I am looking to build a simple proximity circuit but i am not sure where to start or what methods would work best.

General Idea/goal:
circuit 1 is carried by the user and sends (wireless) signal to circuit 2 (in pulses approximately 10 seconds apart to save battery life) circuit 2 is in the object that the user does not want to loose receives signal and (pings) response to circuit 1 in a similar manner (but preferably without need of a power source) If (ping) is not received by circuit 1 it will give a signal to the user. *signal does not need to go farther than 4 ft*

I am pretty sure this is the basic principal behind RFID but i am asking if this can be simplified to something i could make at home without buying or finding a RFID tag or tag reader

*It does not have to work off radio, i do not know but maybe some form of electromagnetism could be used. Any ideas are welcome. If you have one Please contribute*

 

It only needs to be a wireless switch it does not need to transmit data.

http://www.laipac.com/easy_434a_eng.htm

 

The object you don't want to lose will HAVE to communicate with the one you keep with you. Meaning the two devices will require two way communications in the scheme you have envisioned.

A simpler method would be to have the device you don't want to lose, transmit a signal and the device you carry alarms when it can no longer receive that signal.

 

Having the object transmit the signal will work as long as i can make a vary low powered low-duty cycler. ill work on that for the moment but if anyone has done this before and knows of a good circuit, again, let me know. Thanks.