3 meters) and if A pings B and since B is out of range, A will not get response from B. In such case A will sound an alarm. I know we can do device to device communication using RF modules. But I don't know which module or protocol to use for this. My questions (Please answer considering that I am not prototyping but actual product for market): Which protocol and module to use? Can you point me to any module available in market that I can look into? Since A will be pining B at regular interval, will that module drain battery much? If yes, how much long will battery last approximately? Any existing product in market you can think of that does something similar (pinging at regular intervals)? Do I need a micro controller?