How this device "Electronic Anti-lost-Anti-theft-Flashlight" measure distance?
That system doesn't measure distance.
It works like this:


You push a button on the transmitter.
The transmitter sends a coded signal.
The receiver decodes the signal and beeps if it is the correct code for that receiver.


The only "distance measurement" that goes on there is that if the receiver is too far away from the transmitter then it can't pick up the signal from the transmitter and so doesn't beep.

Your only real option is to use a low power transmitter that is on all the time.  When the receiver picks up the signal, then it beeps.  Since the transmitter is low power (REALLY low) the receiver will only beep if you get close enough to it.

You can calibrate the transmit strength to make the receiver beep when you get within a desired range.  Objects (like walls and furniture and people) that get in the way will, of course, block part of the signal and shorten the range.

Anything else, and you are going to need a lot of stuff that gets too expensive and bulky to be using as a key finder - if you can do it reliably at all.



For anti-theft, you keep the transmitter on all the time and invert the operation of the receiver.  

So long as it receives the signal from the transmitter, it stays quiet.
It beeps when it loses the signal.  

You just need to make sure that your beeper has a greater range than the transmitter.