Smart-Key Tag

I was the victim of paying  fine for loosing  the keys in the ground so I have come up with this idea.

How many of us are searching the keys. House key, car key, etc. etc..  Yes, we know there may be  a time   you will be the key of your appliances. But the Solution for the  current problem is,  Just use  the small mobile phone  as a  key-tag.  You can afford  to buy 10USD phone  ok 20USD phone .