[Distance beeper]

After reading what I wrote I don't think I described it correctly.  Think of this way, (just example) I have a child who I'm taking to the mall I stop to talk to someone and the child see something and wander away before I even had change to notice.  Once the child is say 10 feet away from me the device (size of beeper) starts to play a noise to tell me that child is no longer close to me (like leash with no leash).  I was thinking the device would just have to send a signal out to see/hear the other device which it then does nothing, if don

[Distance beeper]

Not sure the terms I need but looking to build two device one transmits that you put on something and the other that starts to beep/buzz when other pass the distance of 10 feet away.  I would even be nice to have it so you can adjust the distance before it beeps.  I know of stuff that beeps when you get to close but trying to make so beep when it gets too far away.  I there something already on the market please let me know.

Thanks